Schematic Reference

If you need to use a headset with your PC, then you will know how frustrating it is continuously swapping over speaker and microphone cables. This is even worse if the PC is parked in a dark corner and the hard-to-read writing on the sound card sockets is covered in dust. This simple switch box eliminates all these problems. It sits on top of the desk and connects to the PC with stereo one-to-one cables.

On the rear of the box are sockets for the PC speaker and microphone connections and the existing speakers. On the front of the box are the sockets for the headset microphone and headphones, an input for an external microphone and two switches. One switch is used to direct the sound card output from the PC to either the existing speakers or the headphones.

Speaker-Headphone Switch Circuit Diagram

The second switch connects either the headset microphone or the external microphone to the input socket of the PC sound card. The switches used were 3 position 4 pole rotary switches with the last pole unused and adjusted for 2-position operation. All sockets were stereo 3.5mm types. This multiple switching arrangement is very flexible and is especially handy if you want to use an external microphone while monitoring with headphones. The ground wire as well as the left and right wires are all switched to prevent noise that could otherwise be induced into the microphone input through joining separate earths. For the same reason, a plastic case is used so that the earths of the sockets are not shorted together as would happen with a metal case. You will require two additional short stereo extension cables to connect the box to the PC.

Author: Leon Williams - Copyright: Silicon Chip Electronics

This accurate one-pulse-per-second clock is made with a few common parts and driven from a 50 or 60 Hertz mains supply but with no direct connection to it. A beep or metronome-like click and/or a visible flash, will beat the one-second time and can be useful in many applications in which some sort of time-delay counting in seconds is desirable. The circuit is formed by a CMos 4024 counter/divider chip and 3 diodes, arranged to divide the frequency of the input signal at pin #1 by 50 (or 60, see Notes). The input impedance at pin #1 is very hight, so simply touching the pin (or a short track or piece of wire connected to it) is usually enough to provide the necessary input signal. Another way to provide an input signal consists in a piece of wire wrapped several times around any convenient mains cable or transformer. No other connection is necessary.

Source : www.redcircuits.com

Although a simple crystal oscillator may be built from one comparator of an LT1720/LT1721, this will suffer from a number of inherent shortcomings and design problems. Although the LT1720/LT1721 will give the correct logic output when one input is outside the common mode range, additional delays may occur when it is so operated, opening the possibility of spurious operating modes. Therefore, the DC bias voltages at the inputs have to be set near the center of the LT1720/LT1721’s common mode range and a resistor is required to attenuate the feedback to the non-inverting input. Unfortunately, although the output duty cycle for this circuit is roughly 50%, it is affected by resistor tolerances and, to a lesser extent, by comparator offsets and timings.

If a 50% duty cycle is required, the circuit shown here creates a pair of complementary outputs with a forced 50% duty cycle. Crystals are narrow-band elements, so the feedback to the non-inverting input is a filtered analogue version of the square-wave output. The crystal’s path provides resonant positive feedback and stable oscillation occurs. Changing the non-inverting reference level can vary the duty cycle. The 2k-680Ω resistor pair sets a bias point at the comparator + (Comparator IC1a) and – (Comparator IC1b) input. At the complementary input of each comparator, the 2k-1.8k-0.1µF path sets up an appropriate DC average level based on the output.

IC1b creates a complementary output to IC1a by comparing the same two nodes with the opposite input. IC2 compares band-limited versions of the outputs and biases IC1a’s negative input. IC1a’s only degree of freedom to respond is variation of pulse width; hence the outputs are forced to 50% duty cycle. The circuit operates from 2.7V to 6V. When ‘scoping the oscillator output signal, a slight dependence on comparator loading, will be noted, so equal and resistive loading should be used in critical applications. The circuit works well because of the two matched delays and rail-to-rail outputs of the LT1720.

Amplifiers which run from 12V DC generally don’t put out much power and they are usually not hifi as well. But this little stereo amplifier ticks the power and low distortion boxes. With a 14.4V supply, it will deliver 20 watts per channel into 4-ohm loads at clipping while harmonic distortion at lower power levels is typically less than 0.03%. This is an ideal project for anyone wanting a compact stereo amplifier that can run from a 12V battery. It could be just the ticket for buskers who want a small but gutsy amplifier which will run from an SLA battery or it could used anywhere that 12V DC is available – in cars, recreational vehicles, remote houses with 12V DC power or where ever.

Because it runs from DC, it will be an ideal beginner’s or schoolie’s project, with no 240VAC power supply to worry about. You can run it from a 12V battery or a DC plugpack. But while it may be compact and simple to build, there is no need to apologise for “just average” performance. In listening tests from a range of compact discs, we were very impressed with the sound quality. Long-time readers might recall that we presented a similar 12V power amplifier design back in May 2001. It was a similar configuration to this one but it is now completely over-shadowed by the much lower distortion and greatly improved signal-to-noise ratio of this new design. In fact, let’s be honest: the previous unit is not a patch on this new design. It used two TDA1519A ICs which resulted in distortion figures above 1% virtually across the board and a signal-to-noise ratio of only -69dB unweighted.

However, by using the TDA­7377 power amplifier IC and making some other improvements, the THD (total harmonic distortion) of the new design is about 50 times better than the older unit (see performance graphs for details). The bottom line is that the THD under typical conditions is around just 0.03% or less. It is also able to deliver more output power due to the improved output transistors in the new power amplifier IC. In addition, its idle power consumption is low – not much more than 1W. As a result, if you don’t push it too hard it will run cool and won’t drain the battery too quickly. And because the IC has self-protection circuitry, it’s just about indestructible. It will self-limit or shut down if it overheats and the outputs are deactivated if they are shorted.

With a 12V supply, the largest voltage swing a conventional solid-state power amplifier can generate is ±6V. This results in a meagre 4.5W RMS into 4O and 2.25W RMS into 8O, without considering losses in the output transistors. Even if the DC supply is around 14.4V (the maximum that can normally be expected from a 12V car battery), that only brings the power figures up to 6.48W and 3.24W for 4O and 8O loads respectively – still not really enough. There are three common solutions to this problem. The first is to boost the supply voltage using a switchmode DC converter. This greatly increases the cost and complexity of the amplifier but it is one way of getting a lot of power from a 12V supply. However, we wanted to keep this project simple and that rules out this technique.

There are variations on the boosting method, such as the class H architecture used in the TDA1562Q IC featured in the Portapal PA Amplifier (SILICON CHIP, February 2003). It is able to achieve 40W/channel but with >0.1% THD. In that case, the amplifier output itself provides the switching for a charge pump. The second method is to lower the speaker impedance. Some car speakers have an impedance as low as 2O, which allows twice as much power to be delivered at the same supply voltage. However, we don’t want to restrict this amplifier to 2O loudspeakers.

Author: Nicholas Vinen - Copyright: Silicon Chip

The ASUS Eee is a fantastic ultra-portable notebook with almost everything required for geeks (and nothing that isn’t). Plus it features fantastic build quality and is very well priced. If you live in New Zealand you can get them from DSE; at the time of writing they are the exclusive supplier. I worked out it’s the same cost as importing one once you include all the duties and tax, plus you get the advantage of a proper NZ-style mains charger. Anyway, being so small I thought it would be nice to be able to carry this around in the car. Unfortunately I couldn’t find a car charger available anywhere at the time so I decided to tackle the problem myself. As a bonus this provides an opportunity for an external high-capacity battery.

I thought at this stage it would be worth noting that a commercial car charger is now available for less than it cost me to build this from Expansys and is available in most countries (select your location on their site). It outputs 9.5v from 10-18v in at up to 2.5A. I’d actually recommend it over the design here is it seems to perform better at lower voltages (that one works down to 10V). However I have kept this page up as a reference for those who enjoy tinkering.

The charger included with the Eee is rated at 9.5v, 2.315A. There isn’t a fixed voltage regulator available for this exact voltage, so the circuit needed to be designed around an adjustable regulator. I decided to design the charger around the LM2576 “Simple Switcher” IC from National Semiconductor. There are tons of ICs like this available, many of which are a bit more efficient, however I selected this one because it is readily available and relatively cheap. It also has a lower drop-out voltage (~2V) than many other chips I looked at which is important when powering the device from a car or 12v SLA battery.

This circuit could have used a standard three pin regulator IC such as the LM317, however most types require an external transistor when handling so much current and not to mention the fact that they are very inefficient; they draw the same amount of current from the input as the load and the difference in power is dissipated as heat. The main problem with using the LM2576 is the fact it needs quite a large inductor due to its somewhat low switching frequency. The inductor I used is made by Pulse Engineering, part number PE92108KNL. I’d prefer a smaller one, however I couldn’t find one capable of supplying the required current that I could purchase in single units. Besides the PE92108KNL is apparently designed specifically to work with the LM257x series.

The circuit also includes a low voltage cut-out based on a 9.1v Zener diode and BC337 transistor that will shut down the regulator if the input voltage is below 11.5V. This prevents unstable operation of the regulator at lower input voltages, and also helps prevent accidental flattening of the supply battery. Substituting this transistor for similar type may affect the cut-out voltage; the Vbe of the transistor should be 1.2v.All of the components used should be pretty readily available in most areas. I got everything from Farnell. Jaycar also sells everything except the inductor. Make sure you specify high temperature, low ESR capacitors as these help result in more stable operation and better efficiency of the charger.

Unfortunately the end result is a charger that is slightly bulkier than I would really like. I attempted to fit this inside an old mobile phone charger case so the whole thing could hang out of the cigarette lighter, however I ran into trouble making the circuit stable enough and dissipating all the heat. Due to the high current involved compared to a mobile phone charger the components are much bulkier so it’s pretty tricky to get all to fit! If I do get it finished I’ll add an update.

Make yourself a PCB using the template below (600dpi). I simply laser print (or photocopy) the design onto OHP transparency sheet and then transfer the toner onto a blank PCB using a standard clothes iron. Any missing spots can be touched up with a permanent marker before etching. This is quick, usually results in pretty tidy boards and hardly costs a thing. There is a tutorial on a variation of this method at http://max8888.orcon.net.nz/pcbs.htm.

Install the components on the PCB and triple check the layout before soldering. It is much easier to start with the low profile components such as resistors and diodes, then install the larger components after-wards. Don’t forget the wire link; this is shows as a red line on the layout guide above. Remember to smear a small amount of heatsink compound on the regulator tab before mounting the heatsink.

For a case I used a small plastic enclosure from DSE, part H2840, as it was all the local store had in stock that was remotely suitable. The PCB is designed to fit into this particular case, however any small box should be suitable. If you have a dead laptop charger lying about it might be worth ripping the guts out of that and salvaging the case. If your enclosure is different you may need to modify the design to suit, so I have provided the schematic and PCB design files for download. They were created using Eagle. The Eee uses a standard 1.7mm DC power connector with a positive tip.

Connect the circuit to a 12v supply. If you use a car or lead acid battery ensure you have a 3A fuse fitted in line with the circuit before connecting it, just in case. Use your multimeter to check that the circuit outputs about 9.45v with no load. Connect a 12V, 21W lamp (e.g. old brake lamp from a car) or similar load across the output and check that the voltage doesn’t vary much. You should now be able to connect your Eee. The circuit design should be good for up to 2.5A, so there is plenty of margin for the Eee to fully function and charge its own battery off this supply.

Jaycar have a really cool carry bag with a shoulder strap designed to perfectly fit a 12v 7AH sealed lead acid battery. The bag features a fused cigarette lighter socket and is the perfect compliment to this charger. It works well with the Eee and provides hours of extra use. The shoulder strap means it’s not too bothersome to carry about and the charger circuit itself zips up neatly inside the bag. The under-voltage cut-off means the battery will never run completely flat, and the Eee will simply cut over to its internal battery once the SLA runs out. I got my SLA battery from Rexel as they are much cheaper (approx NZ$18 including GST last time I bought one) and they don’t sit as long on the shelf as many other suppliers.

This circuit is intended for people who have had experience in constructing electronic projects before. The circuit design and build process are provided simply as a reference for other people to use and I take no responsibility for how they are used. If you proceed with building and/or using this design you do so entirely at your own risk. You are free to use the content on this page as you wish, however I do ask that you include a link or reference back to this page if you distribute or publish any of the content to others.

 Source: Marlborough Wi-Fi

The LM4990 is an audio power amplifier primarily designed for demanding applications in mobile phones and other portable communication device applications. It is capable of delivering 1.25 watts of continuous average power to an 8Ω BTL load and 2 watts of continuous average power (LD and MH only) to a 4Ω BTL load with less than 1% distortion (THD+N+N) from a 5VDC power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. The LM4990 does not require output coupling capacitors or bootstrap capacitors, and therefore is ideally suited for mobile phone and other low voltage applications where minimal power consumption is a primary requirement.

The LM4990 features a low-power consumption shutdown mode. To facilitate this, Shutdown may be enabled by either logic high or low depending on mode selection. Driving the shutdown mode pin either high or low enables the shutdown pin to be driven in a likewise manner to enable shutdown. The LM4990 contains advanced pop & click circuitry which eliminates noise which would otherwise occur during turn-on and turn-off transitions. The LM4990 is unity-gain stable and can be configured by external gain-setting resistors.

Source: www.national.com

This circuit was designed as an aid to installers and maintainers of video systems. It is basically a video sync separator (IC1) followed by a LED and buzzer driver (IC2, Q1 & Q2). In use, the device is connected to a video cable and if there is video present, the LED will flash at about 10Hz. If there is no video, the LED flashes briefly every couple of seconds. A buzzer can also be switched in to provide an audible indication. The buzzer is particularly useful when tracing cabling faults or trying to find a correct cable amongst many, where it is difficult to keep an eye on the LED.

Another use for the buzzer option is to provide a video fault indication. For example, it could be inserted in bridging mode, with switch S1 in high impedance mode (position 2) across a video line and set to alarm when there is no video present. If someone pulls out a cable or the video source is powered off, the alarm would sound. IC1 is a standard LM1881 video sync separator circuit and 75Ω termination can be switched in or out with switch S1a. The other pole of the switch, S1b, turns on the power. The composite sync output at pin 1 is low with no video input and it pulses high when composite sync is detected.

These pulses charge a 100nF capacitor via diode D1. When there is no video at the input, oscillator IC2b is enabled and provides a short pulse every couple of seconds to flash the LED. The duty cycle is altered by including D2, so that the discharge time for the 10μF capacitor is much shorter than the charge time. The short LED pulse is used as a power-on indicator drawing minimal average current. When video is present at the input, IC2b is disabled and IC2d is enabled. The output of IC2d provides a 10Hz square wave signal to flash the LED. The buzzer is controlled by switch S2. In position 2 the buzzer will sound when there is video at the input and in position 1 the buzzer will sound when there is no video at the input.

Author: Leon Williams - Copyright: Silicon Chip Electronics

Today, almost all computers contain logic blocks for implementing a USB port. A USB port, in practice, is capable of delivering more than 100 mA of continuous current at 5V to the peripherals that are connected to the bus. So a USB port can be used, without any trouble, for powering 5V DC operated tiny electronic gadgets. Nowadays, many handheld devices (for instance, portable reading lamps) utilise this facility of the USB port to recharge their built-in battery pack with the help of an internal circuitry.Usually 5V DC, 100mA current is required to satisfy the input power demand. Fig. 1 shows the circuit of a versatile USB power socket that safely converts the 12V battery voltage into stable 5V.

This circuit makes it possible to power/recharge any USB power-operated device, using in-dash board cigar lighter socket of your car. The DC supply available from the cigar lighter socket is fed to an adjustable, three-pin regulator LM317L (IC1). Capacitor C1 buffers any disorder in the input supply.Resistors R1 and R2 regulate the output of IC1 to steady 5V, which is available at the ‘A’ type female USB socket.

Red LED1 indicates the output status and zener diode ZD1 acts as a protector against high voltage. Assemble the circuit on a general-purpose PCB and enclose in a slim plastic cabinet along with the indicator and USB socket. While wiring the USB outlet, ensure correct polarity of the supply. For interconnection between the cigar plug pin and the device, use a long coil cord as shown in Fig. 2. Pin configuration of LM317L is shown in Fig. 3.

A small electronic switch that connects a battery to the equipment for a certain amount of time when a push-button is momentarily pressed. And we have also taken the ambient light level into account; when it is dark you won’t be able to read the display so it is only logical to turn the switch off, even if the time delay hasn’t passed yet. The circuit is quite straightforward. For the actual switch we’re using a well-known MOSFET, the BS170. A MOSFET (T2 in the circuit) used in this configuration doesn’t need a current to make it conduct (just a voltage), which makes the circuit very efficient. When the battery is connected to the battery saver circuit for the first time, capacitor C2 provides the gate of the MOSFET with a positive voltage, which causes T2 to conduct and hence connect the load (on the 9 V output) to the battery (BT1). C2 is slowly charged up via R3 (i.e. the voltage across C2 increases).

This causes the voltage at the gate to drop and eventually it becomes so low that T2 can no longer conduct, removing the supply voltage to the load. In this state the battery saver circuit draws a very small current of about 1 µA. If you now press S1, C2 will discharge and the circuit returns to its initial state, with a new turn-off delay. Resistor R5 is used to limit the discharge current through the switch to an acceptable level. You only need to hold down the switch for a few hundredths of a second to fully discharge C2. In our prototype, connected between a 9 V battery and a load that drew about 5 mA, the output voltage started to drop after about 26 minutes. After 30 minutes the voltage had dropped to 2.4 V. You should use a good quality capacitor for C2 (one that has a very low leakage current), otherwise you could have to wait a very long time before the switch turns off!

The ambient light level is detected using an LDR (R1). An LDR is a type of light sensor that reduces in resistance when the light level increases. We recommend that you use an FW150, obtainable from e.g. Conrad as part number 183547-89. When there is too little light its resistance increases and potential divider R1/R2 causes transistor T1 to conduct. T1 then charges up C2 very quickly through R4, which limits the current to a safe level. This stops T2 from conducting and the load is turned off. The choice of value for R2 determines how dark it has to be before T1 starts to conduct. The battery saver circuit can be added to devices that use 6 or 9 volt batteries and which don’t draw more than 100 mA. The circuit can be built on a piece of experimenter’s board and should be made as compact as possible so that it can be built into the battery powered device.

Copyright : Elektor Electronics

This particular timing circuit can be used to time one-shot events from a few seconds to a few hours. And in standby mode (ie, with RLY1 and LED1 off), its power consumption is very low. The heart of this circuit is a low-cost CMOS 4011 quad NAND gate, with IC1a & IC1b configured as a standard Set/Reset flip-flop. Briefly pressing switch S1 to start the timing sequence pulls pin 1 of IC1a low and, as a result, pin 3 switches high. Two things happen while pin 3 is high: capacitor Cx begins charging via potentiometer Rx; and (2) pin 11 of IC1d will be low, which means that transistors Q3 and Q1 are both on.

As a result, both LED 1 and relay RLY1 are also on. RLY1 and LED 1 remain on until Cx has been charged up to about 70% of Vcc (ie, the supply rail). At this point, pins 8 & 9 of IC1c are pulled high and so its pin 10 output goes low and resets the flip-flop by applying a low to pin 6 of IC1b. This causes pin 3 of IC1a to go low and so LED1 and RLY1 switch off and the timing period ends. At the same time, pin 4 of the flip-flop goes high and this turns on transistor Q2 while ever the flip-flop is held reset. This ensures that Cx is discharged, so that the circuit is ready the next time S1 is pressed.

Diode D1 and its associated 10µF capacitor reset the flip-flop when power is first applied, so that LED1 and RLY1 remain off until S1 is pressed. D4 is included to protect Q1 against the back-EMF that's generated when the relay switches off. Choosing appropriate values for Cx & Rx for a given time delay is straightforward. The formula is T = 1.24 x Rx x Cx, where T is the delay time in seconds. As an example, let's assume that we require a time delay of 10s using a value of 100µF for Cx. Now we just need to calculate the value of Rx as follows:

In this case, an 82kO resistor would be the closest value. You can use either a fixed resistor for Rx or you can use a potentiometer (or trimpot) which can be adjusted to give the required time delay. Note that the value of Rx should not be any more than a few megohms. Power for the circuit can be derived from any 12V DC source. This is then fed to 3-terminal regulator REG1 to derive a 9V rail to power the circuitry. The exception here is the relay circuit, which is powered from the 12V rail. Diode D3 protects the circuit against incorrect supply polarity.

Author: Trent Jackson - Copyright: Silicon Chip Electronics

Integrated AF power amps have seen great improvements in recent years offering improved power and easier use. The TDA1519C from Philips contains two power amplifiers providing 11 W per channel stereo or 22 W mono when the two channels are connected in a bridge configuration. The special in-line SIL9P package outline allows the chip to be conveniently bolted to a suitable heatsink. The TDA1519CSP is the SMD version, in this case the heat sink is mounted over, and in contact with, the top surface of the chip.

11W Stereo Amplifier Circuit Diagram

The operating voltage of this device is from +6V to +17.5V. The two channels of the amplifier are different in that one channel, between pins 1 and 4, is a non-inverting amplifier, while the other between pins 9 and 6 is an inverting amplifier. It is therefore necessary in stereo operation, to wire the speakers so that one of them has its polarity reversed. Each amplifier has an input impedance of 60kΩ and a voltage gain of 40dB, i.e. 100 times. When both amplifier are used in a bridge configuration, the inputs are in parallel so that the input impedance will be 30kΩ.

22W Stereo Amplifier Circuit Diagram

A combined mute/standby function is provided on pin 8. In its simplest form this can be connected to the positive rail via a switch. When the switch is open the amplifier will be in standby mode and current consumption is less than 100µA. When the switch is closed, the amplifier will be operational. A circuit is also shown that uses the mute input to prevent the annoying switch-on plop heard when power amps are first switched on This is caused by the rush of current to charge capacitors C1 and C2.

The circuit shown generates a ramp voltage, which is applied to pin 8. At switch on, as the voltage rises from 3.3 V to 6.4 V, the amplifier will switch out of standby mode and into mute mode allowing C1 and C2 to charge. Only when the ramp voltage on pin 8 reaches 8.5V will the amplifier switch into active mode. Protection built into the TDA1519C would seem to make it almost foolproof. The two outputs can be shorted to either of the supply rails and to each other. A thermal shutdown will prevent overloading and the power supply input is protected against accidental reversal of the supply leads up to 6V.

Author : G. Kleine  - Copyright : Elektor Electronics

Here’s a clap switch free from false triggering. To turn on/off any appliance, you just have to clap twice. The circuit changes its output state only when you clap twice within the set time period. Here, you’ve to clap within 3 seconds. The clap sound sensed by condenser microphone is amplified by transistor T1. The amplified signal provides negative pulse to pin 2 of IC1 and IC2, triggering both the ICs. IC1, commonly used as a timer, is wired here as a monostable multivibrator. Trigging of IC1 causes pin 3 to go high and it remains high for a certain time period depending on the selected values of R7 and C3. This ‘on’ time (T) of IC1 can be calculated using the following relationship: T=1.1R7.C3 seconds where R7 is in ohms and C3 in microfarads. On first clap, output pin 3 of IC1 goes high and remains in this standby position for the preset time.Also, LED1 glows for this period. The output of IC1 provides supply voltage to IC2 at its pins 8 and 4.

Now IC2 is ready to receive the triggering signal. Resistor R10 and capacitor C7 connected to pin 4 of IC2 prevent false triggering when IC1 provides the supply voltage to IC2 at first clap. On second clap, a negative pulse triggers IC2 and its output pin 3 goes high for a time period depending on R9 and C5. This provides a positive pulse at clock pin 14 of decade counter IC 4017 (IC3). Decade counter IC3 is wired here as a bistable. Each pulse applied at clock pin 14 changes the output state at pin 2 (Q1) of IC3 because Q2 is connected to reset pin 15. The high output at pin 2 drives transistor T2 and also energizes relay RL1. LED2 indicates activation of relay RL1 and on/off status of the appliance. A free-wheeling diode (D1) prevents damage of T2 when relay de-energizes.

Author : Mohammad Usman Qureshi - Copyright : Electronics For You May 2003

This simple circuit can be used to program the PIC16F84 and similar "flash memory" type parts. It uses a cheap 555 timer IC to generate the programming voltage from a +5V rail, allowing the circuit to be powered from a computer’s USB port. The 555 timer (IC1) is configured as a free-running oscillator, with a frequency of about 6.5kHz. The output of the timer drives four 100nF capacitors and 1N4148 diodes wir-ed in a Cockroft-Walton voltage multiplier configuration.

The output of the multiplier is switched through to the MCLR/Vpp pin of the PIC during programming via a 4N28 optocoupler. Diodes ZD1 and D5 between the MCLR/Vpp pin and ground clamp the output of the multiplier to about 13.6V, ensuring that the maximum input voltage (Vihh) of the PIC is not exceeded. A 100kΩ resistor pulls the pin down to a valid logic low level (Vil) when the optocoupler is not conducting. The circuit is compatible with the popular "JDM" programmer, so can be used with supporting software such as "ICProg" (see http://www.ic-prog.com).

Author: Luke Weston - Copyright: Silicon Chip Electronics

Have you ever connected two PCs together via modems using a twisted pair cable and nothing happened? That’s because the modems are expecting a phone line with all the signals and voltages supplied by the local telephone exchange. This circuit simulates the DC power and signal isolation but not the "dial tone" or the "ring signal". It suffices to connect two PCs together to communicate and exchange files using HyperTerminal.

The circuit is self-explanatory and needs only one power supply for both modem lines. Although 50V DC is the usual exchange line voltage, this circuit should operate down to 20V. A 600O line transformer (eg. Jaycar cat. MM-1900) provides signal isolation, while the resistors provide current limiting and keep the lines as balanced as possible. When using this set-up with HyperTerminal, you should not select a Windows modem driver in the "Connect To" dialog. Instead, connect directly to the relevant COM port.

Next, verify that the modems are working by sending information commands such as "ATI1" or "ATI3". If you don’t get a response using these commands, try resetting the modem(s) using the "AT&Z" command. Assuming you do get a response, set one in originate mode using the "ATD" command and the other in answer mode with the "ATA" command. If all is well, you should now be able to type in one terminal window and see the results echoed in the second PC’s terminal window. To return to control mode, type "+++". The advantage of using modems instead of a serial cable between COM ports is that the two PCs can be kilometres apart instead of a few metres. For example, you could connect the house PC to the workshop PC on the other side of the farm.

Author: Filippo Quartararo - Copyright: Silicon Chip Electronics

    

Source : www.extremecircuits.net

These days many more audio-visual devices in the home are connected together. This is especially the case with the TV, which may be connected to a DVD player, a hard disk recorder, a surround-sound receiver and often a PC as well. This often creates a problem when earth loops are created in the shielding of the video cables, which may cause hum and other interference. The surround-sound receiver contains a tuner that takes its signal from a central aerial distribution system. The TV is also connected to this and it’s highly likely that the PC has a TV-card, which again is connected to the same system. On top of this, there are many analogue connections between these devices, such as audio cables. The usual result of this is that there will be a hum in the audio installation, but in some cases you may also see interference on the TV screen.

The ground loop problem can be overcome by galvanically isolating the video connections, for example at the aerial inputs of the surround-sound receiver and the TV. Special adaptors or filters are sold for this purpose, known as video ground loop isolators. Good news: such a filter can also be easily made at home by yourself. There are two ways in which you can create galvanic isolation in a TV cable. The first is to use an isolating transformer with two separate windings. The other is to use two coupling capacitors in series with the cable. The latter method is easily the simplest to implement and generally works well enough in practice. The simplest way to produce such a ‘filter’ is as an in-line adapter, so you can just plug it onto either end of a TV aerial cable.

The only requirements are a male and female coax plug and two capacitors. The latter have to be suitable for high-frequency applications, such as ceramic or MKT types. It is furthermore advisable to choose types rated for high voltages (400 V), since the voltages across these capacitors can be higher than you might expect (A PC that isn’t connected to the mains Earth can have a voltage as high as 115 V (but at a very low, safe current), caused by the filter capacitors in its power supply. These capacitors don’t need to be high value ones, since they only have to pass through frequencies above about 50 MHz. Values of 1 nF or 2.2 nF are therefore sufficient. To make the isolator you should connect one capacitor between the two earth connections of the coax plugs and the other between the two signal connections.

The mechanical construction has to be sturdy enough such that the connections to the capacitors won’t break whenever the inline adapter is removed forcibly. A good way to do this is to make a cover from a piece of PVC piping for the central part. Wrap aluminium foil round the outside and connect it to one of the plugs, so that the internal parts are properly shielded from external interference. Make sure that the aluminium foil doesn’t make contact with the other plug, otherwise you lose the isolation. The majority of earth loops will disappear when you connect these filters to all used outputs of the central aerial distribution system where the signal enters the house.

However effective a domestic alarm system may be, it’s invariably better if it never goes off, and the best way to ensure this is to make potential burglars think the premises are occupied. Indeed, unless you own old masters or objects of great value likely to attract ‘professional’ burglars, it has to be acknowledged that the majority of burglaries are committed by ‘petty’ thieves who are going to be looking more than anything else for simplicity and will prefer to break into homes whose occupants are away.

Rather than simply not going on holiday – which is also one solution to the problem (!) – we’re going to suggest building this intelligent presence simulator which ought to put potential burglars off, even if your home is subjected to close scrutiny. Like all its counterparts, the proposed circuit turns one or more lights on and off when the ambient light falls, but while many devices are content to generate fixed timings, this one works using randomly variable durations.

 

So while other devices are very soon caught out simply by daily observation (often from a car) because of their too-perfect regularity, this one is much more credible due to the fact that its operating times are irregular. The circuit is very simple, as we have employed a microcontroller – a ‘little’ 12C508 from Microchip, which is more than adequate for such an application. It is mains powered and uses rudimentary voltage regulation by a zener diode.

A relay is used to control the light(s); though this is less elegant than a triac solution, it does avoid any interference from the mains reaching the microcontroller, for example, during thunderstorms. We mustn’t forget this project needs to work very reliably during our absence, whatever happens. The ambient light level is measured by a conventional LDR (light dependent resistor), and the lighting switching threshold is adjustable via P1 to suit the characteristics and positioning of the LDR.

Note that input GP4 of the PIC12C508 is not analogue, but its logic switching threshold is very suitable for this kind of use. The LED connected to GP1 indicates the circuit’s operating mode, selected by grounding or not of GP2 or GP3 via override switch S1. So there are three possible states: permanently off, permanently on, and automatic mode, which is the one normally used. Given the software programmed into the 12C508 (‘firmware’) and the need to generate very long delays so as to arrive at lighting times or an hour or more, it has been necessary to make the MCU operate at a vastly reduced clock frequency.

In that case, a crystal-controlled clock is no longer suitable, so the R-C network R5/C3 is used instead. For sure, such a clock source is less stable than a crystal, but then in an application like this, that may well be what we’re after as a degree of randomness is a design target instead of a disadvantage. Our suggested PCB shown here takes all the components for this project except of course for S1, S2, and the LDR, which will need to be positioned on the front panel of the case in order to sense the ambient light intensity.

The PCB has been designed for a Finder relay capable of switching 10 A, which ought to prove adequate for lighting your home, unless you live in a replica of the Palace of Versailles. The program to be loaded into the 12C508 is available for free download from the Elektor website as file number 080231-11.zip or from the author’s own website: www.tavernier-c.com. On completion of the solder work the circuit should work immediately and can be checked by switching to manual mode.

The relay should be released in the ‘off’ position and energized in the ‘on’ position. Then all that remains is to adjust the day/night threshold by adjusting potentiometer P1. To do this, you can either use a lot of patience, or else use a voltmeter – digital or analogue, but the latter will need to be electronic so as to be high impedance – connected between GP4 and ground. When the light level below which you want the lighting to be allowed to come on is reached, adjust P1 to read approximately 1.4 V on the voltmeter.

If this value cannot be achieved, owing to the characteristics of your LDR, reduce or increase R8 if necessary to achieve it (LDRs are known to have rather wide production tolerances). Equipped with this inexpensive accessory, your home of course hasn’t become an impregnable fortress, but at least it ought to appear less attractive to burglars than houses that are plunged into darkness for long periods of time, especially in the middle of summer. (www.tavernier-c.com)

COMPONENTS LIST
Resistors
R1 = 1k 500mW
R2 = 4k7
R3 = 560R
R4,R6 = 10k
R5 = 7k5
R 7 = LDR
R8 = 470k to 1 M
P1 = 470k potentiometer
Capacitors
C1 = 470µF 25V
C2 = 10µF 25V
C3 = 1nF5
C4 = 10nF
Semiconductors
D1,D2 = 1N4004
D3 = diode zener 4V7 400 mW
LED1 = LED, red
D4 = 1N4148
T1 = BC547
IC1 = PIC12C508, programmed, see Downloads
Miscellaneous
RE1 = relay, 10A contact
S1 = 1-pole 3-way rotary switch
F1 = fuse 100 mA
TR1 = Mains transformer 2x9 V, 1.2 -3 VA
4 PCB terminal blocks, 5 mm lead pitch
5 solder pins
Downloads:
The PCB layout can be downloaded free from our website www.elektor.com; file # 080231-1.
The source code and .hex files for this project are available free on www.elektor.com; file # 080231-11.zip.

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.

Circuit diagram :

 Heat Sensitive Switch  Circuit Diagram

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.

Author : M.K. Chandra Mouleeswaran & Miss Kalan Priya - Copyright :  EFY

Even if a large number of album titles once available on vinyl are now, little by little, being proposed as CDs, not all are available and far from it. You may have treasures in your collection that you would like to burn on CDs. First, preserving a CD is easier than preserving a vinyl record, and second, we have to admit that turntables are disappearing, even on fully-equipped Hi-Fi systems. From a point of view of software and PCs, converting from vinyl to CD is not a problem. A large number of programs, whether paid for freeware, are available to re-master vinyl records with varying degrees of success and to eliminate pops, crackles and other undesirable noises.

All of these programs work with the sound card of your PC and that, admittedly, is where the problem starts. Most high-quality turntables are equipped with a magnetic cartridge which typically delivers just a few mV. The cartridge signal requires a correction of a specific frequency, called RIAA correction. If our older readers will perfectly recall what RIAA is all about, others from the CD generation may not know what the acronym RIAA stands for, guessing it may have something to do with illegal downloading of music on the Internet. For mechanical reasons related to the vinyl engraving procedure, high-boost frequency correction is carried out while respecting a very precise curve defined a long time ago by the RIAA (Recording Industry Association of America) and, which therefore, quite naturally, was baptized RIAA correction.

Reversing the correction is the role of to the preamplifier for the magnetic cartridge. Since this correction boosts the lowest frequencies, such a preamplifier is very sensitive to all undesirable noises, hums, including, of course, the one coming from the 50-Hz (or 60-Hz) mains power supply. It is important to take that into account while making this project which must be done carefully with respect to grounding and shielding. The schematic of our preamplifier is very simple because it uses a very low-noise dual operational amplifier. Here the NE5532 is used, whose response curve is modelled by R7, R8, C8, and C9 (or R14, R15, C13, and C14 respectively) in order to match the RIAA correction as closely as possible.

 Multimedia RIAA Preamplifier Circuit Diagram

The input has an impedance of 47 kR, which is the standardized value of magnetic cartridges, and its 1,000-Hz gain is 35 dB which allows it to supply an output level of a few hundred mV typically required by for the line input of a PC sound cards. The connection between the cartridge and the input of the amplifier requires shielded wiring to avoid the hum problems discussed above. Likewise we recommend fitting the assembly in a metal housing connected to the electric ground. With respect to the power supply, three solutions are proposed: If you are a purist and you want to rule out any noise whatsoever, you will utilize a simple 9-V battery. Then, the components outlined with a dotted line will not be useful.

Since the circuit only uses a few mA, such a solution is acceptable unless your collection of vinyls is impressive... If you desire a more elegant technical solution that might sometimes cause more undesirable noise on the signals, you may want to wire up the components within in the dotted lines and you can steal the 12 V positive voltage available from your PC. A Y-connector inserted on the power supply of one of the internal drives or peripherals will work very well for that. Finally, you may also use a mains adapter set to 12 V and connect it to the +12-volt point of the drawing in order to benefit from additional filtering, which is not a luxury for some.

Author: Christian Tavernier - Copyright: Elektor Electronics

How often does it happen that you close down Windows and then forget to turn off the computer? This circuit does that automatically. After Windows is shut down there is a ‘click’ a second later and the PC is disconnected from the mains. Surprisingly enough, this switch fits in some older computer cases. If the circuit doesn’t fit then it will have to be housed in a separate enclosure. That is why a supply voltage of 5 V was selected. This voltage can be obtained from a USB port when the circuit has to be on the outside of the PC case. It is best to solder the mains wires straight onto the switch and to insulate them with heat shrink sleeving. C8 is charged via D1.

This is how the power supply voltage for IC1 is obtained. A square wave oscillator is built around IC1a, R1 and C9, which drives inverters IC1c to f. The frequency is about 50 kHz. The four inverters in parallel power the voltage multiplier, which has a multiplication of 3, and is built from C1 to C3 and D2 to D5. This is used to charge C5 to C7 to a voltage of about 9 V.The generated voltage is clearly lower than the theoretical 3x4.8=14.4 V, because some voltage is lost across the PN-junctions of the diodes. C5 to C7 form the buffer that powers the coil of the switch when switching off. The capacitors charge up in about two seconds after switching on. The circuit is now ready for use. When Windows is closed down, the 5-V power supply voltage disappears. C4 is discharged via R2 and this results in a ‘0’ at the input of inverter IC1b. The output then becomes a ‘1’, which causes T1 to turn on.

A voltage is now applied to the coil in the mains switch and the power supply of the PC is turned off. T1 is a type BSS295 because the resistance of the coil is only 24R. When the PC is switched on, the circuit draws a peak current of about 200 mA, after which the current consumption drops to about 300 µA. The current when switching on could be higher because this is strongly dependent on the characteristics of the 5-V power supply and the supply rails in the PC. There isn’t much to say about the construction of the circuit itself. The only things to take care with are the mains wires to the switch. The mains voltage may not appear at the connections to the coil. That is why there has to be a distance of at least 6 mm between the conductors that are connected to the mains and the conductors that are connected to the low-voltage part of the circuit.

Author: Uwe Kardel - Copyright: Elektor Electronics Magazine

This circuit switches a printer’s USB connection from a PC to a laptop. What was needed was a method of allowing a laptop to use the printer occasionally while at all other times the printer would be connected to the PC. Instead of unplugging the printer from the PC and then into the laptop, the circuit switches the USB connection automatically. K1 and K2 are standard type-B USB sockets, while K3 is a USB type-A socket. The USB lead from the laptop plugs into K2 while the PC’s USB lead plugs into K1. A USB cable from K3 connects the printer to this circuit. The cable from the PC is always plugged in while the cable from the laptop is only connected whenever this device needs to print. In normal operation the laptop is not connected to K2, so the USB signal to the printer comes from the PC via K1, the normally closed contacts of relay Re1, through to K3 and from there to the printer.

Whenever the laptop is connected up, the presence of the 5-volt power signal on its USB port causes Re1 to switch over to the printer’s connection to K2 and the laptop. Unplugging the laptop returns control of the printer back to there PC. The circuit was tested on a USB 1.1 compliant printer and a PC and laptop that had USB-2.0 high-speed ports. The PCB traces for D+ and D– should be kept as short as possible and ideally should be the same length. The relay should be a low-power type (5 V at 100 mA coil current) with two changeover (c/o) contacts. Switch S1 is only required in situations where the two computers you want to select between are permanently present and connected up to the circuit. The switch then selects the computer having access to the printer.

Author: Liam Maskey - Copyright: Elektor Electronics

Today telephone has become an integral part of our lives. It is the most widely used communication device in the world. Owing to its immense popularity and widespread use, there arises a need for call recording devices, which find application in call centres, stock broking firms, police, offices, homes, etc. Here we are describing a call recorder that uses very few components. But in order to understand its working, one must first have the basic knowledge of standard telephone wiring and a stereo plug.

In India, landline telephones primarily use RJ11 wiring, which has two wires—tip and ring. While tip is the positive wire, ring is the negative one. And together they complete the telephone circuit. In a telephone line,  voltage between tip and ring is around 48V DC when handset is on the cradle(idle line). In order to ring the phone for an incoming call, a 20Hz AC current of around 90V is superimposed over the DC voltage already present in the idle line.  The negative wire from the phone line goes to IN1, while the positive wire goes to IN2. Further, the negative wire from OUT1 and the positive wire from OUT2 are connected to the phone. All the resistors used are 0.25W carbon film resistors and all the capacitors used are rated for 250V or more.

The negative terminal of ‘To AUX IN’ is connected to pin 1 of the stereo jack while the positive terminal is connected to pins 2 and 3 of the stereo jack. This stereo jack, in turn, is connected to the AUX IN of any recording device, such as computer, audio cas-sette player, CD player, DVD player, etc. Here we shall be connecting it to a computer. When a call comes in, around 90V AC current at 20Hz is superimposed over the DC voltage already present in the idle line.

This current is converted into DC by the diodes and fed to resistor R1, which reduces its magnitude and feeds it to LED1. The current is further reduced in magnitude by the resistor R2 and fed to the right and left channels of the stereo jack, which are connected to the AUX IN port of a computer.  Any audio recording software, such as AVS audio recorder (available at: http://www.avs4you.com/AVS-Audio-Recorder.aspx), Audacity audio recorder (http://audacity.sourceforge.net/), or audio recorder (http://www.audio-tool.net/audio_recorder_for_free.html), can be used to record the call. When a call comes in, one needs to launch the audio recording software and start recording.

For phone recording, simply connect the stereo jack to the AUX IN port of the PC. Install the  Audacity audio recorder (different versions are available for free for different operating systems at http://audacity.sourceforge.net/) on your PC. Run the executable  Audacity file. In the main window, you will find a dropdown box in the top right corner. From this box, select the AUX option. Now you are ready to record any call. As soon as a call comes in, press the record button found in the Audacity main window and then pick up the telephone receiver and answer the call. Press the stop button once the call ends. Now go to the file menu and select the ‘Export as WAV’ option and save the file in a desired location.

You may change the value of resistor R2 if you want to change the output volume. You can use a variable resistor in series with R2 to vary the volume of the output. The recorded audio clip can be edited using different options in the  Audacity software. You can assemble the circuit on a general-purpose PCB and enclose it in a small cabinet. Use an RJ11 connector and stereo jack for connecting the telephone set and computer (for call recording). Telephone cords can be used to connect to the phone line and the circuit. Use of a shielded cable is recommended to reduce disturbances in the recording. These can also be reduced by increasing the value of R2 to about 15 kilo-ohms.

EFY note. Audacity recording software is included in this month’s EFY-CD under ‘Utilities’ section.

Author : AlizishAAn KhAtri  Copyright: w w w. e f y m a g . c o m

This circuit is intended to program AVR controllers such as the AT90S1200 via the parallel port. The circuit is extremely simple. IC1 provides buffering for the signals that travel from the parallel port to the microcontroller and vice versa. This is essentially everything that can be said about the circuit. The two boxheaders (K2 and K3) have the ‘standard’ ISP (in system programming) pinout for the AVR controllers. The manufacturer recommends these two pinouts in an attempt to create a kind of standard for the in-circuit programming of AVR-controllers. These connections can be found on many development boards for these controllers. The software carries out the actual programming task.

It is therefore necessary to have a program (ATMEL AVR ISP), which is available as a free download from http://www.atmel.com. The construction of the circuit will have to made on standard prototype board, since we didn’t design a PCB for this circuit. This should not present any difficulties considering the small number of parts involved. We recommend that inexperienced builders first make a copy of the circuit and cross off each connection on the schematic once it has been made on the board. This makes it easy to check afterwards whether all connections have been made or not.

Author: P. G oossens - Copyright: Elektor Electronics

The operation of the converter is based on the weighted adding and transferring of the analogue input levels and the digital output levels. It consists of comparators and resistors. In theory, the number of bits is unlimited, but each bit needs a comparator and several coupling resistors. The diagram shows a 4-bit version. The value of the resistors must meet the following criteria:

The linearity of the converter depends on the degree of precision of the value of the resistors with respect to the resolution of the converter, and on the accuracy of the threshold voltage of the comparators. This threshold level must be equal, or nearly so, to half the supply voltage. Moreover, the comparators must have as low an output resistance as possible and as high an input resistance with respect to the load resistors as feasible. Any deviation from these requirements affects the linearity of the converter adversely.

Source :www.extremecircuits.net

Charging of the cellphone battery is a big problem while travelling as power supply source is not generally accessible. If you keep your cellphone switched on continuously, its battery will go flat within five to six hours, making the cellphone useless. A fully charged battery becomes necessary especially when your distance from the nearest relay station increases. Here’s a simple charger that replenishes the cellphone battery within two to three hours. Basically, the charger is a current-limited voltage source. Generally, cellphone battery packs require 3.6-6V DC and 180-200mA current for charging. These usually contain three NiCd cells, each having 1.2V rating. Current of 100mA is sufficient for charging the cellphone battery at a slow rate. A 12V battery containing eight pen cells gives sufficient current (1.8A) to charge the battery connected across the output terminals.

The circuit also monitors the voltage level of the battery. It automatically cuts off the charging process when its output terminal voltage increases above the predetermined voltage level. Timer IC NE555 is used to charge and monitor the voltage level in the battery. Control voltage pin 5 of IC1 is provided with a reference voltage of 5.6V by zener diode ZD1. Threshold pin 6 is supplied with a voltage set by VR1 and trigger pin 2 is supplied with a voltage set by VR2. When the discharged cellphone battery is connected to the circuit, the voltage given to trigger pin 2 of IC1 is below 1/3Vcc and hence the flip-flop in the IC is switched on to take output pin 3 high.

When the battery is fully charged, the output terminal voltage increases the voltage at pin 2 of IC1 above the trigger point threshold. This switches off the flip-flop and the output goes low to terminate the charging process. Threshold pin 6 of IC1 is referenced at 2/3Vcc set by VR1. Transistor T1 is used to enhance the charging current. Value of R3 is critical in providing the required current for charging. With the given value of 39-ohm the charging current is around 180 mA.
The circuit can be constructed on a small general-purpose PCB. For calibration of cut-off voltage level, use a variable DC power source. Connect the output terminals of the circuit to the variable power supply set at 7V. Adjust VR1 in the middle position and slowly adjust VR2 until LED1 goes off, indicating low output. LED1 should turn on when the voltage of the variable power supply reduces below 5V. Enclose the circuit in a small plastic case and use suitable connector for connecting to the cellphone battery.

Note. At EFY lab, the circuit was tested with a Motorola make cellphone battery rated at 3.6V, 320 mAH. In place of 5.6V zener, a 3.3V zener diode was used. The charging current measured was about 200 mA.The status of LED1 is shown in the table.

The circuit described here is a testament to the ingenuity of two young designers from  a specialist  technical secondary school. The ‘garage timer’ began as a school electronics project and has now made it all the way to publication in our Summer Circuits special issue of Elektor Electronics. The circuit demonstrates that the application possibilities for the 555 and 556 timer ICs are by no means exhausted. So what exactly is a ‘garage timer’?

When the light switch in the garage is pressed, the light in the garage comes on for two minutes. Also, one minute and forty-five  seconds  after  the  switch is pressed, the outside light also comes on for a period of one minute. The timer circuit is thus really two separate timers. Although the circuit for the interior light timer is relatively straightforward, the exterior light timer has to deal with two time intervals. First the 105 second period must expire; then the exterior light is switched on, and after a further 60 seconds the light is turned off. To realise this sequence of events, a type 556 dual timer device, a derivative of the 555, is used.

The first of the two timers triggers the second after a period of 105 seconds. The second timer is then active for 60 seconds,and it is this timer that controls the exterior light. The interior light timer is triggered at the same moment as the dual timer. In this case a simple 555 suffices, with an output active for just two minutes from the time when the switch is pressed. Push-button S1 takes over the role of the wall-mounted light switch, while S2 is provided to allow power to be removed from the whole circuit if necessary. The circuit could be used in any application where a process must be run for a set period after a certain delay has expired.

For the school project the two garage lights are simulated using two LEDs. This will present no obstacle to experienced hobbyists, who will be able to extend the circuit, for example using relays, to control proper lightbulbs. The principles of operation of type 555 and 556 timers have been described in detail previously in Elektor Electronics, but we shall say a few words about the functions of Ic1a, IC1b and IC2.

When S1 is pressed (assuming S2 is closed!) the trigger inputs of both IC1a and IC2 are shorted to ground, and so the voltage at these inputs (pins 6 and 2 respectively) falls to 0 V.  The outputs of IC1a and IC2 then go to logic 1, and D2 (the interior light) illuminates. Capacitors  C1  and  C8  now  start  to charge via P1 and R2, and R8 and P3 respectively. When the voltage on C8 reaches two thirds of the supply voltage, which happens after 120 seconds, the output of IC2, which is connected as a monostable multivibrator, goes low. D2 then goes out. This accounts for the interior light function.

Likewise, 105 seconds after S1 is closed, the voltage on C1 reaches two thirds of the supply voltage and the output of Ic1a goes low. Thanks to C4, the trigger input of IC1b now receives a brief pulse to ground, exactly as IC1a was triggered by S1. The second monostable, formed by IC1b, is thus triggered. Its pulse duration is set at one minute, determined by C5, R5 and P2. D1 thus lights for one minute. Potentiometers P1, P2 and P3 allow the various time intervals to be adjusted to a certain extent.

If considerably shorter or longer times are wanted, suitable changes should be made to the values of C1, C5 and C8. The period of the monostable is given by the formula T = 1.1 RC where T is the period in seconds, R the total resistance in ohms, and C the capacitance in farads.

Author :Daniel Lomitzky and Mikolajczak Tyrone  Copyright :Electro