An enlarger timer is essential for consistent quality and repeatability. The timer controls the exposure time of the paper in the enlarger.
The NE555 circuit illustrated in Figure 7-22 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 (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 by a 5 MΩ 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 shaft position of the 5 MΩ potentiometer can be made and calibrated in seconds. Longer or shorter full scale times may be achieved by changing the values of the RC timing components.