Electrical circuit finders are small plastic devices that can peer into walls and find open and live electrical circuits. There is a subtle difference between circuit breaker finders and circuit tracers. In very simple terms, professional electricians use tracers while finders by the layman. Tracers can locate wires on both live and open circuits, but the finders can work only on live or energized circuits.
Certain circuit tracers can follow circuits that are live 4 to 5 feet away. Circuit tracers are very sensitive and can, therefore, detect both strong and weak signals. These tools use many different types of techniques to find an electrical circuit, programmed to look for two different types of signals. To detect a complete circuit, it senses an electromagnetic field, but for an open circuit, an electrostatic field.
The various techniques
One technique used for live circuits is to inject a signal burst at the transmission and then control it on the AC waveform. The other one is meant for dead circuits where you have pulses at regular intervals. Picking the right modulation and frequency is a bit tricky, as you need to define a thing that not many things should hinder with. For example, there are too many sources of EM noise in the background, like computers. This means finders and tracers have to be advanced enough to filter these as well.
Whichever technique is used by the electric circuit finders and tracers; the basic principle remains similar. Depending on your requirements, you can choose between the circuit tracer or finder.