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 is that the individual times affect each other. This
circuit completely solves this problem because the time defining
elements — both R and C — are switched over. That means that there is an
RC pair (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 of IC1, pin 12 to be connected to pin 14 and pin 2 to pin 15 of
By contrast, a logical one (relay energized) causes pin 13 to be
connected to pin 14 and pin 1 to pin 15. With the values shown, the
oscillator period (this can be measured at pin 9 of IC2) can be adjusted
from 4 to 200 ms. Since IC2 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 be ﬁlm capacitors or bipolar electrolytics; 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 15V. 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.