Specifications

CIRCUIT
IDEAS
TELEPHONE RINGER
USING TIMER ICs
PRABHASH K.P.
RUPANJANA
for the ringing to start after the switch is
closed. The circuit used also has a provi-
sion 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.
U
sing modulated rectangular
waves of different time periods,
the circuit presented here pro-
duces ringing tones similar to those
produced by a telephone.
The circuit requires four astable
multivibrators for its working. There-
fore 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’ dura-
tion 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 col-
lector of transistor T2 to prevent ca-
pacitor C3 from fully discharging
when transistor T2 is conducting. Pre-
set 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 pre-
ceding multivibrator stages.
When switch S1 is closed, tran-
sistor 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
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