Schematic Reference

This small circuit can be used to control model railway turnouts operated by AC voltages. A logic level in the range of 5–12 V can be used as the control signal. The coils of the turnout are switched using triacs. Changes in the logic level of the input signal are passed on by the buffer stage built around T1 and T2. The buffer stage is included to boost the current available at the gates of the triacs. If the input goes high, this positive change is passed through via C1. That causes a positive current to flow through D2 (D2 is reverse biased) to the gate of T3. That triac switches on, and power is applied to the turnout coil.


This situation persists until C1 is fully charged. No more current flows after that, so the triac does not receive any gate current and switches off. If the input is set low, a negative current flows briefly via C1. It can flow through D2, but not through D1. T4 is switched on now, and the other turnout coil is energised. This circuit takes advantage of the fact that triacs can be triggered by negative as well as positive gate currents. If the turnout coils are energised for too long, you should reduce the value of C1.

If they are not energised long enough, increase the value of C1. The TIC206D can handle several ampères, so it can easily drive just about any type of turnout coil. You can also use a different type of triac if you wish. However, bear in mind that the TIC206 requires only 5 mA of gate current, while most triacs want 50 mA. That will cause the switching times to become quite short, so it may be necessary to reduce the value of R1.

4 - 6 - 8 - 12 - 24 - 48 hours setting, LED or Beep Alert - 9V Battery Supply
A Pills Reminder is a device that operates a flashing LED (and/or a beeper) at a fixed hour interval. A choice of time-intervals as wide as possible is available with this circuit, namely 4, 6, 8, 12, 24 and 48 hours. At first you must choose the hour interval by switching SW1 to the desired value, then apply power by means of SW2. After the hour delay chosen has elapsed the LED will start flashing at 2Hz, i.e. two times per second. This status will last until pushbutton P1 is pressed: then the LED will turn off, but the circuit will continue its counting and the LED will flash again when the same hour interval as before is reached. A noteworthy feature of this circuit, usually not found in similar devices, is that the internal counter is not reset when P1 is pressed: this allows a better time-interval precision.

Let us explain this feature with an example: suppose you have set the time interval to 24 hours and started the Pills Reminder at 8 o'clock. Next day, at 8 o'clock the LED will start flashing, but you, for some reason, notice the flashes at 8:10 and press P1 to stop the LED. With most devices of this kind, the counter will be reset, causing the LED to start flashing next day at 8:10 o'clock. This will not happen with this circuit and the LED will start flashing next day always precisely at 8 o'clock even if you pressed P1 at 9 or 10 o'clock.

Parts:

R1______________10M 1/4W Resistor
R2,R3,R4_______100K 1/4W Resistors
R5,R7___________10K 1/4W Resistors
R6_______________1K 1/4W Resistor
C1,C2___________22pF 63V Ceramic Capacitors (See Notes)
C3______________22µF 25V Electrolytic Capacitor
C4,C5__________100nF 63V Polyester Capacitors
C6_______________1µF 63V Polyester, Multilayer Ceramic or Electrolytic Capacitor
IC1____________4060 14 stage ripple counter and oscillator CMos IC
IC2____________4040 12 stage ripple counter CMos IC
IC3____________4082 Dual 4 input AND gate CMos IC
IC4____________4075 Triple 3 input OR gate CMos IC
IC5____________4520 Dual binary up-counter CMos IC
IC6____________4001 Quad 2 input NOR Gate CMos IC
D1_____________5 or 10mm red LED
XTAL_________32.768 kHz Sub-miniature Watch crystal
P1_____________SPST Pushbutton
SW1____________2 poles 6 ways Rotary Switch
SW2____________SPST Toggle or Slide Switch
B1_______________9V PP3 Battery Clip for PP3 Battery

Alternative Clock Parts:

R8_______________1K 1/4W Resistor
R9_____________330K 1/4W Resistor
R10_____________20K 1/2W Cermet or Carbon Trimmer
R11______________1K 1/2W Cermet or Carbon Trimmer
C7_______________1µF 63V Polyester Capacitor
IC7____________7555 or TS555CN CMos Timer IC

Circuit Operation:

The clock of the circuit is made of a stable oscillator built around two inverters embedded into IC1 and a Watch crystal oscillating at 32.768kHz. This frequency is divided by 16384 by the internal flip-flop chain of IC1 and a 2Hz very stable clock frequency is available at pin #3 of this IC. IC2 counter and IC3A 4 input AND gate are wired in order to divide by 3600 the 2Hz clock, therefore, a pulse every 30 minutes is available at the clock input of IC5. The division factor of this IC is controlled by IC3B and the position of SW1A and B, selecting from six time-intervals fixed to 4, 6, 8, 12, 24 and 48 hours.

The set-reset flip-flop formed by IC6B and IC6C is set through IC4C each time a low to high transition is present at the pin of IC5 selected by SW1B cursor. IC6A and C4 provide to set the flip-flop also when a high to low transition is present at SW1B cursor. When the flip-flop is set, IC6D is enabled and the 2Hz frequency available at pin #3 of IC1 is applied to pin #13 of IC6D causing the flashing LED operation. The flip-flop can then be reset by means of P1. A master reset is automatically done at switch on by means of C6 and R7.

Alternative Clock:

Sometimes, the Watch crystal can be difficult to locate, or could be considered too expensive. For those willing to avoid the use of a Watch crystal and to accept less time accuracy, an alternative clock generator circuit is provided, directly oscillating at 2Hz, thus avoiding the use of divider ICs. A CMos 7555 Timer IC generates a stable 2Hz square wave, whose frequency must be accurately set by means of two trimmers. R10 must be adjusted first for coarse tuning, then R11 for fine tuning. Setting precisely the 2Hz frequency of this oscillator is a rather difficult task, and can be done with great patience and the aid of a clock, a chronometer or, best, a digital frequency meter capable of measuring very low frequencies. In any case, after an accurate setup, this oscillator showed a very stable performance, not affected by battery voltage variations and an accuracy of about ±30 seconds per 24 hours interval.

Notes:

• Wanting the utmost time precision and if a digital frequency meter is available, a 5-50pF 50V Ceramic Trimmer Capacitor can be used in place of C2. It must be adjusted in order to read exactly 32.768kHz on the meter display with the input probe connected to pin #9 of IC1.
• A Piezo sounder (incorporating a 3KHz oscillator) can be added to provide a visual plus audible alert. It must be wired across pin #11 of IC6D and negative ground, respecting polarities. Remove D1 and R6 if the visual alert is not needed.

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 circuit allows an ECG signal to be displayed on an oscilloscope. Opamps IC1a, b and d form an instrumentation amplifier with a gain of 201. IC1c amplifies the common-mode signal by a factor of 31, and supplies this signal to the right leg. The first consequence of this is that the body is brought to a defined common-mode level, so that the signal will not lie outside the range of the instrumentation amplifier. The second consequence is that negative feedback is applied to the common-mode signal, so that the amplitude of this (undesired) signal is reduced even further. Diodes D1 through D4, along with resistors R1 and R5, are added to the circuit to protect the inputs against damage from excessive electrostatic charges. The CMRR (common-mode rejection ratio) of the instrumentation amplifier can be set using P1.

To make this adjustment, connect both inputs of the instrumentation amplifier together, and then connect a 100mV, 50Hz AC signal between the connected inputs and earth. Measure the output signal using an oscilloscope, and adjust P1 to minimize the level of the output signal. It is important that the electrodes make very good contact with the skin. In our test measurements, winding three uninsulated copper wires several times around the index fingers (and the right leg) proved to be sufficient to provide a good signal. The amplitude of the ECG signal measured with this arrangement was 200mV. The current consumption of this circuit is only 2mA, so the batteries should last a long time. This circuit must never be connected to a mains-operated power supply, in consideration of safety precautions that are necessary when making this sort of measurement on the human body.

Dedicated model rail enthusiasts using sophisticated train and points controllers often have the problem that as their layouts get bigger and more complex, the transformer supplying power to the points does not have enough current to switch several points at the same time. The actuators in the points are designed for ac operation so it doesn’t help by rectifying the supply and adding reservoir capacitors, the coils can overheat and burn out if they get jammed during their travel (ac operation actually helps to overcome friction in the mechanism). The circuit shown here solves this problem by using a sequencer to ensure than only one points actuator can be active at any point in time.

During operation the controller will switch all the points on one line at the same time as usual, but the other connection to each coil is connected to the sequencer unit. This circuit will only allow current to flow through one coil at a time. The sequencer circuit consists of a 555 timer configured as an astable multivibrator clocking a 4017 Johnson counter where the ten outputs are used to switch ten triacs in sequence, enough for ten sets of points. P1 alters the oscillator frequency of the 555 timer and can be adjusted so that each time interval of the sequencer is long enough to allow the points to switch.

The switching time varies depending on the type of points but is typically between 1 s and 1.5 s. Any points that jam during switching give out a characteristic humming noise in time to the switching frequency so it makes them easier to find. The eleventh output of the 4017 can be connected to an LED (together with a series resistor). This will flash to give a visual indication of the sequencers operation. Power for the circuit is provided by 15 V ac from the points transformer. The B80C1500 bridge rectifier (80 Vpiv, 1.5 A) and regulator IC1 produce a stabilised 12 V for the circuit. Current consumption is only a few milliamps.

This simple circuit enables you to listen to your heartbeat, for instance, while you are exercising. The transducer used for detecting the pulse is an electret microphone, X1 in the diagram. The model used has two (polarized) terminals. As usual with this type of microphone, it functions via a series resistor, R1. The potential drop across this resistor is applied to op amp IC1a via C1. The amplification of the op amp is set to between ´40 and ´1000 with preset P1. Network R4-C3 in the feedback loop of IC1a is a low-pass filter with a cut-off frequency of 34 Hz. Higher frequencies are not needed for the present application. A pulse rate of 180* is equivalent to a frequency of 3 Hz.

So as to cater for a wide range of pulse rates, the cut-off frequency is made just over 11 times as high as that representing the highest pulse rate. Operational amplifier IC1c, in conjunction with push-pull am-plifier T1-T2, creates a headphone amplifier, whose output resistance is equivalent to the value of R9, that is, 47 Ω. This makes the circuit usable for virtually any kind of headset. The output is short-circuit-proof. In case of certain headphones, such as that used with Sony Walkman™ sets, it is best to connect the two earphones in series. Operational amplifier IC1b is used as an active potential divider. The voltage across the actual divider, R5-R6, is half the supply voltage.
This voltage is buffered by IC1b, taken from the low-resistance output, pin 7, of this op amp and used as reference for IC1a, and as operating voltage for the electret microphone. The voltage is decoupled by C4 to remove any interference signals from it. The supply voltage for the pulse rate monitor is decoupled by capacitor C7, immediately after polarity protection diode D1. Owing to the use of CMOS op amps, the current drain does not exceed 10 mA, so that operation from a 9 V battery is perfectly feasible. A dry alkaline manganese battery will have a life of about 50 hours.

Unless you are a young super-fit top-class athlete, you should see your GP immediately when you find you have a pulse rate of 180. As a general guide, the absolute maximum pulse rate for a young, very fit person is 180, for a middle-aged person, 160, and for an elderly person, 140. When exercising, the pulse rate of a not very fit person should not exceed 60% of these maxima.

This circuit is a toy to encourage young children to count. Power is turned on by switch S1, then S2 is closed. This makes nine LEDs flash slowly. S2 is then opened and the LEDs go out. Pressing push-button PB1 turns on a random number of LEDs - briefly - during which time they are to be counted. The number counted can be checked by pressing PB2 which turns the same LEDs on for as long as needed. Then repeat. The circuit works as follows: IC3 is a 4049 hex inverter connected as three oscillators running at different rates. It is turned on by closing switch S2a. The clock pulses from IC3 drive both halves of IC1 and one half of IC2, both being 4015 dual 4-stage shift registers. Each shift register has four outputs which go high in order: 1, 1 and 2; 1 and 2 and 3; 1 and 2 and 3 and 4. However as output 4 is connected to the reset line of its own half - the shift register resets to zero. Outputs 1, 2 & 3 of all three shift registers are connected to nine LEDs, the cathodes of which go to a common rail.

This rail is connected to ground via S2b when switch S2 is closed. When S2 is opened the three oscillators stop but a random number of LEDs is still connected to the high outputs of the 4015s. That number can be viewed briefly by pressing PB1 which pulses the 7555 timer in monostable mode, to give a short duration output which drives Q1 and connects the LED cathodes to 0V. The viewing time is adjustable by VR1. Checking a count is done by pressing PB2 which holds the same LEDs on as long as desired. The LEDs are set in a 3 x 3 grid with the connection scattered, ie, the first row is not the three LEDs from the first half of IC1. Note that, unlike the usual dice, a number such as 5 can appear in many formats, so pattern recognition is no help. Also note that this is not a nine output true dice - because the numbers do not come up with equal frequency.

Here’s a twist to the age-old game of noughts and crosses. Instead of pen and paper, it uses nine 10mm tri-colour LEDs arranged in a 3 x 3 grid. One player has nine red buttons while the other player has nine green, set out in identical grids. The aim, of course, is to make three LEDs in a row glow the same colour – red or green! Pushing a red button causes the LED in the equivalent position on the grid to glow red in colour. Likewise, pushing a green button lights the equivalent green LED.


If a player pushes a button for a LED that is already glowing red or green, then that LED changes to yellow, exposing the false move! All the LEDs are then turned off, ready for the next game, by pressing the "Clear" button. For simplicity, the circuit shows only one tri-colour LED and a pair of opposing buttons. This circuit fragment must be repeated another eight times to create a complete 3 x 3 grid. A brief press on a button fires the associated SCR and turns on the LED. The common (cathode) lead of all LEDs is connected to the 0V rail via the normally-closed contacts of the "Clear" pushbutton (S3).

This circuit produces a random "Yes" or "No" with a single button press - indicated by the illumination of a red or green LED. The circuit has two advantages over similar circuits. First, it uses just a single momentary contact pushbutton, so no on-off switch is required. When the pushbutton is pressed, an oscillator comprising the 10nF capacitor and 22kΩ resistor at pins 1 & 2 is almost immediately stopped by FET Q1, which pulls the oscillator's timing capacitor to the positive rail. However, the 220nF capacitor and 470kΩ resistor in the gate circuit of Q1 introduce a tenth of a second's delay, so that about 250 oscillations take place before the clock is stopped.

Due to variations in charge on the circuit's capacitors, as well as voltage and temperature variations, and the unpredictability of when the pushbutton will be pressed, randomness is assured. The circuit has a high degree of randomness because it takes advantage of a near-perfect complementary square waveform at pins 10 and 11 of the 4047 IC. The oscillator frequency (available at pin 13) is passed through an internal divide-by-2 circuit in the 4047. This appears at pin 10 (Q), and is inverted at pin 11 (Q-bar), thus assuring a near perfect 50:50 duty cycle for the two LEDs.
Note:

However, that the "impartiality" of the circuit is partly contingent on the value of the 10nF capacitor and on a reasonably equal current flow through both LEDs. Over five trials, the Yes-No Indicator scored 142 Yes, 158 No, with Yes falling behind No in the fourth trial. Because the circuit only works while switch S1 is pressed, standby current is zero, therefore a miniature 12V battery may be used to power it. In this case the circuit could be used thousands of times before the battery would run flat. The circuit has a further potential use. If the LEDs are omitted and a piezo (capacitive) sounder is wired directly to pins 10 and 11, it will produce a loud beep when equipment is turned on, and will continue to draw less than 0.5mA until it is switched off. The frequency of the beep may be changed by altering the value of the 10nF capacitor and its duration by altering the value of the 220nF capacitor.

This circuit is essentially a light chaser but it can also be set to provides heads or tails (Two Up) or a Dice (die). It also has a speaker to simulate the sound of a spinning roulette wheel. Note that the dice and heads/tails features can be deleted if required and rules for the games created to suit individuals; eg, betting can be used or the numbers recorded and then totalled to get the highest score per game. IC1 is a 4046 phase locked loop (PLL) but only the voltage controlled oscillator (VCO) portion of the chip is used to provide the clock pulse for IC2, a 4017 decade counter/divider. In roulette wheel mode, switch S3 is pushed to start the game.


This charges the 10μF capacitor at pin 9 and as the capacitor discharges, the output frequency is slowly reduced to slow the rate of the chaser LEDs driven by IC2. In chaser mode, switch S2 is closed to provide a fixed frequency output from IC1. This can be varied over a wide range with potentiometer VR1. Transistor Q1 is also driven by the oscillator output of IC1 and it drives the speaker. Trimpot VR2 varies the sound level while switch S4 turns it off. Switch S5 selects Die or other (chaser/roulette). In Die mode, pin 6 is connected to the reset, pin 15, so that the circuit only counts to 6 whereas in the other modes it counts to 10 and displays all LEDs. Pin 12 drives transistor Q2 and two LEDs to provide the Heads/Tails function.

This simple dual digital dice is based on three low-cost ICs, a few transistors and a handful of LEDs. IC1a & IC1b operate as an oscillator with a frequency of about 4kHz and this clocks IC2. The frequency of oscillation is not critical - it simply needs to be high enough to prevent cheating. IC2 and IC3 are 4516 binary counters, configured to count in binary from 1-6. A power-on reset is not required here since, if the initial state is outside the correct range, the counters will count into the correct range after a few clock pulses. Let's first consider how IC2 operates. When the counter reaches "7" (ie, 111), the AND gate formed by diodes D1 & D2 and the 47kO resistor applies a high to the PE pin (pin 1).


This presets the counter to 1 (ie, 001) and so PE goes low again. The counter then increments in the normal manner until it reaches "7" again. Counter IC3 operates in the same manner except that the clock signal is derived from IC2's O3 output. The counter outputs (O1, O2 & O3) drive NPN transistors Q1-Q6 and these in turn drive the LEDs (ie, the LEDs indicate the states of the counters). Normally, the counters are incrementing continuously and the LEDs all appear to be lit. However, when push-button switch S1 is pressed, pin 6 of IC1c goes low and pin 9 of IC1d pulls the Ci input of IC2 high, thus stopping the counters. Finally, toggle switch S2 allows the user to choose between having two dice operating simultaneously or just one.

Although 60/40 solder melts at about 200&degC, the tip temperature of a soldering iron should be at about 370&degC. This is necessary to make a good quick joint, without the risk of overheating delicate components because the iron has to be kept on the joint for too long. Unfortunately, at this temperature, the tip oxidises rapidly and needs constant cleaning. That's where this circuit can help - it keeps the soldering tip to just below 200&degC while the iron is at rest.

Oxidisation is then negligible and the iron can be brought back up to soldering temperature in just a few seconds when needed. In addition, normal soldering operation, where the iron is returned to rest only momentarily, is unaffected because of the thermal inertia of the iron. Two 555 timers (IC1 & IC2) form the heart of the circuit. IC1 is wired as a monostable and provides an initial warm-up time of about 45 seconds to bring the iron up to temperature. At the end of this period, its pin 3 output switches high and IC2 (which is wired in astable configuration) switches the iron on - via relay RLY1 - for about one second in six to maintain the standby temperature.


The presence of the iron in its stand is sensed by electrical contact between the two and some slight modification of the stand may be necessary to achieve this. When the iron is at rest, Q1's base is pulled low and so Q1 is off. Conversely, when the iron is out of its stand, Q1 turns on and pulls pins 2 & 6 of IC2 high, to inhibit its operation. During this time, pin 3 of IC2 is low and so the iron is continuously powered via RLY1's normally closed (NC) contacts. Note that the particular soldering iron that the circuit was designed for has its own 24V supply transformer. Other irons may need different power supply arrangements. The warm-up time and standby temperature can be varied by altering R2 and R5, as necessary.

This simple circuit is a two-person game of strategy and speed - and potentially, agility and athletic fitness. Each player has a row of four LEDs before him/her. Beside each LED, there is a push-button which, when pressed, lights up the corresponding LED. The aim of the game is for a player to illuminate all four of their LEDs in a row, in which case the circuit declares a winner. However, there is a catch. As soon as you light one of your own LEDs, the other player's corresponding LED goes out - and vice versa.

The game begins by giving each player two illuminated LEDs. Consider now that this game is scaled up and used in a gym. If the LEDs in the circuit are directly replaced with N-channel power MOSFETs, then 12V globes can be illuminated (a MOSFET's gate is wired in place of a LED's anode, the source goes to negative, and the load is wired between the drain and positive). If four large push-buttons are mounted on one wall and four on another, this could become a game of agility - if not a physical tussle to keep the other player away from critical push-buttons.


Here's how the circuit works:

Schmitt NAND gate IC1a and IC1b (4093) form a simple bistable latch. When one output (pin 3) goes "high", the other output (pin 4) goes "low" and vice versa. The main advantage of using a bistable latch (as opposed to a flipflop) is that it does not suffer from switch bounce. Four such bistable latches are fed to inputs A-D of IC2. However, for the sake of simplicity, only one of these is shown; ie, IC1a-IC1b. We now need to identify when all four bistable latches go either "high" or "low".

This is done using IC2, a 4067 16-channel multiplexer. When inputs A-D are all "low" (binary 0000), this opens decimal channel 0. Conversely, when all are "high" (binary 1111), this opens decimal channel 15. Channels 0 and 15 thus trigger a win for one side or the other, by taking pins 9 or 16 of IC2 "low". Finally, if the game is quite hectic, a win might only last for a fraction of a second before it is lost again. Therefore, IC1c and IC1d are wired as timers, which do not permit any further play until a win has been reported for one or two seconds - either via LED3 or LED4. During this time, however, the players' buttons may be pressed to reset the game to two LEDs all.

Strictly speaking, this simple circuit shouldn't work! How could anyone expect an ordinary light dependent resistor photo cell to 'see' through a fingertip in natural daylight and detect the change in blood flow as the heart pulsates? The secret is a high gain circuit, based on a dual op amp IC which can be either the low power LM358 or the JFET TL072. The LDR is connected in series across the 9V battery supply via a 100kO resistor (R1) and the minute signal caused by the blood pulsing under the skin is fed to the non-inverting (+) input, pin 3, of IC1a via a 0.µF capacitor.

Pin 3 is biased by a high impedance voltage divider consisting of two 3.3MO resistors. The feedback resistors to pin 2 set the gain to 11 times. The output of IC1a is fed via a 0.47µF capacitor and 220kO resistor to IC1b. This is configured as an inverting op amp with a gain of 45 so that the total circuit gain is about 500. The output of IC1b is used to drive an analog meter which may be a multimeter set to the 10V DC range or any panel meter in series with a resistor to limit the current to less than its full-scale deflection. The prototype used an old VU meter with a 47kO resistor fitted in series.


Note that the unit was designed to use the Dick Smith Electronics light dependent resistor (Z-4801). Other LDRs may require a change in the value of resistor R1. A light source such as a high brightness LED is not required. All that is needed is a reasonably well-lit room, preferably natural daylight, to produce a healthy swing of the needle. Only when the hands are very cold does it make it a little more difficult to accurately count the pulses. To check your heart rate, carefully position your thumb or finger over the LDR and count the meter fluctuations for a period of 15 seconds. Then multiply the result by four to obtain your pulse rate. The circuit can not be used if you are walking or running, etc.

This circuit could be used (depending on your circumstances) by a gentleman to summon his butler, a manager his secretary or as in the author’s case to call the kids down to dinner without having to shout above the level of the CD player/TV/games console in their bedroom. Rather than resorting to a full-blown intercom system, a simpler solution was envisaged and while a buzzer could easily fulfil this function, this circuit has the advantage of providing a visual indication of a call as well as confirming to the caller that the ‘message’ has been received.

This is especially useful in the latter case, as the call may be easily drowned out by the music playing in the headphones. The circuit, which requires no complicated switching, uses a simple two-wire connection between the two stations and utilises the fact that the forward voltage drop of a blue (or white) LED is greater than that of a red, green or yellow one. The circuit is based on a two-transistor multivibrator which is used to pulse a red LED (D3) as well as the buzzer Bz1 on and off at about 1.5 Hz when push button S1 is closed. This frequency may of course be altered if required by changing the values of the capacitors.


The diode D1 in series with the collector of transistor T2 is required to isolate the output from the effects of the buzzer circuitry, which would alter the multivibrator frequency. In principle, the multivibrator could be dispensed with but a pulsed buzzer/flashing led is much more noticeable than a continuous signal especially in noisy conditions. Since the voltage across a red LED is typically about 1.5 V while a blue LED requires at least 2.5 V to 3 V to light, the blue LED will remain off when the call button S1 is pressed. Despite being rated for operation at 3-12 V, most piezo sounders can still produce a piercing sound from the pulsed 1.5-V available across the red LED which should get the attention of even the most preoccupied teenager.

When the recipient presses the acknowledge (push to break) switch S2, the red LED/buzzer are disconnected allowing the blue LED to flash at the sending station indicating to the caller that his call has been received. Alternatively, if a blue LED is not available, a red or green type in series with a forward biased silicon diode to raise its forward voltage above that of the red LED in the receiver could be used instead. The circuit may be powered by a 9-V battery, a mains power supply being unnecessary in view of the low power consumption and infrequency of use of the circuit.