Light and Dark Dependent Circuir

This is the circuit of a simple buzzer
that is activated by darkness, the buzzer is off when theres light and
on when it is dark. A general-purpose operational amplifier, the 741, is
used as a comparator that determines whether it is dark enough to turn on a self-oscillating piezoelectric buzzer.

Its inverting input is connected to a photoresistor,
a component whose resistance decreases as more light shines on it.  Its
non-inverting input, on the other hand, is connected to an almost fixed
voltage, i.e., a proportion of the supply voltage as set by timmer
resistor R2. 

 

Figure 1.  Darkness-Activated Buzzer Circuit Diagram.

            If there is ample light shining on the photo-resistor, the buzzer is quiet. As less light shines on the photo-resistor,
its resistance increases and causes the voltage across R1 to decrease. 
At a certain level of lighting, the voltage across R1, which is also
the voltage at the inverting input of the 741, becomes smaller than the
voltage at the non-inverting input. At this point, the 741 is triggered
to output a high level, turning on Q1.  Q1 then activates the
self-oscillating piezoelectric buzzer.