Telephone Ringer using 556 dual timers
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.
Title: Telephone Ringer using 556 dual timers
electronic circuit
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-09
Reads: 485
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