Design & Technology Resources
A transistor is a component that can be used to amplify or switch electrical currents. They are the basis of all modern electronics, a computer will contain literally millions of them (just very small!)
Some Ics (chips) like PICs, 555 timers and logic gates can only supply a small amount of current. This current is enough to power an LED or a small speaker but not much else.
We can use a transistor to interface (connect) a low power circuit to a high power circuit. In this case the transistor is being used as an electronic switch.
This is how a transistor works...
• Normally no current flows through the lamp.
• If the input is connected to, say 5V, a tiny current flows into the base of the transistor. The resistor is needed to stop too much current flowing into the transistor which would otherwise damage it.
• The current flowing into the base switches the transistor on allowing a much larger current to flow through the lamp, into the collector and out through the emitter. The current flowing through the lamp is much larger than what was put into the base. The transistor is therefore amplifying the current.
Small current flowing into the base
What we can use them for
The circuit to the right shows how a transistor can be connected to the output of a circuit, in this case a 555 astable, to allow it to control a large lamp.
A Darlington transistor is effectively two transistors in one.
A normal transistor amplifies current but a Darlington transistor amplifies the current twice which means that it is a very sensitive device. It can amplify extremely small currents into larger ones.
Darlington transistors work as shown below:
A touch sensitive light circuit that will not work
A finger has a very high resistance which means that very little current can flow though it from a 9V battery.
The circuit on the left that doesn’t have a Darlington transistor would not work because not enough current can flow through the finger to make the lamp light up.
A touch sensitive light circuit that will work
• The circuit on the left however shows a circuit that will work correctly.
• Even though a very tiny current will flow through the finger, the Darlington transistor can make the lamp light up.
• The Darlington amplifies the tiny current flowing through the finger by many times allowing the current flowing through the lamp to be much higher. The lamp will therefore light up.
Transistors (NPN Bipolar)
Field Effect Transistors (FETS)