Darkroom exposure timer circuit using 9300

The darkroom exposure timer circuit is shown in the figure below. It can provide timing control from seconds to minutes. After pressing the start button SB to trigger the music integrated circuit, the music signal is rectified and filtered into a DC voltage, which turns on the switch tube VT, closes the relay K, and the load works. When the song ends, VT is cut off due to the loss of bias voltage, K is released, and the load stops working. 　

Adjusting RP can change the music rhythm, that is, adjust the timing time.

VD3 is a protection diode to prevent the reverse voltage of the relay coil from breaking down when VT is turned off. Automatic timing control can be achieved by connecting the A and B terminals of the relay contacts in series to the amplifier (or printer) power supply circuit. No external resistor R0 is required on the small printed board of the music integrated circuit.