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Bredhurst Receiving and Transmitting Society

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Part 2

Practical 6.

Demonstrate that a transistor can be used as a switch in a simple DC circuit.

This can be as easy or as in depth as you the student would like. Your lead instructor should accommodate your wishes.

The actual task is merely to demonstrate that a transistor can be used as a switch in a simple DC circuit.

So you can :

  • either use the circuit that the Lead instructor will have pre-prepared and follow their instructions

  • or build up the circuit your self and demonstrate it. Information on the circuit is given below.

Another little circuit where you will need to demonstrate that you understand the function of the transistor when it is acting as a switch.

Earlier in the course you will have seen the section on transistors and their uses - click here for a re-cap.

In the section were diagrams similar to these:-

Close switch and LED will light

Close switch and LED will go out

This assignment is to ensure that you know the function of the NPN transistor in relation to the base emitter and collector leads when using it as a switch.

To be able to do this you also have to be able to identify which leg of the transistor, an NPN type BC337, is the Base, the Emitter and collector.

For the purpose of the course the emitter leg has been coloured white with a white blob on the side. Such marking are not usually put on transistors so reference to technical data sheet is needed when using a transistor of a different type to check the leg configuration.

When looking at a data sheet it is usual to see a diagram showing in UNDER side of the transistor where the legs enter the body and an indication of the E B and C legs.

Note that there is a flat side to the transistor and a rounded side. This ensures that you can orientate the transistor properly to identify the legs.

You can chose either of the configurations, one will switch on the LED when the switch is closed and the other will turn it off.

Be careful with the choice of R2 as it is the current limiter for the LED - too much current and you will blow the LED too little and you will not see it light. About 10mA to 20 mA would be a reasonable amount of current to flow through the LED so R2 should be 47R to 100R.

With regards to R1 and R3 this is a potential divider to provide the turning on current when R1 is linked to the positive supply and held what is called "low" or to the negative potential level by R3 to ensure the transistor stays turned off when R1 is not connected. Suggested values of R1 is 1k and for R3 10k

This circuit can also be built on the experimental board of previous assignments.

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