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Telephone
Headgear |
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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
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Click
Here For Circuit Diagram
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