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Telephone
Ringer using Timer ICs
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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.
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| 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. |
