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Many
a times one needs an ex- tra 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 telec- om 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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