Schematic Reference

Unplugging or re-connecting equipment to the serial COM or PS2 connector always gives problems if the PC is running. Even if you only need to swap a mouse or changeover from a graphics keyboard to a standard keyboard. The chances are that the connected equipment will not communicate with the PC, it will always be necessary to re-boot. If you are really unlucky you may have damaged the PC or the peripheral device. In order to switch equipment successfully it is necessary to follow a sequence. The clock and data lines need to be disconnected from the device before the power line is removed. And likewise the power line must be connected first to the new device before the clock and data lines are re-connected.

This sequence is also used by the USB connector but achieved rather more simply by using different length pins in the connector. The circuit shown here in Figure 1 performs the switching sequence electronically. The clock and data lines from the PC are connected via the N.C. contacts of relay RE2 through the bistable relay RE1 to connector K3. Pressing push-button S1 will activate relay RE2 thereby disconnecting the data and clock lines also while S1 is held down the semiconductor switch IC1B will be opened, allowing the voltage on C4 to charge up through R4. After approximately 0.2 s the voltage level on C4 will be high enough to switch on IC1A, this in turn will switch on T1 energizing one of the coils of the bistable relay RE1 and routing the clock, data and power to connector K2.


When S1 is released relay RE2 will switch the data and clock lines through to the PC via connector K1. It should be noted that the push-button must be pressed for about 0.5s otherwise the circuit will not operate correctly. Switching back over to connector K3 is achieved similarly by pressing S2. The current required to switch the relays is relatively large for the serial interface to cope with so the energy necessary is stored in two relatively large capacitors (C2 and C3) and these are charged through resistors R1 and R6 respectively. The disadvantage is that the circuit needs approximately 0.5 minute between switch-overs to ensure these capacitors have sufficient charge.

The current consumption of the entire circuit however is reduced to just a few milliamps. The PCB is designed to accept PS2 style connectors but if you are using an older PC that needs 9 pin sub D connectors then these will need to be connected to the PCB via flying leads. In this case the mouse driver software configures pin 9 as the clock, pin 1 as the data, pin 8 (CTS) as the voltage supply pin and pin 5 as earth.

Resistors:
R1 = 2kΩ2
R2 = 47kΩ
R3 = 10kΩ
R4 = 4kΩ7
R5 = 1kΩ
R6 = 1kΩ2
Capacitors:
C1 = 10µF 10V radial
C2 = 1000µF 10V radial
C3 = 2200µF 10V radial
C4 = 2µF2 10V radial
Semiconductors:
D1-D5 = 1N4148
T1 = BC547
IC1 = 4066 or 74HCT4066
Miscellaneous:
RE1 = bistable relay 4 c/o contacts
RE2 = monostable relay 2 c/o contacts
K1,K2,K3 = 6-way Mini-DIN socket (pins at 240°, PCB mount
S1,S2 = push-button (ITTD6-R)

Here is the circuit diagram of superb mini audio power amplifier, That can be power with 4.5 volt dc to 18 volts dc (maximum). This amplifier is based on TDA1015, Product of NXP Semiconductors formerly PHILIPS Semiconductors. The TDA1015 is a monolithic integrated audio amplifier circuit in a 9-lead single in-line (SIL) plastic package. The device is especially designed for portable radio and recorder applications and delivers up to 4 watt in a 4 ohm load impedance. The very low applicable supply voltage of 3,6 V permits 6 V applications.

click on the images to enlarge

Parts:

R1330K
R25.6K
R34.7R
C11uF-25V
C21uF-25V
C3100pF
C4100nF-63V
C5182pF
C6224pF
C7100uF-25V
C8100nF-63V
C910uF-25V
C101KuF-25V
IC1TDA1015

Applications:

• In-car use
• Your own unique application
• Power amplifier for audio projects
• For use with portable audio equipment
• Small but powerful multi-purpose amplifier

Special features:

• Low current drain
• High output power
• Thermal protection
• High input impedance
• Separated preamplifier and power amplifier
• Limited noise behavior at radio frequencies
• Single in-line (SIL) construction for easy mounting

Specification:

• Quiescent current : 12mA
• Thermal and short circuit protection
• Frequency Response : 60Hz - 15Khz
• Max. output power : 3W (4ohm/12V)
• Input sensitivity : 20-15mV selectable
• Power supply : 4.5 - 15V DC @ 400mA

The circuit shows one way of obtaining a voltage of 90V from a 1.5V battery supply. The LT1073 switching regulator from Linear Technology (www.linear-tech.com) operates in boost mode and can work with an input voltage as low as 1.0 V. The switching transistor, which is hidden behind connections SW1 and SW2, briefly takes one end of choke L1 to ground. A magnetic field builds up in the choke, which collapses when the transistor stops conducting: this produces a current in diode D1 which charges C3. The diode cascade comprising D1, D2, D3, C2, C3 and C4 multiplies the output voltage of the regulator by four, the pumping of C2 causing the voltage developed across C4 via C3, D2 and D3 to rise.

Finally, the regulator control loop is closed via the potential divider (10 MΩ and 24 kΩ). These resistors should be 1 % tolerance metal film types. With the given component values, fast diodes with a reverse voltage of 200 V (for example type MUR120 from On Semiconductor www.onsemi.com) and a choke such as the Coilcraft DO1608C-154 (www.coilcraft.com) an output voltage of 90 V will be obtained. The output of the circuit can deliver a few milliamps of current.

The circuit depicted here forms one half of a device that will prove extremely handy when tracing the path of electrical wiring in a building or to locate a break in a wire. The system is based on similar equipment that is used by technicians in telephone exchanges. The operation is straightforward. You require a generator that delivers an easily recognizable signal which, using a short antenna, is inductively coupled to a simple, but high gain, receiver. To create a useful transmitter it would suffice to build a simple generator based on a 555. But as the adjacent diagram shows, a 556 was selected instead. The second timer (IC1a) is used to modulate the tone produced by IC1b.

The output frequency alternates between about 2100 Hz and 2200 Hz. This is a very distinctive test signal that is easily distinguished from any other signals that may be present. Resistor R6 is connected to a piece of wire, about ten centimeters long, that functions as the antenna. The ground connection (junction C2-C3) is connected to ground. When the antenna is connected directly to a cable, it is possible to determine at the other end of the cable, with the aid of the receiver, which conductor is which (don’t do this with live conductors!). The schematic for the matching receiver may be found elsewhere in this website.

The circuit depicted is the receiver device of a transmitter/receiver combination that will prove extremely handy when tracing the path of electrical wiring in a building or to locate a break in a wire. The corresponding transmitter may be found elsewhere in this website. The transmitter produces a distinctive tone which alternates between 2100 Hz and 2200 Hz. The matching receiver for the wire tracer is possibly even simpler than the transmitter, as is shown by the schematic. It consists of no more than a short wire antenna (a piece of wire, 10 cm long is adequate), a high-pass filter (C1-R1), an amplifier stage (IC1), an output stage (T1) and a loudspeaker.

The prototype used a high impedance loudspeaker from a telephone handset, and this worked remarkably well. The purpose of P1 is to adjust the amplification. At the highest amplification, the wire energized by the transmitter can be traced from several tens of centimeters away. A direct electrical connection is therefore not required. However, it is important that you hold the ground connection (earth) in your hand.

This tester is intended to quickly check whether a transistor is functional or not and possibly also select two or more transistors with (approximately) equal gains. This is about the simplest conceivable test circuit, so don’t expect super accuracy. The circuit has been designed only to quickly carry out a brief check, when there is no time or equipment to carry out a thorough test. The operation is simple: in the position ‘battery test’ (S2 closed), the 10mA moving coil meter M1 in series with a 600 Ω resistor (R4 + R5) is connected to a 6 V battery. A current of 10mA will flow, resulting in full-scale deflection of the meter. When a transistor is being tested (S2 open, S3 in position 2 or 3) a current will flow through the base-emitter junction of the transistor under test, the value of which can be computed by dividing the voltage across R1 or R2 by its resistance.

With S3 in position 2 this will be (6 V – 0.6 V)/560 kΩ = approx. 10µA. If the transistor has a gain of 1000 it will cause a collector current (and therefore a meter current) of 10mA, causing full-scale deflection of the moving coil instrument. Therefore, the value indicated by the meter, when S3 is in position 2, has to be multiplied by a factor of 100 to obtain the gain of the transistor. In position 3 the base resistor is 10 times lower (R1 = 56 kΩ), so in this case the reading has to be multiplied by 10 to obtain the gain. It will be clear that position 2 of S3 is intended for high gains of up to 1000 and position 3 for gains of 0 to 100. The purpose of S1 is to reverse the polarity: the upper position drawn is for NPN transistors, the bottom for PNP types. If you have no moving coil instrument available, it is of course also possible to replace M1 with a digital meter.

More and more electronic devices are portable and run off batteries. It is no surprise, then, that so many flat batteries find their way into the bin - and often far too early. When a set of batteries can no longer run some device - for example, a flashgun - the cells are not necessarily completely discharged. If you put an apparently unserviceable AA-size cell into a radio-controlled clock with an LCD display it will run for months if not years. Of course not every partially discharged cell can be put in a clock. The circuit presented here lets you squeeze the last Watt-second out of your batteries, providing a bright ‘night light’ - for free!

The circuit features a TBA820M, a cheap audio power amplifier capable of operating from a very low supply voltage. Here it is connected as an astable multivibrator running at a frequency of around 13 kHz. Together with the two diodes and electrolytic capacitor this forms a DC-DC converter which can almost double the voltage from between four and eight series-connected AA-, C- or D-size cells, or from a PP3-style battery. The DC-DC converter is followed by a constant current source which drives the LED. This protects the expensive white LED: the voltages obtained from old batteries can vary considerably.

With the use of the DC-DC converter and 20 mA constant current source a much greater range of usable input voltages is achieved, particularly helpful at the lower end of the range when old batteries are used. With the constant current source on its own the white LED would not be adequately bright when run from low voltages. An additional feature is the ‘automatic eye’. The LDR detects when the normal room lighting is switched on or when the room is lit by sunlight: its resistance decreases. This reduces the UBE of the transistor below 0.7 V, the BC337 turns off and deactivates the LED.

This prolongs further the life of the old batteries. A further LDR across capacitor C reduces the quiescent current of the circuit to just 4mA (at 4V). Light from the white LED must of course not fall on the LDR, or the current saving function will not work.

Attention: This Circuit is using high voltage that is lethal. Please take appropriate precautions

Using this circuit you can convert the 12V dc in to the 220V Ac. In this circuit 4047 is use to generate the square wave of 50hz and amplify the current and then amplify the voltage by using the step transformer.

How to calculate transformer rating

The basic formula is P=VI and between input output of the transformer we have Power input = Power output. For example if we want a 220W output at 220V then we need 1A at the output. Then at the input we must have at least 18.3V at 12V because: 12V*18.3 = 220v*1
So you have to wind the step up transformer 12v to 220v but input winding must be capable to bear 20A.

A number of people have been unable to find the transformer needed for the Black Light project, so I looked around to see if I could find a fluorescent lamp driver that does not require any special components. I finally found one in Electronics Now. Here it is. It uses a normal 120 to 6V stepdown transformer in reverse to step 12V to about 350V to drive a lamp without the need to warm the filaments.

Parts:

C1 100uf 25V Electrolytic Capacitor
C2,C3 0.01uf 25V Ceramic Disc Capacitor
C4 0.01uf 1KV Ceramic Disc Capacitor
R1 1K 1/4W Resistor
R2 2.7K 1/4W Resistor
Q1 IRF510 MOSFET
U1 TLC555 Timer IC
T1 6V 300mA Transformer
LAMP 4W Fluorescent Lamp
MISC Board, Wire, Heatsink For Q1

Notes:

1. Q1 must be installed on a heat sink.
2. A 240V to 10V transformer will work better then the one in the parts list. The problem is that they are hard to find.
3. This circuit can give a nasty (but not too dangerous) shock. Be careful around the output leads.

The LP2951 regulator is manufactured by National Semiconductors. The choice of values is from an application note "Battery Charging", written by Chester Simpson. Diode D1 can be any diode from the 1N00x series, whichever is conveniently available. It functions as a blocking diode, to prevent a back flow of current from the battery into the LP2951 when the input voltage is disconnected. Charging current is about 100+mA, which is the internally-limited maximum current of the LP2951. For those wondering, this is compatible with just about any single-cell li-ion battery since li-ion can generally accept a charging current of up to about 1c (i.e. charging current in mA equivalent to their capacity in mAh, so a 1100mAh li-ion cell can be charged at up to 1100mA and so on).

A lower charging current just brings about a correspondingly longer charge time. IMHO 100mA is quite low, low enough that the circuit can be used for an overnight charger for many typical single-cell li-ion batteries. The resistors are deliberately kept at large orders of magnitude (tens/hundred Kohm and Mohm range) to keep the off-state current as low as possible, at about 2?A. Resistor tolerances should be kept at 1% for output voltage accuracy. The 50k pot allows for an output voltage range between 4.08V to 4.26V - thus allowing calibration as well as a choice between a charging voltage of 4.1V or 4.2V depending on the cell to be charged. The capacitors are for stability, especially C2 which prevents the output from ringing/oscillating.

Parts List:

IC1 = LP2951, voltage regulator
D1 = 1N4002, General purpose diode
R1 = 2M, 1%, metal-film
R2 = 806K, 1%, metal-film
P1 = 50K, potentiometer
C1 = 0.1uF, polyester
C2 = 2.2uF/16V, electrolytic
C3 = 330pF, ceramic

The above pictured schematic diagram is just a standard constant current model with a added current limiter, consisting of Q1, R1, and R4. The moment too much current is flowing biases Q1 and drops the output voltage. The output voltage is: 1.2 x (P1+R2+R3)/R3 volt. Current limiting kicks in when the current is about 0.6/R1 amp. For a 6-volt battery which requires fast-charging, the charge voltage is 3 x 2.45 = 7.35 V. (3 cells at 2.45v per cell). So the total value for R2 + P1 is then about 585 ohm. For a 12 V battery the value for R2 + P1 is then about 1290 ohm. For this power supply to work efficiently, the input voltage has to be a minimum of 3V higher than the output voltage. P1 is a standard trimmer potentiometer of sufficient watt for your application. The LM317 must be cooled on a sufficient (large) coolrib. Q1 (BC140) can be replaced with a NTE128 or the older ECG128 (same company). Except as a charger, this circuit can also be used as a regular power supply.

Parts List:

R1 = 0.56 Ohm, 5W, WW
R2 = 470 Ohm C2 = 220nF
R3 = 120 Ohm
R4 = 100 Ohm
C1 = 1000uF/63V
Q1 = BC140
Q2 = LM317, Adj. Volt Reg.
C3 = 220nF (On large coolrib!)
P1 = 220 Ohm

Except for use as a normal Battery Charger, this circuit is perfect to 'constant-charge' a 12-Volt Lead-Acid Battery, like the one in your flight box, and keep it in optimum charged condition. This circuit is not recommended for GEL-TYPE batteries since it draws to much current. The above circuit is a precision voltage source, and contains a temperature sensor with a negative temperature coλficient. Meaning, whenever the surrounding or battery temperature increases the voltage will automatically decrease. Temperature coλficient for this circuit is -8mV per °Celcius. A normal transistor (Q1) is used as a temperature sensor. This Battery Charger is centered around the LM350 integrated, 3-amp, adjustable stabilizer IC. Output voltage can be adjusted with P1 between 13.5 and 14.5 volt.
T2 was added to prevent battery discharge via R1 if no power present. P1 can adjust the output voltage between 13.5 and 14.5 volts. R4's value can be adjusted to accommodate a bit larger or smaller window. D1 is a large power-diode, 100V PRV @ 3 amp. Bigger is best but I don't recommend going smaller. The LM350's 'adjust' pin will try to keep the voltage drop between its pin and the output pin at a constant value of 1.25V. So there is a constant current flow through R1. Q1 act here as a temperature sensor with the help of components P1/R3/R4 who more or less control the base of Q1. Since the emitter/base connection of Q1, just like any other semiconductor, contains a temperature coλficient of -2mV/°C, the output voltage will also show a negative temperature coλficient.

That one is only a factor of 4 larger, because of the variation of the emitter/basis of Q1 multiplied by the division factor of P1/R3/R4. Which results in approximately -8mV/°C. To prevent that sensor Q1 is warmed up by its own current draw, I recommend adding a cooling rib of sorts. (If you wish to compensate for the battery-temperature itself, then Q1 should be mounted as close on the battery as possible) The red led (D2) indicates the presence of input power.Depending on what type of transistor you use for Q1, the pads on the circuit board may not fit exactly (in case of the BD140).

Four timing controls - 12V supply, Suitable for Halloween or Christmas props

This circuit, designed on request for a Halloween prop, allows fine control of a pulsing Super Bright white LED. The four potentiometers or trimmers will set precisely: on, off, ramp up and ramp down time-delays respectively. Ramp up and ramp down time-delays can be set roughly in the 1 - 15 seconds range, whereas on and off time-delays can range from a few seconds to about one minute. A 12V battery or regulated power supply is required, provided it is reasonably stable. Total current drawing is about 25 - 30mA when the LED reaches maximum brightness.

Parts:

R1,R5,R12,R13___10K 1/4W Resistors
R2,R5___________10K 1/2W Trimmers or Lin. Potentiometers
R3______________47K 1/4W Resistor
R4______________22K 1/4W Resistor
R6_______________1K 1/4W Resistor
R7,R8,R9,R14___100K 1/4W Resistors
R10,R11__________2M2 1/2W Trimmers or Lin. Potentiometers
R15____________220R 1/4W Resistor
C1,C2__________100nF 63V Polyester or ceramic Capacitors
C3,C4___________22µF 25V Electrolytic Capacitors
C5_____________220µF 25V Electrolytic Capacitor
D1,D2________1N4148 75V 150mA Diodes
D3______________LED Super Bright white (e.g. RL5-UV2030)
Q1____________BC337 45V 800mA NPN Transistor
IC1___________LM324 Low Power Quad Op-amp IC
IC2____________4093 Quad 2 input Schmitt NAND Gate IC

Notes:

• Wanting to use two white LEDs, the second device must be wired across the Emitter of the transistor and negative ground with its own limiting resistor wired in series, like R15 and D3 in the circuit diagram.
• If common red, yellow or green LEDs are required, please wire two of them in series, in order to present roughly the same voltage drop of one white or blue LED.
• Please note that the unused sections in both ICs must have their inputs tied to negative ground whereas the outputs must be left open, as shown at the bottom of the diagram.
• All time-delays can be increased by changing the value of C3 and C4 to 47µF 25V or even higher. Please vary the value of these capacitors only, as the values of the resistors wired to the four control pots are rather critical and should not be changed.

Can drive up to 15 LEDs or LED-clusters, Selectable Bar-length

This circuit, designed on request, allows up to 15 LED clusters to illuminate in bar-mode sequence. LED sequencing will start at power-on and, after reaching the desired output, all the LEDs turn off and sequence restarts. The number of LEDs or clusters forming the bar can be selected by connecting R7 to the appropriate output pin of IC2 or IC3.

Parts:

R1,R5,R9_________1K 1/4W Resistors
R2______________33K 1/4W Resistor
R3_____________100K 1/2W Trimmer Cermet
R4_______________1M 1/4W Resistor
R6,R10__________10K 1/4W Resistors
R7,R8___________22K 1/4W Resistors
R11______________4K7 1/4W Resistor
R12_____________33R 1/4W Resistor (See Notes)
C1______________10µF 25V Electrolytic Capacitor
C2_____________100nF 63V Polyester Capacitor
C3_____________470µF 25V Electrolytic Capacitor
D1--D14________LEDs (See Notes)
Q1___________2N3819 General-purpose N-Channel FET
Q2,Q3,Q5______BC547 45V 100mA NPN Transistors
Q4____________BC337 45V 800mA NPN Transistor
IC1____________7555 or TS555CN CMos Timer IC
IC2,IC3________4094 8-stage shift-and-store bus register IC

Notes:

• R5 and D1 are optional: they could be of some utility in monitoring the sequence frequency set by means of R3.
• The terminal of R7 bearing an arrow must be connected to the desired output pin of IC2 or IC3 in order to select the number of LEDs or clusters forming the bar.
• For example: if you want to drive seven LEDs or clusters connect R7 to pin#11 of IC2 (Output 8) and the LED or cluster drivers to Outputs 1 to 7 respectively.
• Clusters can be formed by up to 12 LEDs as shown in the circuit diagram, right side. Common cluster types usually range from 5 to 10 LEDs.
• Up to 15 of these cluster driver circuits, each formed by the LEDs, two transistors and three resistors can be built and connected to the progressively numbered outputs of IC2 (the first eight clusters) and IC3 (the remaining clusters).
• If a number of clusters up to 7 is required, IC3 can be omitted.
• Constant output current value for the LEDs can be changed by varying R10.
• The formula is: R = 0.6/I (I expressed in Amperes).
• Wanting to drive only one LED per output instead of a cluster, the above mentioned cluster driver can be substituted by a single transistor, as shown in the circuit formed by D2, Q3, R8 and R9.

It normally takes two transistors to build a blinker circuit (in order to make positive feedback possible). However, you can also use a photo-resistor (LDR) that is illuminated by an LED. The feedback takes place here by means of light rays. The circuit is easy to understand. When light falls on the LDR, the current increases. The capacitor then charges, and this increases the base current. This causes the transistor to switch the LED fully on. The stable ‘on’ state switches to the ‘off ’ state as soon as the capacitor is fully charged. The LED is then completely off, the base voltage goes negative and the transistor is cut off.

The circuit cannot switch back to the ‘on’ state until the capacitor has been discharged via the base resistor. The circuit naturally reacts to external light sources as well. You will have to test it in different light environments to see whether it will work. In any case, it will not work in full sunlight. With an ultra-bright LED and a very low-resistance LDR, it might be possible to build a blinker without using a transistor. The combination of the LED and the LDR would have to provide the gain that is needed to produce oscillations.

Nearly always when a remote control doesn’t work, the underlying problem is elementary: the unit does not emit light. The cause may be dry solder joints, defective LEDs etc., but also a flat battery (perhaps due to stuck key). The human eye is unable to perceive infra-red light. By contrast, an ordinary photo transistor like the BP103 has no problems working in the infrared spectrum, so in the circuit here it simply biases the BC558 which, in turn, makes LED D1 flash in sympathy with the telegram from the remote control. The preset in the circuit determines the sensitivity.

If you have an application in which a MOSFET is already used to switch a load, it is relatively easy to add short-circuit or overload protection. Here we make use of the internal resistance RDS(ON), which produces a voltage drop that depends on the amount of current flowing through the MOSFET. The voltage across the internal resistance can be sensed using simple comparator or even a transistor, which switches on at a voltage of around 0.5V. You can thus avoid the use of a sense resistor (shunt), which usually produces an undesirable extra voltage drop. The comparator can be monitored by a microcontroller. In case of an overload, the software can initiate suitable countermeasures (PWM regulation, alarm, emergency stop etc.). It is also conceivable to connect the comparator output directly to the gate of the MOSFET, in order to immediately cut off the transistor in case of a short circuit.

The schematic diagram shows an audio stage with a common-collector circuit. This does not damp the tuned circuit, but instead actually increases its response. This yields good sensitivity and selectivity. Due to the low supply voltage, the subsequent audio amplifier needs three transistor stages. The volume is adjusted using the potentiometer. This radio works well using an internal ferrite rod (around 1 cm diameter and 10 cm long) with a winding of around 50 turns of enameled copper wire. With a two-meter external wire aerial, you can receive even more stations. This radio is not only economical in terms of components, it also needs very little ‘juice’: since the current consumption is only 10mA, an alkaline AA cell will easily last for around 200 hours of operation.


The specifications, very briefly stated, are:

• medium-wave receiver with ferrite aerial
• optional supplementary aerial
• power supply 1.5 V/10 mA
• 4 transistors
• loudspeaker output

This simple adapter circuit is specially intended for use with the USB Audio DAC published in this website elsewhere. With an easily implemented modification, it is possible to make the output of the D/A converter pseudo-symmetric, so that it can be connected to professional equipment having XLR line inputs. This will do even more justice to the high quality of the USB Audio DAC. The modification actually amounts to just adding a single resistor (R11a) and changing the value of the existing resistor at the output of the audio DAC (R11) from 100 Ωto 68Ω. Components C14 and R12 remain unchanged. It is not difficult to make this change on the printed circuit board of the audio DAC, but a bit of improvisation is necessary. After replacing R11 with a 68-Ω version, unsolder R12 and connect R11a in series with it. Bring out the junction of these two resistors to act as the signal return connection (pin 3 of the XLR socket). The same operation must also be carried out on the right channel, where the affected resistors are labelled R16, R16a and R17.

Here is a deluxe version of the simple charge rate limiter, using the same idea but with the ability to charge two packs simultaneously from a single wall charger. For circuit description and parts list, see the simple charger page. Since wall chargers provide about 55mA, you should not use this dual circuit to charge batteries at rates greater than 27mA (for a total of 54mA).

This ultra-simple remote control extender is ideal for use with a hidden video recorder. The recorder is a Panasonic NV-SD200 and is used as part of a camera surveillance system. A PICAXE-08-based circuit is used to detect events and control the recorder. It also flashes a LED near the monitor to indicate the number of events since last viewing.
Click for larger image

Strangely, the NV-SD200 model refused to work with a number of commercial infrared remote control extenders, hence the need for this design. As a bonus, it uses less power than a traditional extender (no plugpacks) and the remote can still be used in the normal manner.

As shown, an additional 5mm infrared LED is mounted directly in front of the equipment to be controlled. This is cabled back to a convenient location near the monitor and terminated in a 3.5mm plug.

To modify the remote control unit, break the circuit to the anode of the existing infrared LED and wire in a 3.5mm headphone socket. In most cases, the LED will be accessible without dismantling the circuit board. The purpose of the socket is to allow the existing infrared LED to operate normally when the jack is unplugged.

If the socket won’t fit inside the case, then a very short flying lead with a moulded in-line socket can be used instead. By using light-duty figure-eight cable, the transmitting LED could be 30m or more from the hand-held remote control without problems.

The circuit as shown in the figure employs 14 bi-colour (red and green) LEDs having three terminals each. Different dancing colour patterns are produced using this circuit since each LED can produce three different colours. The middle terminal (pin 2) of the LEDs is the common cathode pin which is grounded. When a positive voltage is applied to pin 1, it emits red light. Similarly, when positive voltage is applied to pin 3. it emits green light. And when positive voltage is simultaneously applied to its pins 1 and 3, it emits amber light. The circuit can be used for decorative lights. IC1 (555) is used in astable mode to generate clock signal for IC2 and IC3 (CD4518) which are dual BCD counters.

Both counters of each of these ICs have been cascaded to obtain 8 outputs from each. The outputs from IC2 and IC3 are connected to IC4 through IC7 which are BCD to 7-segment latch/decodor/driver ICs. Thus we obtain a total of 14 segment outputs from each of the IC pairs consisting of IC4 plus IC5 and IC6 plus IC7. While outputs from former pair are connected to pin No. 1 of all the 14 bi-colour LEDs via current limiting resistors, the ouputs of the latter pair are similarly connected to pin No.3 of all the bi-colour LEDs to get a magical dancing lights effect.

Such jobs as jewel cutting and polishing require the workers to switch on/off two electrical appliances one after another repeatedly for two different services on the same workpiece. This is cumbersome as they need to fully concentrate on delicate handwork on precious jewels. Switching in such situations cannot be done by hand, and doing it by foot using ordinary switches is too tedious. This is mainly because of the difficulty in sensing and controlling the switch position by foot. Ordinary pushbutton switches make or break a contact momentarily, and they can not hold the keypress status. You need a bistable multivibrator with two independent trigger inputs to solve this problem. Here’s a smart foot switch based on dual negative-edge triggered master slave JK flip-flop IC 74LS76 (IC1).

J1 and J2 inputs are conneted to 5V through resistors R2 and R5 (each 10k), respectively. K1 and K2 inputs are grounded. Preset pins 2 and 7 are shorted and connected to 5V via resistor R7 (10k). Push-to-on switch S3 connected to the preset inputs is also grounded. Clock and clear inputs of the two flip-flops are cross-connected, i.e. CLK1 (pin 1) is conneted to CLR2 (pin 8) and CLR1 (pin 3) is connected to CLK2 (pin 6). Clock input pins 1 and 6 are pulled up high through resistors R1 and R4 (each 4.7k), respectively. Push-to-on switches S1 and S2 are connected between clock and ground of the flip-flops. Switch S1 activates device 1, while switch S2 activates device 2. Switch S3 activates both device 1 and device 2 simultaneously.

Device status is indicated by LED1 and LED2. Glowing of LED1 and LED2 indicates that device 1 and device 2, respectively, are in on condition. The LEDs are connected from +5V to Q1 (pin 14) and Q2 (pin 10) of IC1 through resistors R3 and R6, respectively. Initially when the power supply is switched on, Q1 and Q2 outputs of the JK flip-flops are at low level (logic 0). When switch S1 is pressed for the first time, the high level (logic 1) present at J1 input is transferred to Q1 output on the trailing edge of clock (CLK1). The high level (logic 1) at Q1 activates relay RL1 through pin 16 of IC ULN2003 (IC2), turning on device 1 via its normally-opened (N/O) contacts. Clock CLK1 of flip-flop IC1(A) is also connected to clear input CLR2 of flip-flop IC1(B) so as to clear it asynchronously.

Switch debounces don’t affect the circuit as the same J1 state is being transferred to Q1 output on succeeding trailing edges. At the same time, device 2 is switched off. When switch S2 is pressed, flip-flop IC1(A) gets cleared via CLR1 and the high state of J2 input of flip-flop IC1(B) is transferred to its Q2 output on the trailing edge of clock (CLK2). This high level (logic1) activates relay RL2 through pin 15 of IC2, turning on device 2 via its N/O contacts. At the same time, device 1 is switched off. Now if you want to turn on both the devices simultaniously, press switch S3 momentarily. Switch S3 provides ground to preset inputs PRE1 and PRE2 of flip-flops IC1(A) and IC1(B), making their Q1 and Q2 outputs high, which energises both the relays turning on the two devices. LEDs glow to indicate that both the devices are ‘on.’ Place all the three switches (S1 through S3) where you can easily press them by foot when required. The LEDs can also be mounted at a convenient location to know whether the devices are turned on.

This circuit is very useful while checking/operating counters, stepping relays, etc. It avoids the procedure of setting a switch for the required number of pulses. By pressing appropriate switches S1 to S9, one can get 1 to 9 negative-going clock pulses, respectively. Schmitt trigger NAND gate N1 of IC2, resistor R1, and capacitor C1 are wired to produce clock pulses. These pulses are taken out through NAND gate N3 that is controlled by decade counter CD4017 (IC1). Initially no switch from S1 to S9 is depressed and the LED is glowing. As pins 5 and 6 of NAND gate N2 are pulled up by resistor R3, its output pin 4 goes low. This disables NAND gate N3 to take its output pin 10 to high state, and no pulse is available. IC1 is a decade counter whose Q outputs normally remain low.

When clock pulses are applied, its Q outputs go high successively, i.e. Q0 shifts to Q1, Q1 shifts to Q2, Q3 shifts to Q4, and so on. If any one of switches S1 through S9, say, S5 (for five pulses), is momentarily depressed, pins 5 and 6 of NAND gate N2 go low, making its output pin 4 high, which fully charges capacitor C2 via diode D. At the same time, this high output of N2 enables NAND gate N3 and clock pulses come out through pin 10. These are the required number of pulses used to check our device. The clock pulses are fed to clock-enable pin 13 of IC1, which starts counting. As soon as output pin 1 (Q5) of IC1 turns high, input pins 5 and 6 of NAND gate N2 will also become high via switch S5 because high-frequency clock allowed five pulses during momentary pressing.

This high input of N2 provides low output at pin 4 to disable NAND gate N3 and finally no pulse will be available to advance counter IC1. Before the next usage, counter IC1 must be in the standby state, i.e. Q0 output must be in the high state. To do this, a time-delay pulse generator wired around NAND gate N4, resister R4, diode D, capacitor C2, and differentiator circuit comprising C3 and R5 is used. When output pin 4 of NAND gate N2 is low, it discharges capacitor C2 slowly through resistor R4. When the voltage across capacitor C2 goes below the lower trip point, output pin 11 of NAND gate N4 turns high and a high-going sharp pulse is produced at the junction of capacitor C3 and resistor R5.

This sharp pulse resets counter IC1 and its Q0 output (pin 3) goes high. This is represented by the glowing of LED. Ensure the red LED is glowing before proceeding to get the next pulse. Press any of the switches momentarily and the LED will glow. If the switch is kept pressed, the counter counts continuously and you cannot get the exact number of pulses.

This circuit is especially designed for those who often need to wake up early in the morning. Ordinary alarms in electronic watches are not loud enough and very often they fail to wake up. The switch circuit described here will come handy; it can be used to switch on a TV, radio or tape recorder etc, which will not allow even the laziest amongst us to ignore their sound for too long. Besides, this time switch can also be used to switch on/off any other electric or electronic gadget at any time. What you need is a simple analogue electronic clock with alarm facility and a small circuit to implement the time switch. This time switch has two modes. One is ‘time-on’ mode and the other is ‘time-off ’ mode. In time-on mode, you set up the alarm in your clock as per normal procedure and at the set time this switch turns on the gadget connected at the output socket-1.

In time-off mode, it turns your gadget off at the set time. The optional output socket-2 is wired in such a way that when you use this socket, the mode changes without having to flip the mode switch (i.e.mode switch can be omitted). Please refer to the back panel diagram of a typical analogue clock and the audio jack, to see how the existing buzzer of the clock is required to be wired to the audio output from the clock. This will ensure that when plug is inserted in the audio jack, the clock’s buzzer will remain off and not consume any power unnecessarily. The audio alarm output from the clock is coupled to the AF detector built around low-power switching transistor T1. During alarm, the collector of transistor T1 will fluctuate around ground level and Vcc.

During absence of audio alarm input, the collector of transistor T1 is held at Vcc potential. The next stage consists of an S-R latch built around NAND gates N1 and N2. Capacitor C2 and resistor R4 are used for power-on-reset. On switching the power supply, gate N2 output will acquire logic 1 and that of gate N1 logic 0. This is the initial state, irrespective of the position of mode switch. At the time of alarm, when point A connected to collector of transistor T1 passes through logic 0 state, the output logic state of both the gates will toggle. Assuming that mode switch is flipped to ‘Mode Off’ position at power-on-reset (when point D is at logic 1), initially diode D1 would be in blocking state and transistor T2 would be forward biased via resistor R5 and diodes D2 and D3. As a result, the relay is in energised state, which makes output power available at output socket1 and cuts it off from socket-2.

At alarm time, the audio signal toggles logic output states of both gates N1 and N2. As a result, point D goes to logic 0 state. Diode D1 conducts, taking the voltage at junction of diodes D1 and D2 to near about 1 volt. Diode D3 ensures that its series combination with diode D2 puts them in blocking mode. Capacitor C3 meanwhile discharges via resistor R6 and the voltage at base of transistor T2 approaches towards ground level, cutting off transistor T2 and de-energising relay RL1. Now the power at output socket-1 would be cut off while it becomes available in socket-2. If the above operation is repeated with switch S1 in ‘Mode On,’ the power would initially not be available in socket-1 (but available in socket-2). But after the alarm, the power would become available in socket-1 and not in socket-2.

Cordless telephones are very popular nowadays. But they have a major drawback, i.e. they cannot be operated during power failure. Therefore usually another ordinary telephone is connected in parallel to the cordless telephone. This results in lack of secrecy. UPS is a permanent solution to this problem. Since the UPS is meant only for the cordless telephone, its output power is limited to around 1.5W. This is sufficient to operate most cordless telephones. as these employ only small capacity adapters (usually 9V/12V, 500mA), to enable the operation of the circuit and to charge the battery present in the handset. The UPS presently designed is of online type. Here the inverter is ‘on’ throughout, irrespective of the presence of the AC mains.

When the AC mains is present, the same is converted into DC and fed to the inverter. A part of the mains rectified output is used to charge the battery. When the mains power fails, the DC supply to the inverter is from the battery and from this is obtained AC at the inverter output. This is shown in fig.1. The circuit wired around IC CD4047 is an astable multivibrator operating at a frequency of 50 Hz. The Q and Q outputs of this multivibrator directly drive power MOSFETS IRF540. The configuration used is push-pull type. The inverter output is filtered and the spikes are reduced using MOV (metal oxide varistor). The inverter transformer used is an ordinary 9V-0-9V, 1.5A mains transformer readily available in the market.

Two LEDS (D6 and D7) indicate the presence of mains/battery. The mains supply (when present) is stepped down, rectified and filtered using diodes D1 through D4 and capacitor C1. A part of this supply is also used to charge the battery. In place of a single 12V, 4Ah battery, one may use two 6V, 4Ah batteries (SUNCA or any other suitable brand). The circuit can be easily assembled on a general-purpose PCB and placed inside a metal box. The two transformers may be mounted on the chassis of the box. Also, the two batteries can be mounted in the box using supporting clamps. The front and back panel designs are shown in the Fig. 3. The same circuit can deliver up to 100W, provided the inverter transformer and charging transformer are replaced with higher current rating transformers, so that the system can be used for some other applications as well.

You can take this white LED-based night lamp on your picnic outings. The lamp has sound trigger and push-to-on facilities and gives ample light during a walk at night. It will also prove useful in locating the door of your tent in the darkness. A click of the fingers will switch on the lamp for three minutes to help you in a strange place. The circuit uses low-power ICs to save the battery power. JFET op-amp TL071 (IC1) amplifies the sound picked up by the condenser microphone. Resistor R1 and low-value capacitor C1 (0.22µF) make the amplifier insensitive to very low-frequency sounds, eliminating the chance of false triggering. VR1 is used to adjust the sensitivity of the microphone and VR2 adjusts the gain of IC1. The amplified output from IC1 is coupled to trigger pin 2 of IC2, which is a monostable multivibrator built around low-power CMOS timer IC 7555.

Resistor R4 keeps trigger input pin 2 of the monostable normally high in the absence of the trigger input. Timing elements R6 and C4 give a time delay of three minutes. Reset pin 4 of IC2 is connected to the positive rail through R5 and to the negative rail through C2 to provide power-on-reset function. The output of IC2 powers the white LED (LED1) through ballast resistor R7. The circuit can be easily assembled on a perforated board. Make the circuit assembly as compact as possible to enclose in a small case. Use three 1.5V pen-light cells to power the circuit. Adjust VR1 and VR2 suitably to get sufficient sensitivity of IC1. Toggle switch S1 can be used to switch on the lamp like a torch.

Using this circuit, you can change the speed of the fan from your couch or bed. Infrared receiver module TSOP1738 is used to receive the infrared signal transmitted by remote control. The circuit is powered by regulated 9V. The AC mains is stepped down by transformer X1 to deliver a secondary output of 12V-0-12V. The transformer output is rectified by full-wave rectifier comprising diodes D1 and D2, filtered by capacitor C9 and regulated by 7809 regulator to provide 9V regulated output. Any button on the remote can be used for controlling the speed of the fan. Pulses from the IR receiver module are applied as a trigger signal to timer NE555 (IC1) via LED1 and resistor R4. IC1 is wired as a monostable multivibrator to delay the clock given to decade counter-cum-driver IC CD4017 (IC2).

Out of the ten outputs of decade counter IC2 (Q0 through Q9), only five (Q0 through Q4) are used to control the fan. Q5 output is not used, while Q6 output is used to reset the counter. Another NE555 timer (IC3) is also wired as a monostable multivibrator. Combination of one of the resistors R5 through R9 and capacitor C5 controls the pulse width. The output from IC CD4017 (IC2) is applied to resistors R5 through R9. If Q0 is high capacitor C5 is charged through resistor R5, if Q1 is high capacitor C5 is charged through resistor R6, and so on. Optocoupler MCT2E (IC5) is wired as a zero-crossing detector that supplies trigger pulses to monostable multivibrator IC3 during zero crossing. Opto-isolator MOC3021 (IC4) drives triac BT136.

Resistor R13 (47-ohm) and capacitor C7 (0.01µF) combination is used as snubber network for triac1 (BT136). As the width of the pulse decreases, firing angle of the triac increases and speed of the fan also increases. Thus the speed of the fan increases when we press any button on the remote control. Assemble the circuit on a general-purpose PCB and house it in a small case such that the infrared sensor can easily receive the signal from the remote transmitter.

Several intercom circuits have appeared in www.circuit-lab.com using integrated circuits. The circuit described here uses three easily available transistors only. Even a beginner can easily assemble it on a piece of veroboard. The circuit comprises a 3-stage resistor-capacitor coupled amplifier. When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals. These ring signals are amplified by transistor T3 to drive the speaker of earpiece. Current consumption of this intercom is 10 to 15mA only. Thus a 9-volt PP3 battery would have a long life, when used in this circuit. For making a two-way intercom, two identical units, as shown in figure, are required to be used.

Output of one amplifier unit goes to speaker of the other unit, and vice versa. For single-battery operation, join corresponding supply and ground terminals of both the units together. The complete circuit, along with microphone and earpiece etc, can be housed inside the plastic body of a toy cellphone, which is easily available in the market. Suggested cellphone cabinet, with the position of switches, speakers and mike etc is shown.

This automatic door opener can be made using readily available components. The electromagnetic relay at the output of this gadget can be used to control the DC/AC door-opener motor/solenoid of an electromechanical door opener assembly, with slight intervention in its electrical wiring. A laser diode (LED1) is used here as the light transmitter. Alternatively, you can use any available laser pointer. The combination of resistor R1 and diode D1 protects the laser diode from over-current flow. By varying multi-turn trimpot VR1, you can adjust the sensitivity. (Note that ambient light reflections may slightly degrade the performance of this unit.) Initially, when the laser beam is falling on photo-transistor T1, it conducts to reverse-bias transistor T3 and the input to the first gate (N1) of IC1 (CD4001) is low.

The high output at pin 3 of gate N1 forward biases the LED-driver transistor (T4) and the green standby LED (LED2) lights up continuously. The rest of the circuit remains in standby state. When someone interrupts the laser beam, photo-transistor T1 stops conducting and transistor T3 becomes forward-biased. This makes the output of gate N1 go low. Thus LED-driver transistor T4 becomes reverse-biased and LED2 stops glowing. At the same time, the low output of gate N1 makes the output of N2 high. Instantly, this high level at pin 4 of gate N2 triggers the monostable multivibrator built around the remaining two gates of IC1 (N3 and N4). Values of resistor R8 and capacitor C1 determine the time period of the monostable.

The second monostable built around IC2 (CD4538) is enabled by the high-going pulse at its input pin 12 through the output of gate N4 of the first monostable when the laser beam is interrupted. As a result, relay RL1 energizes and the door-opener motor starts operating. LED3 glows to indicate that the door-opener motor is getting the supply. At the same time, piezo-buzzer PZ1 sounds an alert. Transistor T5, whose base is connected to Q output (pin 10) of IC2, is used for driving the relay. Transistor T6, whose base is connected to Q output of IC2, is used for driving the intermittent piezo-buzzer. ‘On’ time of relay RL1 can be adjusted by varying trimpot VR2. Resistor R9, variable resistor VR2 and capacitor C3 decide the time period of the second monostable and through it on time of RL1.

The circuit works off 12V DC power supply. Assemble it on a general-purpose PCB. After construction, mount the laser diode and the photo-transistor on opposite sides of the door-frame and align them such that the light beam from the laser diode falls on the photo-transistor directly. The motor connected to the pole of relay contacts is the one used in electromechanical door-opener assembly. If you want to use a DC motor, replace mains AC connection with a DC power supply.

At the heart of this heat-sensitive switch is IC LM35 (IC1), which is a linear temperature sensor and linear temperature-to-voltage converter circuit. The converter provides accurately linear and directly proportional output signal in millivolts over the temperature range of 0°C to 155°C. It develops an output voltage of 10 mV per degree centigrade change in the ambient temperature. Therefore the output voltage varies from 0mV at 0°C to 1V at 100°C and any voltage measurement circuit connected across the output pins can read the temperature directly. The input and ground pins of this heat-to-voltage converter IC are connected across the regulated power supply rails and decoupled by R1 and C1. Its temperature-tracking output is applied to the non-inverting input (pin 3) of the comparator built around IC2. The inverting input (pin 2) of IC2 is connected across the positive supply rails via a voltage divider network formed by potentiometer VR1.

Since the wiper of potentiometer VR1 is connected to the inverting input of IC2, the voltage presented to this pin is linearly variable. This voltage is used as the reference level for the comparator against the output supplied by IC1. So if the non-inverting input of IC2 receives a voltage lower than the set level, its output goes low (approximately 650 mV). This low level is applied to the input of the load-relay driver comprising npn transistors T1 and T2. The low level presented at the base of transistor T1 keeps it nonconductive. Since T2 receives the forward bias voltage via the emitter of T1, it is also kept non-conductive. Hence, relay RL1 is in de-energised state, keeping mains supply to the load ‘off’ as long as the temperature at the sensor is low. Conversely, if the non-inverting input receives a voltage higher than the set level, its output goes high (approximately 2200mV) and the load is turned ‘on.’ This happens when IC1 is at a higher temperature and its output voltage is also higher than the set level at the inverting input of IC2.

So the load is turned on as soon as the ambient temperature rises above the set level. Capacitor C3 at this pin helps iron out any ripple that passes through the positive supply rail to avoid errors in the circuit operation. By adjusting potentiometer VR1 and thereby varying the reference voltage level at the inverting input pin of IC1, the temperature threshold at which energisation of the relay is required can be set. As this setting is linear, the knob of potentiometer VR1 can be provided with a linear dial calibrated in degrees centigrade. Therefore any temperature level can be selected and constantly monitored for external actions like turning on a room heater in winter or a room cooler in summer. The circuit can also be used to activate emergency fire extinguishers, if positioned at the probable fire accident site. The circuit can be modified to operate any electrical appliance. In that case, relay RL1 must be a heavy-duty type with appropriately rated contacts to match the power demands of the load to be operated.

Quiz-type game shows are increasingly becoming popular on television these days. In such games, fastest finger first indicators (FFFIs) are used to test the player’s reaction time. The player’s designated number is displayed with an audio alarm when the player presses his entry button. The circuit presented here determines as to which of the four contestants first pressed the button and locks out the remaining three entries. Simultaneously, an audio alarm and the correct decimal number display of the corresponding contestant are activated. When a contestant presses his switch, the corresponding output of latch IC2 (7475) changes its logic state from 1 to 0. The combinational circuitry comprising dual 4-input NAND gates of IC3 (7420) locks out subsequent entries by producing the appropriate latch-disable signal. Priority encoder IC4 (74147) encodes the active-low input condition into the corresponding binary coded decimal (BCD) number output.

The outputs of IC4 after inversion by inverter gates inside hex inverter 74LS04 (IC5) are coupled to BCD-to-7-segment decoder/display driver IC6 (7447). The output of IC6 drives common-anode 7-segment LED display (DIS.1, FND507 or LT542). The audio alarm generator comprises clock oscillator IC7 (555), whose output drives a loudspeaker. The oscillator frequency can be varied with the help of preset VR1. Logic 0 state at one of the outputs of IC2 produces logic 1 input condition at pin 4 of IC7, thereby enabling the audio oscillator. IC7 needs +12V DC supply for sufficient alarm level. The remaining circuit operates on regulated +5V DC supply, which is obtained using IC1 (7805). Once the organiser identifies the contestant who pressed the switch first, he disables the audio alarm and at the same time forces the digital display to ‘0’ by pressing reset pushbutton S5. With a slight modification, this circuit can accommodate more than four contestants.

This motor starter protects single-phase motors against voltage fluctuations and overloading. Its salient feature is a soft on/off electronic switch for easy operation. The transformer steps down the AC voltage from 230V to 15V. Diodes D1 and D2 rectify the AC voltage to DC. The unregulated power supply is given to the protection circuit. In the protection circuit, transistor T1 is used to protect the motor from over-voltage. The over-voltage setting is done using preset VR1 such that T1 conducts when voltages goes beyond upper limit (say, 260V). When T1 conducts, it switches off T2. Transistor T2 works as the under-voltage protector. The under-voltage setting is done with the help of preset VR2 such that T2 stops conducting when voltage is below lower limit (say, 180V). Zener diodes ZD1 and ZD2 provide base bias to transistors T1 and T2, respectively. Transistors T3 and T4 are connected back to back to form an SCR configuration, which behaves as an ‘on’/‘off’ control.

Switch S1 is used to turn on the pump, while switch S2 is used to turn off the pump. While making over-/under-voltage setting, disconnect C2 temporarily. Capacitor C2 prevents relay chattering due to rapid voltage fluctuations. Regulator IC 7809 gives the 9V regulated supply to soft switch as well as the relay after filtering by capacitor C4. A suitable miniature circuit breaker is used for automatic over-current protection. Green LED (LED1) indicates that the motor is ‘on’ and red LED (LED2) indicates that the power is ‘on’. The motor is connected to the normally-open contact of the relay. When the relay energizes, the motor turns on.

Today, most telephone equipment use a DTMF receiver IC. One common DTMF receiver IC is the Motorola MT8870 that is widely used in electronic communications circuits. The MT8870 is an 18-pin IC. It is used in telephones and a variety of other applications. When a proper output is not obtained in projects using this IC, engineers or technicians need to test this IC separately. A quick testing of this IC could save a lot of time in research labs and manufacturing industries of communication instruments. Here’s a small and handy tester circuit for the DTMF IC. It can be assembled on a multipurpose PCB with an 18-pin IC base. One can also test the IC on a simple breadboard. For optimum working of telephone equipment, the DTMF receiver must be designed to recognise a valid tone pair greater than 40 ms in duration and to accept successive digit tone-pairs that are greater than 40 ms apart.

However, for other applications like remote controls and radio communications, the tone duration may differ due to noise considerations. Therefore, by adding an extra resistor and steering diode the tone duration can be set to different values. The circuit is configured in balanced-line mode. To reject common-mode noise signals, a balanced differential amplifier input is used. The circuit also provides an excellent bridging interface across a properly terminated telephone line. Transient protection may be achieved by splitting the input resistors and inserting zener diodes (ZD1 and ZD2) to achieve voltage clamping. This allows the transient energy to be dissipated in the resistors and diodes, and limits the maximum voltage that may appear at the inputs. Whenever you press any key on your local telephone keypad, the delayed steering (Std) output of the IC goes high on receiving the tone-pair, causing LED5 (connected to pin 15 of IC via resistor R15) to glow.

It will be high for a duration depending on the values of capacitor and resistors at pins 16 and 17. The optional circuit shown within dotted line is used for guard time adjustment. The LEDs connected via resistors R11 to R14 at pins 11 through 14, respectively, indicate the output of the IC. The tone-pair DTMF (dual-tone multi-frequency) generated by pressing the telephone button is converted into binary values internally in the IC. The binary values are indicated by glowing of LEDs at the output pins of the IC. LED1 represents the lowest significant bit (LSB) and LED4 represents the most significant bit (MSB). So, when you dial a number, say, 5, LED1 and LED3 will glow, which is equal to 0101. Similarly, for every other number dialled on your telephone, the corresponding LEDs will glow. Thus, a non-defective IC should indicate proper binary values corresponding to the decimal number pressed on your telephone keypad.

To test the DTMF IC 8870/KT3170, proceed as follows:

1. Connect local telephone and the circuit in parallel to the same telephone line.
2. Switch on S1. (Switch on auxiliary switch S2 only if keys A, B, C, and D are to be used.)
3. Now push key ‘*’ to generate DTMF tone.
4. Push any decimal key from the telephone keypad.
5. Observe the equivalent binary as shown in the table.
6. If the binary number implied by glowing of LED1 to LED4 is equivalent to the pressed key number (decimal/A, B, C, or D), the DTMF IC 8870 is correct.

Keys A, B, C, and D on the telephonekeypad are used for special signalling and are not available on standard pushbutton telephone keypads. Pin 5 of the IC is pulled down to ground through resistor R8. Switch on auxiliary switch S2. Now the high logic at pin 5 enables the detection of tones representing characters A, B, C, and D.

The State Jal Boards supply water for limited duration in a day. Time of water supply is decided by the management and the public does not know the same. In such a situation, this water alarm circuit will save the people from long wait as it will inform them as soon as the water supply starts. At the heart of this circuit is a small water sensor. For fabricating this water sensor, you need two foils—an aluminium foil and a plastic foil. You can assemble the sensor by rolling aluminium and plastic foils in the shape of a concentric cylinder. Connect one end of the insulated flexible wire on the aluminium foil and the other end to resistor R2. Now mount this sensor inside the water tap such that water can flow through it uninterrupted. To complete the circuit, connect another wire from the junction of pins 2 and 6 of IC1 to the water pipeline or the water tap itself. The working of the circuit is simple.

Timer 555 is wired as an astable multivibrator. The multivibrator will work only when water flows through the water tap and completes the circuit connection. It oscillates at about 1 kHz. The output of the timer at pin 3 is connected to loudspeaker LS1 via capacitor C3. As soon as water starts flowing through the tap, the speaker starts sounding, which indicates resumption of water supply. It remains ‘on’ until you switch off the circuit with switch S1 or remove the sensor from the tap. The circuit works off a 9V battery supply. Assemble the circuit on any general-purpose PCB and house in a suitable cabinet. The water sensor is inserted into the water tap. Connect the lead coming out from the junction of 555 pins 2 and 6 to the body of the water tap. Use on/off switch S1 to power the circuit with the 9V PP3 battery.

Continuous monitoring of the mains voltage is required in many applications such as manual voltage stabilisers and motor pumps. An analogue voltmeter, though cheap, has many disadvantages as it has moving parts and is sensitive to vibrations. The solidstate voltmeter circuit described here indicates the mains voltage with a resolution that is comparable to that of a general-purpose analogue voltmeter. The status of the mains voltage is available in the form of an LED bar graph. Presets VR1 through VR16 are used to set the DC voltages corresponding to the 16 voltage levels over the 50-250V range as marked on LED1 through LED16, respectively, in the figure. The LED bar graph is multiplexed from the bottom to the top with the help of ICs CD4067B (16-channel multiplexer) and CD4029B (counter).

The counter clocked by NE555 timer-based astable multivibrator generates 4-bit binary address for multiplexer-demultiplexer pair of CD4067B and CD4514B. The voltage from the wipers of presets are multiplexed by CD4067B and the output from pin 1 of CD4067B is fed to the non-inverting input of comparator A2 (half of op-amp LM358) after being buffered by A1 (the other half of IC2). The unregulated voltage sensed from rectifier output is fed to the inverting input of comparator A2. The output of comparator A2 is low until the sensed voltage is greater than the reference input applied at the non-inverting pins of comparator A2 via buffer A1.

When the sensed voltage goes below the reference voltage, the output of comparator A2 goes high. The high output from comparator A2 inhibits the decoder (CD4514) that is used to decode the output of IC4029 and drive the LEDs. This ensures that the LEDs of the bar graph are ‘on’ up to the sensed voltage-level proportional to the mains voltage.The initial adjustment of each of the presets can be done by feeding a known AC voltage through an auto-transformer and then adjusting the corresponding preset to ensure that only those LEDs that are up to the applied voltage glow.

Note.
It is advisable to use additional transformer, rectifier, filter, and regulator arrangements for obtaining a regulated supply for the functioning of the circuit so that performance of the circuit is not affected even when the mains voltage falls as low as 50V or goes as high as 280V. During Lab testing regulated 12-volt supply for circuit operation was used.)

Normally the base of a cordless phone has an adaptor and the handset has Ni-Cd cells for its operation. The base unit becomes in-operative in case of power failure. Under such conditions, it is better to provide a backup using Ni-Cd cells externally. Here is a simple power supply back-up circuit which can be used with cordless phone SANYO CLT-420 or similar sets. The working is simple. When AC mains is present, Ni-Cd cel ls are charged through IC LM317L, which is wired as a current source. Also, diode D3 is reverse-biased, which keeps Ni-Cd cells isolated from positive rail. When AC mains goes off, the Ni-Cd cells provide supply to the cordless phone base unit through diode D3. A green LED is used to indicate the presence of AC mains. Each Ni-Cd cell costs around Rs 34, and the cost of the backup unit, including the box and cells, would not exceed Rs 300. Hence the circuit is well worth the investment.