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. |
