Finding problems in electrical circuits can be a difficult and tedious task. Using a multimeter around the electrical circuit is the best method, but if you don't have a multimeter you can still test the electrical circuit by building your own electrical circuit tester. The chances are you have the few items needed to build the tester in your garage.
Check the voltage of the electric circuit you need to test. This determines the type of lightbulb you need for building your electric circuit tester. For example, if you want to test a 12-volt circuit you need a 12-volt lightbulb, and if the circuit's 24 volts you need a 24-volt lightbulb.
Cut two strips of AWG 16-gauge wire using a small knife. Cut each strip about 12 inches long.
Use a small knife or strippers to remove 1/4 inch of outer plastic off one end of both strips of wire. Remove 1 inch of outer plastic from the other end of both wires. Twist the exposed metal wire so the strands stay together.
Attach the two wire ends that you removed ¼ inch of plastic from to the terminals on the bulb holder. It doesn't matter which wire attaches to which terminal. Loosen the terminal screws using a screwdriver, then insert the wires under the screws and retighten.
Insert your lightbulb into the bulb holder. To test a circuit, simply attach the ends of the two wires you removed 1 inch of plastic from onto the terminals or wires you're testing. If the lightbulb illuminates, the circuit is fine. It the lightbulb doesn't illuminate, then there's a problem.