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The circuit can be
used as data transmitter and receiver system. Frequency
Modulation is used and the center frequency is 50Hz.
Transmitter
The pulse repetition
rate is relatively insensitive to temperature and power supply
voltage and is a linear function of Vin the modulating voltage.
Useful information transfer was obtained in free air ranges of
12 feet (4m). Lenses or reflectors at the light emitter and
detector increases range and minimizes stray light noise
effects. Greater range can also be obtained by using a higher
power output IRLED such as the F5D1 in combination with the
L14P2 phototransistor. Average power consumption of the
transmitter circuit is less than 3 watts.
Receiver
For maximum range, the receiver must be designed in the same
manner as a radio receiver front end, since the received signals
will be similar in both frequency component and in amplitude of
the photodiode current. The major constraint on the receiver
performance is signal to noise ratio, followed by e.m.
shielding, stability, bias points, parts layout, etc. These
become significant details in the final design. This receiver
circuit consists of a L14G2 detector, two stages of gain, and a
FM demodulator which is the tachometer circuit, modified to
operate up to 100 kHz. Better sensitivity can be obtained using
more stages of stabilized gain with AGC, lower cost and
sensitivity may be obtained by using an H23A1 emitter-detector
pair and/or by eliminating amplifier stages. For some
applications, additional filtering of the output voltage may be
desired.
Schematic
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