Command Center

So what is voltage anyhow? Well, its a pretty abstract term but a lot of people like to use the term "potential energy" which is that thing you heard about in high school physics and then forgot immediately.
Some people like to draw an analogy to water to describe voltage. A water pump is like a voltage supply (also known as a battery).
The pump pushes water through a hydraulic system, and the voltage supply pushes electrons through an electronic system.
The higher the rated pressure of the pump, the more 'work' the water can do.
Likewise, the higher the voltage the more 'work' (Watts) the electrons can do.
Voltage is used to provide power (via a battery or wall plug) and its also used as a way of transmitting data. For example, music is recorded from a microphone as an analog voltage signal, if that voltage waveform is applied to a speaker the voltage performs the work of making air move and produces sound.
Voltage is also used to in digital circuits to talk back and forth in binary, usually 5V or 3.3V is a "1" and 0V is a "0", by alternating the 1's and 0's millions of times a second, data can be moved around rather quickly.

Not just an 80's hair metal band! Voltage comes in two flavors (yum): Alternating Current (AC) and Direct Current (DC). Here is a quick tour of the differences.
Direct current voltage is what comes out of batteries. The battery is at 9V, and it pretty much keeps that voltage constant, until it dies. The chemical reactions inside the battery creates DC voltage.
Electronic circuits really like DC voltage.
Alternating current voltage is what comes out of the wall. The generator at the US power plant creates a voltage that oscillates, going from -120V to 0 to +120V to 0 again, 60 times a second. At the European power plant its -240V to +240V at 50 times a second. (Note that those voltages are 'RMS' - Root Mean Square - which means that the peak voltage is actually about 1.4x higher, but since multimeters show RMS voltages, its easier to just use those)
AC voltage is great for power plants because its easy to transform AC voltages (using a transformer) up to 50KV for long distance travel and then down to 240V or 120V to safely power your home. Those big honking grey things that you see next to buildings that hum are the huge transformers.
Motors (like your washing machine and refrigerator compressor pump) like running off of AC voltage.
You can turn AC voltage into DC voltage very easily by using a very small transformer to bring the 120V down to a reasonable level like say 16VAC and rectifier. This is basically what's inside a wall wart plug or your laptop power supply.
Its much harder to turn DC into AC, you will need an inverter which are more expensive than transformers/rectifiers.
Batteries only supply DC voltage and wall plugs only supply AC voltage. However, it is totally possible to have both AC and DC voltage at a certain point:
If an AC voltage is oscillating between -60V and +60V it has 120V AC and 0V DC because the average voltage of -60V and +60V is 0V.
If an AC voltage is oscilating between 0V and 120V then it has 120V AC and 60V DC because the average voltage of 0V and 120V is 60V.

Voltage testing is very common, you'll use it a lot

• Test if your power supply is working, are you getting 5V out of that 7805 regulator?
• Verify that your circuit is getting enough power: when all of the blinky lights are on, is the power supply drooping too low?
• Verify signals to and from chips to make sure they are what you expect once the circuit is up and running
• Testing batteries, solar cells, wall plugs, and power outlets (carefully!)
• With a current sense resistor you can perform current testing on a project without possibly damaging your meter.

You can only test voltage when the ciruit is powered If there is no voltage coming in (power supply) then there will be no voltage in the circuit to test! It must be plugged in (even if it doesn't seem to be working)
Voltage is always measured between two points There is no way to measure voltage with only one probe, it is like trying to check continuity with only one probe. You must have two probes in the circuit. If you are told to test at a point or read the voltage at this or that location what it really means is that you should put the negative (reference, ground, black) probe at ground (which you must determine by a schematic or somewhere else in the instructions) and the positive (red) probe at the point you would like to measure.
If you're getting odd readings, use a reference voltage (even a 9V battery is a reasonable one) to check your voltage readings. Old meter batteries and wonky meters are the bane of your existence but they will eventually strike! Good places to take reference voltages are regulated wall plugs such as those for cell phones. Two meters might also be good :)
Voltage is directional If you measure a battery with the red/positive probe on the black/negative contact and the black probe on the positive contact you will read a negative voltage. If you are reading a negative voltage in your ciruit and you're nearly positive (ha!) that this cannot be, then make sure you are putting the black probe on the reference voltage (usually ground)
DC voltage and AC voltage are very different Make sure you are testing the right kind of voltage. This may require pressing a mode button or changing the dial.
Unless otherwise indicated, assume DC voltages

Multimeters have different input impedences that affect readings of high impedence circuits For example, measuring a sensor that has 1Mohm impedence with a 1Mohm impedence meter will give you only half the correct reading

There are often two seperate modes for AC and DC voltage. Both will have a V but one will have two lines, one dashed and one solid (DC) and one with have a wave next to it (AC).
When the probes are not connected to anything, they should display 0V. They might flicker a bit if they pick up ambient voltage (your home is a big radiator of 60Hz voltage which can couple into your meter probes).

Testing batteries is a super useful skill and is one of the best ways to practice with your multimeter
The first battery we'll test is a new 1.5V alkaline. This one is a AAA but a AA, C or D cell will be the same voltage. Set the range to 2V DC .
Next, we measure a 9V alkaline battery. If we still have the range set to 2VDC we will get a mysterious "1. " display, indicating is it over-range.
For this new battery we get 9.6V. Remember that battery voltage is nominal, which means that the "9V" is just the average voltage of the battery. In reality, it starts out as high as 9.5V and then drops down to 9 and then slowly drifts to 7V. You can check out the discharge curve in the Duracell 9V datasheet
If we want to check a rechargeable AA battery, and it's set to a 20VDC range, we will read 1.3V, which is about what a fully charged NiMH battery will measure.

Testing wall adapters is also very handy, especially when you build your own circuits.
The first kind we will test is a transformer-based adapter.
Note that the label says Transformer, its also blocky and heavy which indicates a transformer as well. It requires 120VAC input, US power only. The nominal output is 9VDC at 300mA. The polarity symbol shows that the middle is positive, the outside is negative, thus we place the ground (black) probe on the outside and the positive (red) probe on the inside.

Lastly, we'll test a 9VAC adaptor, which outputs AC voltage instead of DC. Basically this means that there's still a transformer inside, but no rectifier. This is also an unregulated supply

This is the 'easiest' test, just shove the two probes into a wall socket. If you're clumsy and think you'll somehow electrocute yourself, don't do this. Many people freak out about this test, but ironically it's what the multimeter was designed to do.