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This circuit is very simple and it was mainly designed to be used as a photo timer. It is useful for providing controlled on times for any type of low power electrical equipment.
The relay is normally held off. A momentary touch on switch S1 energizes the relay which is held closed (on) for a time 1.1*(R1+R2)*C1 and then released. Time is set by pot R2. With C1 of 100uF, R2 provides a range of 1 to 100 second.
The circuit is based on the famous 555 timer which is used in monostable mode of operation and acts as a one shot timer. Once triggered the relay is switched on and will remain in this state until the one-shot time is elapsed even if it is triggered again.
Posted by: A.G