Transistor as an Automatic Switch

The Transistor as an Automatic Switch

1. Transistor can be used as automatic switches.
   
2. 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 become large.
3. We have learned that in a transistor, the collector current is controlled by the base current, or the base voltage.
4. The greater the base voltage is, the greater the base current, and hence the greater the collector current.
5. Therefore the bulb can be switched on and off by varying the voltage supplied to the base.
6. The voltage across the base can be controlled a potential divider.
7. According to the potential divider rule, the voltages across the resistor R₁ and R₂ are given by the following equations:

$$V_1=\frac{R_1}{R_1+R_2}\times V$$ and $$V_2=\frac{R_2}{R_1+R_2}\times V$$

8. 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.