Photo exposure timer

The attached circuit is a photo exposure timer made of 555 monostable circuit. It uses a manually started monostable circuit.

Working principle:

After the power is turned on, the timer enters the steady state. At this time, the voltage of the timing capacitor CT is: VCT=VCC=6V. For the equivalent trigger 555, both inputs are high level, that is, VS=0. The relay KA is not attracted, the normally open point is open, and the exposure lighting HL is not on.

After pressing the button switch SB, the timing capacitor CT is immediately discharged to zero voltage. So at this time, the input of the equivalent trigger of the 555 circuit becomes: R=0, S=0, and its output becomes high level: V0=1. Relay KA is attracted, the normally open contact is closed, and the exposure light is lit. The button switch is released immediately after being pressed, so the power supply voltage charges the capacitor CT through RT, and the temporary steady state begins. When the voltage on the capacitor CT rises to 2/3VCC, that is, 4 volts, the timing time has expired, and the input of the 555 equivalent circuit trigger is: R=1, S=1, so the output flips to a low level again: V0=0. Relay KA is released, and the exposure light HL goes out. The temporary steady state ends and returns to the steady state.
The exposure time calculation formula is: T=1.1RT*CT. The delay time provided by this circuit is about 1 second to 2 minutes, which can be adjusted and set by the potentiometer RP.

The relay in the circuit must use a product with a pull-in current no greater than 30mA, and the relay contact capacity should be selected according to the capacity of the load (HL).