Light to Frequency Converter

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.

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.

Click on Schematic below to view PDF version of this Circuit