Sleep-switch cum Wake-up Timer

Here is a sleep-switch circuit that can be easily converted into a wake-up timer. A dual-mode time setting makes the system versatile. The circuit is low-cost and can function as a precise timer. 

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 The heartbeat produced by IC1 is a sharp 1Hz square wave signal having a duty cycle of 50 per cent. This is achieved by using a 4.194304MHz crystal in combination with discrete components around it. The 1Hz output of IC1 is connected to IC2 as well as one of the terminals of switch S1. IC2 is configured as divideby- 6 counter while IC3 further divides the output of IC2 by ten to produce one-minute output at its pin 12. This is brought to the second terminal of twoway switch S1 to help select either the ‘minutes’ or the ‘seconds’ mode of operation for IC4.

 The decade counter IC4 provides binary output as it counts up the input pulses and IC5 decodes/converts them to 1-of-10 outputs (units). Similarly, the IC6- IC7 pair provides tens output since IC6 clock input pin is connected to D output pin of IC4.

  Rotary switches S2 and S3 can be set to select any time between either 0 to 99 seconds or 0 to 99 minutes, depending upon the position of mode switch S1. Switches S2 and S3 could also be replaced by thumb-wheel type switches or 10-position DIP switches with one of their side terminals shorted together to serve as a pole. Please note that IC5 and IC7 (74145) have active low outputs.

 The outputs from switches S2 and S3 are input to a two-input OR gate inside IC8 (7432) to obtain active low output on completion of the set time delay to deactivate relay RL1 through relay driver transistor T1 (normally conducting) when set time is reached. When transistor T1 cuts off, its collector goes high to reset oscillator IC1, and thus count at output of IC4 and IC6 gets locked. For resetting or restarting, the power supply to the circuit should be switched off and then switched on again.

 The BCD outputs of IC4 and IC6 are converted to seven-segment outputs by IC9 and IC10 to drive the units and tens displays respectively for indicating elapsed time continuously. The relay contacts (normally open and normally closed) can be suitably used to energise or de-energise an alarm after the preset delay. It can thus be used as wake-up alarm or sleep timer.

  If you want to de-energise the relay, say after 30 minutes, then set switch S1 to minutes mode, S2 to 0 and S3 to 3, and then switch on the supply to the circuit. After 30 minutes the outputs at poles of switches S2 and S3 will go low and so also the output of OR gate (IC8). As a result, transistor T1 will be cut-off to de-energise the relay.

 One can easily add 0-99 hours capability by cascading two counters similar to the minutes counter section comprising IC2 and IC3. Input clock for hours counter would be the minutes clock available at pin 12 of IC3.

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Computerised Universal Timer

This simple and flexible timer is more accurate than the real-time clock of the computer used for the purpose. It can be used in laboratories, dark rooms, kitchens, and for competitions in educational institutes. The program written in Q-Basic is self-explanatory.

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 Generally, a universal timer provides the facility for switching on an electrical/ electronic device after elapse of a certain time period, say, 5 minutes. The software does the same job here.

 When the program is executed, it displays 0:0:0 on the monitor, indicating 0 hour, 0 minute, and 0 second. The display time 0:0:0 is increased by 10 seconds each time function key F1 of the computer keyboard is depressed. So by depressing function key F1 the required time is set for which the electrical or electronic device is to be switched on. However, in debate competitions the time allowed for a candidate to speak is filled the way it is discussed above. The program may be changed as indicated by REM statements and the single quote (‘) in the beginning of a program line may be accordingly removed in the program.

 Now, after setting the time in the manner as discussed above, function key F2 of the computer keyboard is depressed to switch on any device. Simultaneously, the countdown of the time in the display box starts. The device will remain on until the display box shows 0:0:0 and then it will get switched off. The figure shows the relay interface circuit connected between D0 line (pin 2) and ground line (pin 25) of 378H output port of LPT1 printer port of the computer.

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Dark Room Timer

The timer circuit described here provides a pleasant musical tone in your darkroom at 1-second intervals. The circuit takes up very little space and can be easily converted into a metronome.

 Unijunction transistor (UJT) T1 functioning as a relaxation oscillator triggers the phase-shift audio oscillator circuit built around transistor T2, turning it on and off. As capacitor C1 is charged through preset VR1 and resistor R1, the emitter voltage of UJT rises toward the supply voltage.

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When the emitter voltage becomes sufficiently positive, the emitter becomes forward biased and discharges capacitor C1 through the emitter-base 1 (B1) junction and resistor R2.  The voltage drop across R2 forward biases transistor T2 and turns it on. As capacitor C1 becomes discharged, the current through resistor R2 drops and transistor T2 is cut off.

 A tone signal is generated by transistor T2 and R-C coupled phase-shift oscillator. Part of the signal taken from the collector of transistor T2 is coupled to a small speaker through a transistor-radio type output transformer.

  The 22-kilo-ohm value of resistor R3 represents a compromise between tone duration and intensity. You can use resistors having a value anywhere between 10 kilo-ohms and 25 kilo ohms for different durations and intensities of the output signals.

Since the unijunction transistor is functioning as the oscillator trigger, changing the values of one or more components in the UJT circuit will change the rate of the tone burst. The tone frequency can be varied by changing the value of any or more of capacitors C2 through C4 and resistors R5 and R6 in the phase-shift network.

 The primary winding of transformer X1 can be tuned for a slight increase in the output, using capacitor values between 0.05 and 0.25 μF for C5 by trial-and-error method. Tone pulses should begin about ten seconds after the unit is turned on.

After a minute or so, adjust preset VR1 for 1-second beats by comparing the timing of the beats with the seconds needle on your wristwatch.

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