Darkroom-enlarger-timer

  
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 Kl 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-M!lpotentiometer, R,. All timer -driven relay circuits should use a reverse clamping diode, such as D 1, across the coil.
Darkroom-enlarger-timer - schematic

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-M!l 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.




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