555 darkroom printing timed exposure device circuit diagram

This exposure circuit is used for enlarging photos in the darkroom, and the exposure timing can be adjusted from a few seconds to three minutes. The exposure circuit consists of a step-down rectifier circuit, a timing circuit and a thyristor control circuit. The circuit is shown in the figure.

T in the picture is an AC step-down transformer, which forms a step-down bridge rectifier circuit with D1~D4. The rectified voltage is stabilized by the three-terminal regulator IC 1 to output a +9V voltage. IC 2 uses 555 integrated circuit , which forms a monostable timing circuit with RP1, R1 and C3. SN is a timer switch. Normally, SN is disconnected and pin 6 of 555 is at high potential, putting 555 in reset state. When you want to expand the photo, press SN, pin 2 of 555 will be at low level, so that 555 will be set, and pin 3 will be at high level. The duration of the high level is the monostable timing time td=1.1 (RP1+R1) C3. Adjusting RP1 can change the exposure time. The temporary stable time range of the parameters shown in the figure is 10 to 186 seconds.

During the 555 timing period, the thyristor SCR is triggered to conduct, lighting the exposure bulb, and exposing the photo; when the timing is up, the 555 automatically flips and resets, pin 3 turns to low potential, the SCR is cut off, and the H light goes out. At the same time, the indicator LED lights up, indicating that the exposure is complete. The transformer T in the figure uses a 0.5W AC transformer with a secondary voltage of 12~15V; the thyristor SCR can use a bidirectional thyristor with a withstand voltage above 400V, such as 3CTS-1A/400V; the LED uses a 5mm red light-emitting diode.

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