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FM
Transmitter For Channel Changer
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is the circuit of an FM transmitter which can be used
in conjunction with my circuit ‘TV Channel Selection Using
Remote Control’ published under Circuit Ideas section
in Feb. ’98 issue of EFY. The transmitter is capable of
being tuned to any frequency within 88-108 MHz FM band.
The circuit consists of two main sections: 1. Low frequency
AF oscillator 2. VHF oscillator. The AF oscillator configured
around transistor T1 utilises secondary of a 3V transistor
radio’s driver transformer as collector load. The positive
feedback is applied to base of transistor T1 via capacitor
C1. The same output is also coupled to VHF oscillator
section via capacitor C2. The audio oscillator output
causes frequency modulation of the VHF oscillator frequency.
The frequency modulated (FM) output from the tank circuit
of transistor T2 is coupled to a short (15cm to 30cm)
wire antenna via capacitor C9. Any readymade FM receiver
kit can be easily tuned to the transmitter frequency.
Alternately, FM carrier frequency of the transmitter can
be changed with the help of trimmer capacitor C3 to align
itself to the FM receiver’s frequency. For use with the
TV channel changer circuit published in EFY Feb. ’98,
the transmitter will transmit short duration FM signal
when push-to-on switch S1 is momentarily depressed. The
signal will be received by the FM receiver forming a part
of the TV channel changer circuit. Selection of the desired
channel is already explained in the above-mentioned circuit
idea published in EFY Feb.’98. |
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