Audio Visual Indicator for Telephones
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.
Title: Audio Visual Indicator for Telephones
electronic circuit
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-01
Reads: 712
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