One of the most important parts of any piece of Electrical or Electronic work is knowing how you are going to power it.
In the relative comfort of an Electronics Lab, variable power supplies are common place but if you are tinkering at home, you may find that a Battery of Cells is just as useful (it is important to remember that a Cell only becomes a Battery when two or more are connected together).
For the purposes of what we are doing in this booklet we will concentrate on Direct Current (DC) power but be aware that we can also use Alternating Current (AC) power sources and/or waveform generators as input sources for our work. The difference between them is that AC current reverses its direction whilst going around a circuit, while DC current does not. DC power comes from a source such as a battery whereas AC comes directly from a mains supply.
DC is safer to use in circuits such as the ones we are building here. You may wish to research AC and DC power in more detail.