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Electronic Projects: Transistor

This project introduces the Transistor - the building block of almost all modern electronics. To make them work in your own circuit, you need to buy them along with some other basic parts such as batteries and leads to join everything up. Below is a list of the parts and a brief description of what they do.

1. 9V PP3 type battery. An alkaline one works best, but any will do.

2. Crocodile test leads for joining the circuit together. About 10 leads should be enough for most experiments.

3. Two Blue high quality LEDs. I say high quality to mean high brightness at a low current. With this type of LED you can make it light up even with a very very small amount of electricity. See the LED project. DO NOT CONNECT AN LED DIRECTLY ACROSS THE BATTERY. YOU MUST USE FINGERS OR THE CIRCUIT BOARD AS SHOWN IN THE DIAGRAMS, OTHERWISE YOU MIGHT BREAK THE LED.

4. A 6B pencil. It needs to be a very high B number, that is very soft, to make it easy to make dark thick lines.

5. Some cardboard - about 1mm thick. Can be any type really, but we will connect the crocodile leads to it, so a thickish type is easiest to use. 50mm x 50mm square is enough.

6. A general purpose small signal transistor such as a BC108.

The transistor being used has three terminals called:

  • base
  • collector
  • emitter

These name are not important right now, but they do mean that each terminal is special in its own specific way. If you connect them the wrong way round, they will not work. If your circuit does not work, then most likely it is not connected correctly.

The circuit below uses a transistor to make the small amount of electricity flowing through you enough to make another LED very bright instead of only dim. There are two parts to the circuit.The first part - cicuit 1 - consists of you, the base, collector and first LED. This is similar to the LED project circuit where you could make an LED light with just a smallamount of electricity. Now we are using that small amount of electricity to make the second LED in circuit 2 very bright. The second part of the circuit consists of the collector, emitter, and second LED.  

The leads on the transistor must be connected correctly. The picture show the transistor from the bottom where the leads come out from.



Which LED is brighter? What is the transistor doing?



Transistors can be considered as switches or amplifiers or multipliers.

  • Multipliers are probably the easiest word for most people to understand. The number of electrons flowing through the base is amplified or multiplied by the gain of the transistor. This result is then how many electrons will flow through the collector and the emitter
  • The gain of a transistor is a characteristic of each transistor, based on its fabrication style and parameters. The BC108 transistor being used has a gain of about 300. So for every electron that flow through the base, about 300 will flow though the collector/emitter part of the circuit.
  • In the transistor circuits, you can see that the transistor has amplified the number of electrons flowing through your fingers or PCB, because of the difference in the brightness of the two LEDs in the circuit.
Example of multiplication of electrons in the circuit – for every electron flowing through the Base, 300 or so will be flowing through the Collector/Emitter.



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