Telephone Ringer Using Timer ICs


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
two ringing tones are heard in the loudspeaker 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 connection 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.

Note.

To resemble the actual telephone ringing a 400 Hz tone is switched
on in the following sequence: 400ms on, 200ms off, 400ms on and 2000ms
off and then repeat.


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