audio visual indicator for telephone circuit diagram


Audio Visual
Indicator for Telephones

Audio visual indicator for telephone circuit
Click here for the circuit diagram

Many
a times one needs an extra telephone ringer in an ad
joining room to know if there is an incoming call. For
example, if the telephone is installed in the drawing
room you may need an extra ringer in the bedroom. All
that needs to be done is to connect the given circuit
in parallel with the existing telephone lines using
twin flexible wires. This circuit does not require any
external power source for its operation. The section
comprising resistor R1 and diodes D5 and LED1 provides
a visual indication of the ring. Remaining part of the
circuit is the audio ringer based on IC1 (BA8204 or
ML8204). This integrated circuit, specially designed
for telecom application as bell sound generator, requires
very few external parts. It is readily available in
8-pin mini DIP pack.
Resistor R3 is used for bell
sensitivity adjustment. The bell frequency is controlled
by resistor R5 and capacitor C4, and the repeat frequency
is controlled by resistor R4 and capacitor C3. A little
experimentation with the various values of the resistors
and capacitors may be carried out to obtain desired
pleasing tone. Working of the circuit is quite simple.
The bell signal, approximately 75V AC, passes through
capacitor C1 and resistor R2 and appears across the
diode bridge comprising diodes D1 to D4. The rectified
DC output is smoothed by capacitor C2. The dual-tone
ring signal is output from pin 8 of IC1 and its volume
is adjusted by volume control VR1. Thereafter, it is
impressed on the piezo-ceramic sound generator.


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