If you need an adjustable ‘on’ or ‘off’ time for some application, then this is the circuit you have been looking for. A problem that often occurs when adjusting timers are that the individual times affect each other. This circuit completely solves this problem because the time defining elements — both Rand C — are switched over. That means that there is an RCpair (P1 + R3 and C1) for the ‘off’ time and another pair (P2 +R4 and C2) for the ‘on’ time. The relay is not energized when there is a logical zero at the base of T1. This same zero causes, via input pins 10 and 11 ofIC1, pin 12 to be connected to pin 14 and pin 2 to pin 15 ofIC1.
By contrast, a logical one (relay energized) causes pin 13to be connected to pin 14 and pin 1 to pin 15. With the values shown, the oscillator period (this can be measured pin 9 of IC2) can be adjusted from 4 to 200 ms. SinceIC2 divides the frequency by 8,192 the resulting time period is adjustable from 32.8 seconds to 27.3 minutes. If a shorter period of time is desired C1 (or C2) has to be reduced, increase for a longer period. C1 and C2 need to film capacitors or bipolar electrolytes; if these are not available it is possible to make one yourself by connecting two ordinary electrolytics in series, with the positive terminals together. The power supply voltage for the timer may range from 5 to 15 V. It is preferable that you choose the same value as the rated operating voltage of the relay. The relay shown in the parts list is a 12-V type that is able to switch 230 VAC at several amps. The PCB for this project is unfortunately not available ready-made.