light sensitive astable multivibrator

The circuit of an a table multi- vibrator using a Schmitt trigger and a single RC time constant has fre- quently been used in Elektor. Figure I shows how this circuit can be modified so that the frequency of oscillation is dependent upon the intensity of light faLUng on a pholotransistor. Assuming that Cl is initially uncharged, the output of SI will be high. I will now charge via R I, D2 and TI at a rate depending on the leakage current of T l, which is in turn dependent on the light level. When the voltage on C I has reached the upper threshold voltage of the Schmitt trigger the output of S 1 will go low and C I will now discharge through DI and R I until the lower threshold voltage is reached , when the output of SI will go high again and the cycle will repeat. With the component values shown the frequency will vary between about 5 kHz and I O kHz, depending on the light level incident on the phototransistor.