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The light-to-frequency converter has a photodiode whose output drives an current controlled oscillator (similar to a VCO). The frequency is therefore proportional to the intensity of the light falling on the photodiode. As the light level increases the frequency increases.
Light to frequency converter is a circuit which senses the light and produces frequency proportional to the light intensity. As the light level increases the frequency increases.
This circuit 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.
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.