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Telephone
Ringer Using 556 Dual Timers |
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Using modulated
rectangular waves of different time periods, The circuit presented
here produces ringing tones similar to those produced by a
telephone.
The circuit requires four astable multivibrators for its working.
Therefore two 556 ICs are used here. The IC 556 contains two
timers (similar to 555 ICs) in a single package. One can also
assemble this circuit using four separate 555 ICs. The first
multivibrator produces a rectangular waveform with 1-second ‘low’
duration and 2-second ‘high’ duration. This waveform is used to
control the next multivibrator that produces another rectangular
waveform.
A resistor R7 is used at the collector of transistor T2 to prevent
capacitor C3 from fully discharging when transistor T2 is
conducting. Preset VR1 must be set at such a value that the two
ringing tones are heard in one second. The remaining two
multivibrators are used to produce ringing tones corresponding to
the ringing pulses produced by the preceding multivibrator stages.
When switch S1 is closed, transistor T1 cuts off and thus the
first multivibrator starts generating pulses. If this switch is
placed in the power supply path, one has to wait for a longer time
for the ringing to start after the switch is closed. The circuit
used also has a provision for applying a drive voltage to the
circuit to start the ringing.
Note that the circuit is not meant for connecting to the telephone
lines. Using appropriate drive circuitry at the input (across
switch S1) one can use this circuit with intercoms, etc. Since
ringing pulses are generated within the circuit, only a constant
voltage is to be sent to the called party for ringing.
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Click
Here For Circuit Diagram
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