Other DIY Testers
Active vs Passive Testers
An "active" tester is one that generally has its own battery or other power source, and it provides some kind of electrical pulse or current to the circuit being tested. "Active" testers are generally used when a device is not powered on. Care must be taken in selecting what parts of a circuit are being tested so that power from the tester itself doesn't damage any sensitive electronic components on the device being tested. Active testers are therefore widely useful for testing physical electrical devices and not necessarily sensitive electronic devices such as IC chips. A simple way to think of an active tester is one that PUSHES electricity into the device being tested.
A "passive" tester is one that generally gets its power or signal from the device being tested, which means the device being tested must be powered on. As such, great care must be taken so that short circuits aren't created, which could not only be dangerous to the user, but damaging to both the device and the tester. An example of a passive tester might be an oscilloscope or volt meter (DVM/VOM). A simple way to think of a passive tester is one that PULLS electricity from the device being tested.
Simple circuit continuity tester
You can build a simple circuit continuity tester with just a few parts. This is an active-type tester and would allow testing a cut or broken extension cord to discover which wire connects to the wide terminal (common) and which wire is the narrow terminal (hot) at the plug end. (Of course, disconnect the extension cord from the wall socket first!). You could also use it to test the ends of a communication cable to discover which wire connects to which pin. If there's "continuity" (i.e. "a connection") then the LED would light. If not, the LED would remain dark. A continuity tester can also be used to check various electrical hardware components such as switches, physical connectors, plugs and jacks, etc. However, you can't use a continuity tester on everything. For example, don't use it to check a PIC chip, transistor, diode or other sensitive electronic part because they can be damaged by the tester's voltage.
Simple passive tester
Here's an example of a simple passive tester you can make. Notice the difference from the active tester? Yes, no battery! This is just a simple tester than can tell you if voltage is present or not; it won't tell you what the voltage is. It can be handy to keep in your pocket if you're out in the yard and a controller isn't working and your voltmeter isn't readily available -- there are some circuits on the controller that you can test with this little gizmo.
Also note that the test leads are marked as either negative or positive, because an LED will only light when connected the proper way. Also notice that no value has been specified for the resistor. Here's a guideline: for low voltages (3-5 volts) you can get by with a 330-680 ohm resistor. For higher voltages (6-15 volts) you might use a 750-1000 (1K) ohm resistor; in the 16-24 volt range, try a 1200-1800 ohm resistor (1.2k to 1.8k). Once you determine what the range of voltage might be, connecting the test leads across the ground (negative) and the +v will light the LED. CAUTION: This is not a tester for checking A/C power from your wall socket!
For example, the logic circuitry on most any controller generally uses 5 volts DC power, such as the controller's processor chip, the voltage regulator, etc. By touching the negative lead to what should be a ground pin on the chip and the + lead to the chip's power pin, the LED will light if there is 5v power there. There are many other places where you can safely test a controller with this, and as you gain experience, you'll really understand how convenient simple little tools like this can be.