Light to Frequency Converter Circuit

    Here is the circuit diagram of a effective light to frequency converter circuit that can
be used for variety of applications such as light intensity measurement,fun etc.
The circuit is based on TLC555, the CMOS version of famous timer IC NE 555. A photo
diode is used for sensing the light 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

source: http://www.circuitstoday.com