A resistor is used to restrict the flow of electric current in a circuit. It is not an electronic component, but is used in almost every electronic circuit. It changes the amount of current in that circuit. This is necessary because too much current will damage other components, for example, light-emitting diodes (LEDs), fixed to the circuit.

Resistor value

Resistors are rods made of carbon wrapped inside an insulating material (one that prevents the flow of electricity). They each have two terminals. They come in a range of different values measured in the unit of resistance: ohms. The higher the resistor value, the lower the flow of electricity. The coloured bands on the resistor tell you its value (see caption to illustration). High values are usual, so we use the letter k for kilo (thousand) and M for mega (million) in front of the symbol for ohms, Ω.

The voltage value we need is not simply the voltage of the battery. An LED, like all components, will take up a certain amount of voltage as the current runs through it. This is called the voltage drop. Check the spec sheet (datasheet) of your LED to find what its voltage drop is. Let's say this is 2 V. If we are using a 9 V battery to power the circuit, this means there will be a 7 V (subtracting 2 from 9) drop over the resistor.
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If too much current is going through your LED, it will burn out at once, or stop working after a while. If too little current is going through it, the LED will not light up. Again, the spec sheet will tell you what the appropriate current for it is. Let’s say the LED needs 20 mA (milliamps) or 0.02 A.

We can now use Ohm's Law to find the resistor value:
7 V ÷ 0.02 A = 350 Ω
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