Circuit basics for beginners: voltage, current and resistance

Before you build a robot that moves or a smart device that responds to its surroundings, you need to understand the language electronics speaks: voltage, current and resistance. Many beginners skip this stage and start connecting wires as shown in pictures, then are puzzled when an LED burns out or a sensor does not work. In this article we explain the basic ideas with simple comparisons, learn Ohm’s law and apply it to a real example, compare series and parallel connections, meet the breadboard and the multimeter, and finish with safety rules and exercises that fix the ideas in place.

The water comparison: the easiest way to understand electricity

Electricity cannot be seen, so it helps to compare it with water flowing through pipes. The comparison is not accurate in every detail, but it is excellent for a start:

• Voltage is like the water pressure in a tank. The higher the pressure, the more strongly the water can push. It is measured in volts (V). An Arduino’s USB supply gives 5 volts, and an ordinary AA battery about 1.5 volts.
• Current is like the amount of water passing through the pipe each second. It is measured in amperes (A), and in beginner projects we usually deal with small fractions called milliamperes (mA); one ampere equals a thousand milliamperes.
• Resistance is like the narrowness of the pipe. A narrow pipe reduces the amount of water that gets through. It is measured in ohms (Ω).

For water to flow, the path must be closed: from the tank through the pipes and back to the tank. The same goes for electricity: current only flows in a closed circuit that starts at the positive terminal of the power source and returns to its negative terminal, which on the Arduino we call GND, or ground.

Ohm’s law: the relationship that ties it all together

In the nineteenth century the German physicist Georg Ohm described a simple relationship between the three, which has become the most important equation you will use in electronics:

In practice: if you keep the voltage the same and increase the resistance, the current falls. And if you want a particular current from a known voltage, you can calculate the resistance you need precisely.

A practical example: why do we put a resistor with an LED?

An LED does not limit its own current. If you connect it straight to a 5-volt pin, it will try to draw a large current that damages it, and it may damage the board’s pin too. The fix is a resistor in series. We calculate it as in the diagram: a red LED uses about 2 volts (an approximate value that varies with colour and type, so check the LED’s datasheet if you have one), leaving 3 volts across the resistor. If we want a safe current of 15 milliamps, the resistance = 3 ÷ 0.015 = 200 ohms. Resistors are sold in standard values, so we choose the nearest higher one, which is 220 ohms. That is why you will find this value in almost every beginner tutorial.

Electrical power: why do parts get hot?

There is a second useful equation: power = voltage × current, measured in watts (W). Power tells you how much energy is turned into heat, light or motion each second. In our example the resistor has 3 volts across it and 0.015 amps through it, so its power is 0.045 watts. Common small resistors are rated for a quarter of a watt, so it is safe here by a wide margin. But if you design a circuit that loads a resistor beyond its rating, it will heat up and may burn out. That is why we always calculate power in circuits carrying larger currents, such as motor circuits.

Series and parallel

When you use more than one part in a circuit, there are two basic ways to connect them:

The breadboard: the playing field you build on

A breadboard is a plastic board full of holes, with metal strips underneath connecting some holes together, so you can build and change circuits without soldering. The most important rule: in the middle section, each group of five holes in one short row is connected together, and the channel down the middle separates the two sides. Along both edges run two long power rails, usually marked + and −, connected along the length of the board; we use them to distribute 5 volts and ground. Understanding this rule solves half of all “my circuit doesn’t work” problems.

The multimeter: your eyes in the world of electricity

A multimeter is a cheap, essential tool that measures voltage, current and resistance, and it has a “continuity” mode that beeps when two points are connected. Use it to check a resistor’s value before fitting it, or to find out whether voltage reaches a particular point in your circuit. An important warning: to measure current, the meter must be placed in series in the circuit; putting it by mistake straight across a power source while it is set to current can damage it or blow its fuse.

The following video from Afrotechmods is a quick, practical introduction to Arduino and circuits:

Reading resistor colours

Small resistors do not have their value printed in numbers but in coloured bands. Each colour stands for a digit, and on four-band resistors the third band sets the number of zeros. For example, a 220-ohm resistor is red, red, brown, followed by a gold band that shows the tolerance. At first you do not need to memorise every colour; use a printed chart or the multimeter to check.

Safety rules

• Always work with low voltage: a USB port or small batteries. Stay away from mains electricity.
• Disconnect the power before changing any connection.
• Never connect positive straight to negative without a load; that is a “short circuit”, which can heat the wires and damage parts.
• If a part feels hot or you smell burning, disconnect the power at once and check the circuit.
• Mind the orientation of polarised parts such as LEDs: the longer leg is usually the positive one.

Exercises to practise

1. Calculate the right resistor for a green LED of about 2.2 volts, with a 5-volt supply and a current of 15 milliamps.
2. Two resistors of 100 and 220 ohms are in series: what is the total resistance?
3. On the breadboard, build a circuit of two LEDs in parallel, each with its own resistor, then disconnect one: what happens to the other?

With these basics, you are ready to write your first Arduino program. If you have not met the board yet, start with what is Arduino. In the robotics and embedded systems course at Badr Academy Education we apply these ideas to real circuits under the instructor’s supervision. For further resources visit the learning library, and try the Find your path tool.