One of the darkroom timed exposure circuits

One of the darkroom timed exposure circuits

It emits an audible signal when the development time is approaching. This circuit can be used for photo development and amplification
timing, and can also be used in many other situations.
    The given time link is composed of field effect transistor VTi, resistor, capacitor, etc., and
the sound signal link is composed of transistor VTz, transformer T, speaker B, etc.
    When the potentiometer RP is adjusted to the resistance value of zero, the timer delay is 1 to 1.5 minutes; when the resistance value of the potentiometer RP is adjusted to the maximum,
the timer delay is 10 to 15 minutes. The tone of the audio signal depends on the capacity of the capacitor Cz, and the delay time depends on
the capacity of the capacitor Cl.
    Turn on the switch s, the exposure starts (the exposure lamp circuit is not shown), and when it ends, the horn emits a sound signal. At this time, the power supply should
be cut off immediately to avoid wasting electric energy.