Crocodile Clips - Tutorial 2

Transistors, Capacitors and Diodes

In this tutorial, you will learn about Transistors, capacitors and diodes. All three enable you to make more advanced circuits, and they are the most advanced components available in CC before you start using ICs.

Diodes are small electronic components which only enable electricity to flow in one direction. This is very useful in many circuits, such as the one below:

If you push the left-hand PUSH switch, then both LEDs will light, but if you push the right hand PUSH switch, then only the right-hand LED will light. This is because the diode stops the current from flowing in the opposite direction and lighting the first LED.

Diodes can be found in the "Discrete Semi-conductors" section. Make sure that you get the correct one, the arrow points in the direction of the flow of the current.

Capacitors can be treated like small batteries. They can charge up with electricity, and then discharge it before restarting that sequence again. You can see that they are charging by adding a Voltmeter around the capaciter. A voltmeter measures the voltage across two set points, you can find out more about them here.

As the capacitor charges, the voltmeter will slowly rise until it reaches a value of about that of the power supply (8.96v in this circuit). As it does so, the capaciter will be charging, and that can be seen by the slow animation of a series of red "+" and blue "-" signs. To increase the speed of the filling up, decrease the size of either the capaciter or resistor.

Finally, I come to transistors. These components are like sensitive switches which active when they have a certian voltage flowing into them. A simple transistor circuit would look like this:

Transistors are the first component that you have come across which have more than 2 legs. The top leg is the collector, the middle the base and the lower the emitter. When the voltage at the base reaches a certian level (usually about 0.6v), the transistor allows current to flow into the collector and out of the emitter. In this circuit above, the base has 0v whilst the switch is off, so the LED does not light. When the switch turns on, a voltage greater than 0.6v flows to the base, and the transistor allows current to flow through the collector and out of the emitter, lighting the LED.

A more advanced transistor circuit is shown below:

This time, the circuit is triggered by an LDR. When the LDR is receiving a lot of light, the current passes through it and not the 1k resistor, so bypassing the base of the transistor. However, when the LDR receives only a little light, the resistance is high, so the current diverts through the 1k resistor, activating the transistor at the base and turning on the LED.

Now you should be familiar with the fundamental, discrete components of electronics. In the next tutorial, you can find out about Thyristors and 555 Intergrated Circuits.


1. Which LED will light in this circuit?

2. What would you use to see a capacitor filling up?

3. Which leg on a transistor is in the base, the top, middle or bottom leg?

4. Design a circuit in which a capacitor charges up before switching on an LED. You will probably need to use a transistor.