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Handy
Telephone Receiver
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interesting circuit for a me- dium performance handy elec-
tronic telephone receiver, suited for receiving incoming
calls, is described here. This circuit can be connected
to almost all types of telephone exchange lines. To simplify
the gadget, the dialler section has been omitted. The
circuit can be constructed on a medium size veroboard.
Wiring and components layout are not very critical. For
compactness, enclose the wired circuit in a plastic cabinet
as shown in Fig.(a) here. In order to prevent undesired
acoustic feedback, it is necessary to adjust the orientation
of ear-piece (LS) and mouth-piece (MIC) at the final stage
of construction. The circuit consists of four sections:
ringer, voltage regulator, transmitter and receiver. The
ringer section is built around capacitor C1, resistor
R1 and electronic buzzer BZ1. No readymade ringer ICs
are used here, deliberately, to minimise the space. Transistor
T1 is the main interface transistor. Transistor T2, in
conjuction with resistor R6 and zener diode D6, works
as a constant voltage regulator which provides an output
of 6V DC. This supply is used to bias the remaining circuit.
Capacitor C2 is used as the filter capacitor. Receiver
section is wired around transistors T3 and T4 which form
a high-gain direct coupled amplifier. Voice input from
the collector of transistor T1 is connected to the base
of transistor T3 via capacitor C3 and preset VR1 and capacitor
C4. Preset VR1 is for receiver level adjustment. A small
loudspeaker is used as the ear-piece. Circuit comprising
transistor T5, condenser mic and associated components
functions as the transmitter circuit. Output signals from
this section are applied to the base of transistor T1
via capacitor C7 and preset VR2. Preset VR2 is for transmit
level control. Microphone muting is provided by push-to-off
switch S2. Initially, set the presets VR1 and VR2 in their
respective mechanical middle positions and readjust both
to get the best performance using trial and error method.
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