Light-Controlled Oscillator

Figure 1.  Light-Controlled Oscillator Circuit Diagram

This circuit is a light-controlled oscillator, i.e., an oscillator whose output frequency increases with the amount of light shining on it. The main components of this circuit are the 741, a general purpose operational amplifier IC, and the light-dependent resistor or photocell, which serves as the circuit's light collector.

The 741 in this circuit is configured as an oscillator driving a piezoelectric buzzer.  The output voltage of the 741 is used as input voltage to the inverting input, which forces the output of the 741 to go back to the opposite level every time it changes state, causing it to toggle between 'low' and 'high' continuously. 

When the output of the 741 becomes low, this is fed back to the inverting input, causing the 'high' voltage at the non-inverting input to 'dominate' and force the 741 output to go back to 'high'.  When the output of the 741 goes 'high',  this is again fed back to the non-inverting input, which drives the output to go low, and the cycle starts over again.

The rate at which the output oscillates depends on how fast capacitor C1 charges and discharges, which in turn depends on the resistance across the light-dependent resistor or photocell. The more light shining on the photocell, the lower is its resistance.  The lower the resistance, the faster capacitor C1 charges or discharges, and the higher is the frequency at which the 741's output oscillates.  This is why this circuit is a light-controlled oscillator - its frequency of oscillation increases as the light shining on it increases.