A transistor can be used as automatic switches.
In the diagram above, the bulb is off when the collector current is off or very small. It is switched on when the collector current becomes large.
We have learned that in a transistor, the collector current is controlled by the base current or the base voltage.
The greater the base voltage is, the greater the base current, and hence the greater the collector current.
Therefore the bulb can be switched on and off by varying the voltage supplied to the base.
The voltage across the base can be controlled a potential divider.
According to the potential divider rule, the voltages across the resistor R₁ and R₂ are given by the following equations:
Therefore, by varying the resistance of R₁ and R₂, we can control the voltage across the base V2, and hence switch the bulb on and off.

Light Controlled Switch

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

In a light operating switch, we connect an LDR to the potential divider.
As a result, the voltage across the base varies according to the presence or absence of light.
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.

Heat Controlled Switch

In a heat operated switch, the LDR is replaced by a thermistor.
A thermistor is a resistor which its resistance changes as the temperature changes.
There are 2 types of thermistor:

For the PTC thermistor, the resistance of the thermistor increases as the temperature increases whereas for the NTC thermistor, the resistance of the thermistor decreases as the temperature increases.
In SPM, we assume all the thermistor used is the NTC thermistor, unless it is stated otherwise.

The circuit of a heat operated switch is similar to the light operated switch, except that the LDR is replaced by an NTC thermistor.
If heat is applied to the thermistor, its resistance drops. As a result, the base voltage will increase and the transistor is switched on and the bulb lights.

The figure above shows the circuit design of a sound controlled switch.
The microphone is used to convert sound to electric current.
The variable resistor is adjusted as such that the transistor is switched on when sound is detected by the microphone.
The function of the capacitor is to prevent the direct current from the cell to flow in the base circuit.