The NE555 circuit is a basic one-shot timer with a relay connected between the output and ground. It is triggered with the normally open momentary contact switch, which when operated, grounds the trigger input at pin 2. This causes a high output to energize K1 which closes the normally open contacts in the lamp circuit. They remain closed during the timing interval, then open at time out. Timing is controlled b a 5-MW potentiometer, Rt. All timer-driven relay circuits should use a reverse clamping diode, such as D1, across the coil. The purpose of diode D2 is to prevent a timer output latch-up condition in the presence of reverse spikes across the relay.
With the RC time constant shown, the full-scale time is about 1 minute. A scale for the 5-MW potentiometer shaft position can be made and calibrated in seconds. Longer or shorter full-scale times can be achieved by changing the values of the RC timing components.