Clock & Timer Circuit DiagramsPower Supplies

On/Off Timer Schematic Circuit Diagram

Adjustable On and Off Times: A Breakthrough Circuit Solution

If you’re searching for a circuit offering adjustable ‘on/off’ times for specific applications, your quest ends here. Often, adjusting timers leads to a common issue: the interdependence of individual timings. This circuit provides a definitive solution by cleverly addressing this problem. Here, the time-defining components — both R and C — are switched, creating distinct RC pairs (P1 + R3 and C1) for the ‘off’ time and another pair (P2 + R4 and C2) for the ‘on’ time. Notably, the relay remains unpowered when a logical zero is present at the base of T1. This same zero, through input pins 10 and 11 of IC1, establishes a connection between pin 12 and pin 14, as well as pin 2 and pin 15 of IC1.

On Off Timer Schematic Circuit Diagram 1

On Off Timer Schematic Circuit Diagram 2

By Contrast: Modifying the Oscillator Period

In this configuration, a logical signal (relay energized) establishes a connection between pin 13 and pin 14, as well as between pin 1 and pin 15. The oscillator period, measurable at pin 9 of IC2, can be adjusted from 4 to 200 ms with the given values. Since IC2 divides the frequency by 8,192, the resulting time period is variable from 32.8 seconds to 27.3 minutes. To achieve a shorter time span, reduce C1 (or C2); conversely, increase it for a longer duration. C1 and C2 must be film capacitors or bipolar electrolytes.

If these are unavailable, a makeshift version can be created by connecting two ordinary electrolytes in series, aligning the positive terminals. The timer’s power supply voltage can range from 5 to 15 V. Ideally, this voltage should match the rated operating voltage of the relay, which, in this case, is a 12-V type capable of switching 230 VAC at several amps. Unfortunately, a ready-made PCB for this project is not commercially available.

Capacitors and Their Fundamental Principle

A capacitor functions as a device designed to store electrical charges within a circuit. Its operation relies on the principle that the capacitance of a conductor significantly increases when it is brought near an earthed conductor. Consequently, a capacitor consists of two plates separated by a distance, each carrying equal and opposite charges.


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