This circuit could function as a simple alarm (light activated circuit). Without feedback, a 741 op-amp acts as a comparator. The input voltage (pin 2) is compared with a reference voltage (pin 3). You can see that there are two voltage dividers here (100K resistor + 4.7K resistor and 100K resistor and LDR). The sensitivity of the circuit is controlled by the 4.7K resistor that connects to pin 3. When light shines onto the LDR, its resistance falls. The voltage applied to pin 3 will be higher than the voltage at pin 2. The buzzer will sound. The buzzer should turn off when light is blocked.
The resistance of an LDR is high in the dark and at low light levels. In this dark activated circuit, the LED lights up when no light is present. When this occurs, the voltage at pin 3 is higher than the voltage at pin 2.
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