Transistor as an Automatic Switch - Light Controlled Switch

The LDR

1. A light-dependent resistor (LDR), or photoresistor, is a resistor sensitive to light.
2. In darkness, the LDR has a resistance about 1 million Ohm.
3. In bright light however, the resistance of the LDR falls to only a few hundred Ohms.

Light Operating Switch

1. In a light operating switch, we connect an LDR to the potential divider.
2. As a result, the voltage across the base vary according to the presence or absence of light.
3. Example 1 and 2 below shows how the resistance of the LDR, the base voltage, the base current and the collector current change in different conditions.

Example 1

Bright Surrounding: Resistance of LDR: Low Base voltage: High Base current: High Collector current: High Bulb: ON

Dark Surrounding Resistance of LDR: High Base voltage: Low Base current: Low Collector current: Low Bulb: OFF

Conclusion
The bulb will be switched on when the surrounding is bright and switched off when the surrounding is dark.

Example 2

Bright Surrounding: Resistance of LDR: Low Base voltage: Low Base current: Low Collector current: Low Bulb: OFF

Dark Surrounding Resistance of LDR: High Base voltage: High Base current: Low Collector current: High Base current: ON

Conclusion
The bulb will be switched on when the surrounding is dark and switched off when the surrounding is bright.