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# On/Off Timer(BC517)

2017-08-11 12:05
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This article briefly describes the On / Off Timer (BC517). Although this circuit is very easy to understand, but also very practical. Circuit elements in depth understanding of the grasp of the principle is very effective. Do you know the circuit, you can see and purchase these components: BC517 .

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 IC1.

#### Figure 2

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.

#### Figure 3

C1 and C2 need to be ?lm 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 230VACat several amps.