Light to Frequency converter circuit

Here is the schema diagram of a effective light to frequency converter schema that can be used for variety of applications such as light intensity measurement,fun etc.






The schema is based on TLC555, the CMOS version of famous timer IC NE 555. A photo diode is used for sensing the ligt intensity.The timer IC is wired in astable mode.The leakage current of the reverse biased photo diode is proportional to the light intensity falling on it.This leakage current charges the capacitance C1.When the capacitor voltage reaches 2/3 of the supply voltage the out put (pin 3) goes low.As a result the capacitor discharges through photo diode .When the capacitor voltage reaches 1/3 the supply voltage the out put (pin 3) of IC goes high.This cycling continues and we get a frequency at pin 3 proportional to the light intensity falling on the photo diode.


Notes.

* With the given components the frequency varies from 1KHZ @ complete darkness to 24 Khz @ bright sunlight.The frequency range can be changed by using different values for C1.

* Use any general purpose photo diode for D1.