Darkroom exposure timer light circuit

(1) Introduction to the circuit principle As shown in Figure 3-32, it is a darkroom exposure timer lamp circuit suitable for black and white photography to develop photos. The NE555
time base circuit, potentiometer RP, and capacitor C form a monostable trigger. Normally, the circuit is in reset state, the ③ pin of NE555 outputs low level,
the relay K does not operate, and the exposure light EL
does not light up. The light-emitting diodes I and ED2 are work indicators, which can illuminate the dial of
the potentiometer RP .
At this time, the internal discharge tube of NE555 is turned on, and the capacitor c] is short-circuited and cannot be charged. K can use JZC-22F, DC12V small medium power electromagnetic relay .     (2) Component parameter selection The selection of components is shown in Figure 3-32. There are no special requirements. (3) The darkroom exposure timing light circuit is shown in Figure 3-32.