DARKROOM TIMER

Many times when when we do experiments in Dark room we use stop watch but
due to no light we can't see the time properly that's why we used
digital with led light stop watch or timer or mobile timer . Instead
of using we present you a special timer i.e stop watch for dark room
The timer circuit described here provides a pleasant
musical tone in your darkroom at 1-second intervals. The circuit takes
up very little space and can be easily converted into a metronome.
Unijunction transistor (UJT) T1 functioning as a relaxation oscillator
triggers the phase-shift audio oscillator circuit built around
transistor T2, turning it on and off. As capacitor C1 is charged
through preset VR1 and resistor R1, the emitter voltage of UJT rises
toward the supply voltage.

When the emitter voltage becomes sufficiently positive,
the emitter becomes forward biased and discharges capacitor C1 through
the emitter-base 1 (B1) junction and resistor R2. The voltage drop
across R2 forward biases transistor T2 and turns it on. As capacitor
C1 becomes discharged, the current through resistor R2 drops and
transistor T2 is cut off. A tone signal is generated by transistor T2
and R-C coupled phase-shift oscillator. Part of the signal taken from
the collector of tran sistor T2 is coupled to a small speaker through
a transistor-radio type output transformer. The 22-kilo-ohm value of
resistor R3 represents a compromise between tone duration and
intensity. You can use resistors having a value anywhere between 10
kilo-ohms and 25 kilo-ohms for different durations and intensities of
the output signals.
Since the unijunction transistor is functioning as the
oscillator trigger, changing the values of one or more components in
the UJT circuit will change the rate of the tone burst. The tone
frequency can be varied by changing the value of any or more of
capacitors C2 through C4 and resistors R5 and R6 in the phase-shift
network. The primary winding of transformer X1 can be tuned for a
slight increase in the output, using capacitor values between 0.05 and
0.25 μF for C5 by trial-and-error method. Tone pulses should begin
about ten seconds after the unit is turned on. After a minute or so,
adjust preset VR1 for 1-second beats by comparing the timing of the
beats with the seconds needle on your wristwatch

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