Discovering Semiconductors

Discovering Semiconductors

Experiment E4.

Study the circuit diagram. It shows two separate circuits which each switch on an LED. The two circuits are linked with a diode D3. We will start by testing the circuits without the linking diode.

Wiring the circuit.....

Switch off the 5 volt supply then follow these instructions:-

1. If you are following on from experiment E3 remove the 100uF capacitor C1, the 470 ohm (was 10k) resistor R1, and the push button switch S2. If you are starting with a blank plugboard follow the wiring instructions 1, 4 & 5 for experiment E3.

2. Bend and trim the leads of a 1k resistor as shown in the first experiment. Fit R1 between I3 & I7.

3. Fit bare wire link from J11 to Z11.

4. Prepare a red LED as shown in the first experiment. Fit the LED D1 with the short lead in Y7 and the long lead in J7.

5. Examine the push button switch. The four leads are arranged in pairs, each pair coming out of opposite faces of the square body. Fit the push button switch with two leads from one of these faces in H9 & H11 and the other two leads from the other face in F9 & F11.

Testing the circuit.....

Switch on the 5 volt supply to the programmer module.

Press S1. LED D1 will glow. Release S1. Press S2. LED D2 will glow. Press both push buttons and both LEDs will glow. Press the buttons several times. Each circuit is totally independent.

6. Examine a 1N4148 diode. Notice that one end is marked with a black band. Bend and trim the diode leads with 0.3 inch spacing between the leads. Fit the diode with the black band end in J6 and the other end in J3.

7. Fit yellow link from H6 to J9.

Press S1 on its own and both LEDs will glow. Press S2 on its own and just D2 will glow.

Look at the circuit. A diode is a one way device (a semiconductor). It allows current to flow from the unmarked end to the black band end but not the other way round. When S1 is pressed the diode allows current to pass though it because the current flows the correct direction. When S2 is pressed the diode blocks the flow of current because it is trying to flow the wrong direction.

Reversing the diode.....

Leave the 5 volt supply to the plugboard turned on.

8. Unplug the diode and fit it the other way round so that the black band end is in J3 and the other end in J6.

This time S2 will switch on both LEDs and S1 will only switch on D1. The current is now only allowed to flow the opposite direction.

Think about it.....

A diode is a one way device. It allows current to flow one way and not the other. It is what we call a semiconductor.

Maybe that idea will not excite you just yet if you are new to electronics but as time goes on you will realise that every aspect of modern electronics relies on semiconductors. A diode is the simplest semiconductor and has countless uses. Amongst other things it is the device which makes simple radio reception possible.

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