555 photo exposure timer circuit diagram

The picture shows the photo exposure timer circuit .

After power is turned on, the timer enters steady state. At this time, the voltage of the timing capacitor CT is: VCT=VCC=6V. For the 555 equivalent flip-flop, both inputs are high level, that is, VS=0. Relay KA does not close, the normally open point is open, and the exposure light HL does not light up. After pressing the button switch SB, the timing capacitor CT is immediately put to zero voltage. So at this time, the equivalent trigger input of the 555 circuit becomes: R=0, S=0, and its output becomes high level: V0=1. Relay KA actuates, the normally open contact closes, and the exposure light lights up. 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 or 4 volts, the timing time has expired. The input of the 555 equivalent circuit trigger is: R=1, S=1, so the output flips to low level again: V0= 0. Relay KA is released and exposure light HL goes out. The temporary steady state ends and returns to the steady state.