One of the most useful pieces of test equipment that you can own is a multimeter.

A multimeter is a device that can measure a variety of different electrical properties, including voltage, current, and resistance. Many multimeters also add other features, such as continuity testing, transistor testing, and measuring capacitance.

Early multimeters used an analog scale with a needle that moved across the scale, but almost all multimeters today are digital multimeters, sometimes referred to as a DMM. A standard hand-held DMM has three main areas:

Display. This is where you see the results of measurements.

Controls. Use these to select the mode and range.

Connectors. Plug in probes for connection to your circuit.

To connect your multimeter to a circuit, you'll need a pair of probes. Probes almost always come in black and red, to make it easy for you to see which one you're connecting. Other than the colour, the probes are exactly the same so it doesn't make any functional difference. It's just to save you getting confused about which one is which.

The "common" lead is the black lead. Multimeters have a single probe connector labelled "COM" or "COMMON", which is for the black lead.

Most multimeters then have multiple other probe connectors for the red lead, and you need to insert it into the correct connector depending on the type of measurement that you will be doing. 

For the most common types of measurement, including voltage, resistance, and low current, the red probe goes into a general purpose socket that will probably be labelled "V/R/mA" or similar.

For more unusual types of measurement, such as a high-current range, some meters have a special high-current probe socket that will be labelled for that purpose. When using that range, the red lead needs to be moved to the correct socket.

Continuity Testing

Continuity testing is the simplest possible thing that a multimeter can do, but it's also the most common use! All continuity testing does is check whether there is a direct electrical connection between two points.

Continuity test mode activates an audible beeper that will make a sound whenever there is a direct connection between the probes. If you touch the probes together, or put them at either end of a piece of wire, the multimeter will beep.

This is very handy when you're checking whether there is a short circuit between two parts of your project, or to make sure that a connector is working properly. You can keep your eyes on the test probes and just listen for the beep, so you don't even need to look at the multimeter.

To put your multimeter into continuity test mode, look for a symbol that looks like sound waves.

Voltage Measurement

Multimeters have two voltage measurement types: one for DC voltages, one for AC voltages. For hobby projects you will almost always use the DC range, which has a symbol like "V-". The straight line indicates DC, which is a flat voltage. The AC range has a symbol like "V~", which indicates a voltage that alternates between positive and negative.

Auto-ranging multimeters simply require you to select voltage mode, and they take care of the rest. However, many multimeters don't have auto-ranging. Instead, they have a number of different voltage ranges that you can select from. The voltage range is the maximum voltage that it will be able to measure and display, so you need to select a range that's larger than the voltage you expect to see.

For example, if you're working on a 5V circuit there's no point selecting the 2V range. Instead, you should select the 20V range since it's the next range up that's higher than 20V.

If you select a range that's too high or too low it doesn't matter: you won't damage the multimeter. If you select a range that's too low, such as measuring a 5V signal on the 2V range, the display will simply show that it's out of range. If you select a range that's too high, such as measuring a 5V signal on the 200V range, the multimeter will give you a reading but it will probably not have the same amount of accuracy and can't display it to the same number of decimal places.

Voltage is measured by placing the probes between the two points in the circuit where you expect to see a voltage difference. For example, you could put the common (black) probe on the GND (or 0V) header on an Arduino, and the red probe on the 3.3V header.

Voltage is a parallel measurement, which means it's very easy to take measurements without adjusting your circuit. The circuit needs to be powered as normal, since the multimeter can't supply power to the circuit and can only measure voltage that is already present.

If you connect the multimeter leads backwards, it won't damage the multimeter. It will just show a negative voltage reading instead, because a voltage reading is the difference between the two points being measured. If the red probe is connected to a higher voltage than the black probe, the reading is positive. If the red probe is connected to a low voltage than the black probe, the reading is negative.

Voltage measurements are all relative to the points being measured. If you put the black probe on the 3.3V header of an Arduino, and the red probe on the 5V header, you will see a reading of 1.7V because that's the difference between the two points.

Current Measurement

The current measurement range is usually marked "A" or "mA". Just like with voltage measurements, you may have to select a specific range that's high enough for the current you expect to measure unless your multimeter has auto-ranging.

While voltage is measured in parallel with an existing circuit, current can only be measured by being in series with the circuit. That means you need to put a break in your existing circuit at the point where you want to measure the current, and then connect your probes to either side of the break.

For example, if you wanted to measure how much current an Arduino is using, you could connect the GND (or negative) to your power source, but leave the positive side disconnected so current can't flow directly to the Arduino. Then, connect the red probe to the positive output of the power source, and the black lead to the positive input of the Arduino.

Now power can flow from the power supply, through your multimeter, and on to the Arduino. The Arduino will run as usual but your multimeter will be able to measure the current that's flowing and report it to you.

Just as with voltage measurements, you need to provide power to the circuit for current measurements.

Resistance Measurement

Resistance mode is usually labeled with an "R" or an Ω (omega) symbol.

Unlike voltage and current measurements, the circuit should not be powered when you perform resistance measurements. Resistance measurements are taken by connecting the probes at either end of the part of the circuit or the component that you want to measure.

Once again, select a range that's higher than the value you expect to see.