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.