Darkroom Timer Circuit

The electronic darkroom timer is built around a 555 oscillator/timer, a pair of general-purpose transistors, a buzzer, and an LED. The 555 (Ul) is configured as an astable multivibrator (free-running oscillator). The frequency of the oscillator is determined by the values through i3 and Cl through C4. Switch SI is used to divide the capacitor network to vary the time interval between beeps; when SI is closed, the circuit beeps at intervals of 30 seconds. With SI closed, it beeps at 15-second intervals. When power is applied to the circuit (by closing switch S2), the output of Ul at pin 3 is initially high.
Darkroom Timer Circuit - schematic

That high is applied to the base of transistor Ql (an MPS2907 general-purpose pnp device), keeping it turned off. That high is also applied to the anode of LED 1 (which is used as a power on indicator) through resistor~R7, turning it on. Timing capacitors Cl through C5 begin to charge through timing resistors Rl through R3. dc voltage is applied to BZl`s driver input through R5 and to its feedback terminal (through R4), which is also connected to Q2`s base terminal. The V+ voltage that applied to Q2`s base causes it to turn on, tying BZl`s common terminal high. When the timing capacitors are sufficiently charged, a trigger pulse is applied to pin 2 (the trigger input) of U2, causing Ul`s output to momentarily go low. This causes LED1 to go out and transistor Ql to turn on. That, in turn, grounds the common lead of buzzer BZ1, causing BZ1 to sound. Afterward, the output of Ul returns to the high state, turning off Ql, and turning on LED1, until another time interval has elapsed and the process is repeated. The circuit is powered by a 9-Vac adapter, which plugs into a standard 117-V household outlet. Because the circuit draws only about 10 to 15 mA, a 9-V alkaline transistor-radio-battery can also be used to power the circuit.

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