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Acompact, inexpensive and
low component count telecom head- set can be constructed using two readily available
transistors and a few other electronic components. This circuit is very useful
for hands-free operation of EPABX and pager communication. Since the circuit
draws very little current, it is ideal for parallel operation with electronic
telephone set. Working of the circuit is simple and straightforward. Resistor
R1 and an ordinary neon glow- lamp forms a complete visual ringer circuit. This
simple arrangement does not require a DC blocking capacitor because, under idle
conditions, the telephone line voltage is insufficient to ionise the neon gas
and thus the lamp does not light. Only when the ring signal is being received,
it flashes at the ringing rate to indicate an incoming call. The bridge rectifier
using diodes D1 through D4 acts as a polarity guard which protects the electronic
circuit from any changes in the telephone line polarity. Zener diode D5 at the
output of this bridge rectifier is used for additional circuit protection. Section
comprising transistor T1, resistors R2, R3 and zener diode D6 forms a constant
voltage regulator that provides a low voltage output of about 5 volts. Dial
tone and speech signals from exchange are coupled to the receiving sound amplifier
stage built around transistors T2 and related parts, i.e. resistors R7, R6 and
capacitor C5. Amplified signals from collector of transistor T2 are connected
to dynamic receiver RT-200 (used as earpiece) via capacitor C7. A condenser
microphone, connected as shown in the circuit, is used as transmitter. Audio
signals developed across the microphone are coupled to the base of transistor
T1 via capacitor C3. Resistor R4 determines the DC bias required for the microphone.
After amplification by transistor T1, the audio signals are coupled to the telephone
lines via the diode bridge. The whole circuit can be wired on a very small PCB
and housed in a medium size headphone, as shown in the illustration. For better
results at low line currents, value of resistor R2 may be reduced after testing |
