Meet the components

Open an old radio or look at the circuit board of a broken toy: you will see dozens of small components with colourful rings, black caps and thin legs. Each one has a clear job. The black caps, for example, are capacitors, they store electric charge like a tiny battery. From physics you already know the circuit and Ohm's law $U = R \cdot I$. Here we start with the three components you need most often: resistor, diode and LED.

The resistor: a brake for the current

The resistor is the simplest component: it slows the current down to a desired value. Its value is measured in ohms and is printed on the body as a colour code made of rings. A resistor has no direction, you can fit it either way round. With Ohm's law you work out how much current flows at a given voltage. Rearranged for the current it reads $I = U / R$: at 100 ohms and 5 V that is 5 divided by 100, so 0.05 A flows.

Diode and LED: current in one direction only

A diode is a one way sign for current: it lets current through in one direction and blocks it in the other. The LED is a light emitting diode, a diode that gives off light as it conducts. That is why an LED only lights up when you connect it the right way round. The longer leg belongs on the plus side. And very important: an LED must never go straight onto the battery. It barely limits the current itself and would burn out. It always needs a series resistor that limits the current to roughly 20 mA, which is 0.02 A. The LED itself takes a fixed voltage of about 2 V, and all the rest of the battery voltage drops across the series resistor.

Reading circuit symbols

So that circuits do not have to be drawn as pictures, there are circuit diagrams with standardised symbols. The resistor is a plain rectangle, in many American diagrams a zigzag line instead. The diode is a triangle with a bar, and the triangle points in the direction the current may flow. The LED adds two small arrows that indicate the emitted light. The battery is a long line for plus and a short line for minus. Once you know these symbols, you can read any simple circuit diagram like a map.

The breadboard: building without soldering

You do not need a soldering iron to experiment. A breadboard has many holes that are invisibly connected in short rows. If you push two legs into the same row, the components are connected. The groove down the middle separates the two halves, nothing is connected across it. Long rails for plus and minus run along the edges. This lets you build a circuit in minutes, test it and simply replug it if something is not right.