Transistors can be used as a Switch or Transducer Driver.
Many transducers need quite a lot of current to operate.
This transducer driver circuit amplifies the small base current from a low
power circuit and provides enough current to operate the transducer.
A small base current is used to switch on a much larger collector current.
If the load is inductive, the diode is needed to protect the transistor from "Back EMF" damage. It works by allowing the inductor current to die away gradually when the transistor is suddenly switched off.

• R1 limits the base current to a few milliamps
• Q1 is an NPN transistor. For example a BC182L with a current gain of about 300
• R2 is the load. This could be a bulb, an LED, a relay, a small motor, a buzzer and so on.
• D1 is only needed if the load is inductive. Inductive loads are electromagnetic devices such as motors, relays and some buzzers. When the current through an inductor is turned off suddenly, a large Back EMF is produced. This could be enough to destroy the transistor. By adding the diode, the current through the inductor can die away gradually (by flowing through the diode). This prevents the large back EMF.
• When the transistor is saturated (fully turned on), V_be = 0.7V (the base emitter voltage)
• When the transistor is saturated (fully turned on), V_ce = 0.2V (the collector emitter voltage)
• When the transistor is cut off (fully turned off), V_ce = V_s.

Alternative Circuits

Use a MOSFET. This switches faster. Bigger currents can be handled. A minute input power is needed to operate the switch. The MOSFET is voltage operated. This is often an advantage.

Use a relay. This is mechanical, slow and relatively unreliable.

Use a thyristor or a triac. These are not on the AQA specification so you will need to find the details elsewhere.