The simple transistor based non contact AC mains tester circuit will allow you to test the presence of mains AC hum from a distance of about 3 inches. The LED and an optional buzzer will provide the required mains phase presence.
How the circuit Works
As we can see in the above figure, 3 transistors are connected in a cascaded manner such that emitter of the first conects with the base of the second transistor and subsequently, the second transistor's emitter joins the base of the third.
This configuration enables the circuit to attain a gain of about a million hFe.
This happens because the transistors are configured in the popular Darlingtom mode, however here we have 3 BJTs tied up in the instaed of two which is normally seen in conventional Darlington pairs.
Connecting 3 BJTs in the Darlington form causes the wiring tio become extremely sensitive and thus enables it detect even the smallest of RF disturbances that may be present in the atmosphere.
The proposed non contact mains AC phase tester or detector circuit can be seen powered with a 6V battery, which could be formed using a couple of 3V button cells in series.
However even a single 3V button would be able to operate the circuit as effectively.
This entire contactless wiring tester circuit or contactless mains phase tester circuit could be housed inside an empty screwdriver device and used for locating fault in house wiring without having to practically touch the wiring or without the need of opening the wiring. The detection of the broken wiring or faulty wiring could be done simply by hovering the sensor point close to the wiring lines and locating the missing or broken wiring points.
The piezo can be also seen in the circuit, this is in the form of a 27mm piezo element connected parallel with the LED...the piezo will begin producing a mild sound whenever a AC mains is detected along with the LED which will become illuminated.