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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Tuesday, November 29, 2011 08:14 PM
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More Voltage to
Frequency Converters
Freespace Communications
VCO Circuits
Light Sensor Circuits |
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Light to Frequency Converter
designed by David A. Johnson, P.E. |
| This circuit uses a
CMOS version of the classic 555 timer, to form a light intensity to frequency
converter. A small PIN photo diode is used as the light detector. The
pulses produced are short, so in some applications you may want to stretch them or
feed them through a flip/flop to produce a square wave signal. Although the
circuit shown is designed for a 5v supply, it could operate from almost any
voltage from 3v to 15v. |
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| The
555 timer circuit is configured as a free running oscillator. When a PIN
photodiode is reversed biased, it leaks current proportional to the light intensity
hitting lt. The photodiode leakage current charges the 0.01uF capacitor.
When the voltage of the capacitor reaches about 2/3 of the supply voltage, the pin 3
output of the 555 timer swings low. This state quickly discharges the capacitor
through the photo diode, until the capacitor voltage is less than 1/3 of the supply
voltage. This causes the pin 3 output of the 555 to swing high again, for
another charge cycle. With the component value chosen, the frequency of the
oscillator will range from about 1Hz in total darkness to about 25KHz in sunlight.
Other frequencies are possible by changing the value of the 0.01uF capacitor. |
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Click on Drawing Below to view PDF version of Schematic |
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