Position hidden behind a wall gas or water pipes will easily be able to detect the proposed detector. Resistor RV1 achieve when establishing generation mode just above the threshold of self-excitation (with LEDs LD1 barely lit).
When approaching the coils L1L2 to the metal tube, the generation fails, the transistor TR2 prescribed, a TR3 fully open, causing an increase the brightness of the led. Resistor RV2 is possible to adjust the sensitivity device (detection range).
Coils L1 and L2 wound on a ferrite rod (with a magnetic permeability 400...600) with diameter of 8 mm and a length of 50 mm wire PEL of 0.3. L1 contains 120 turns L2 coil 43, the installation must comply with those specified on the circuit pattern the connection of the beginning and end of both windings.
Consumed from the battery type "Crown" current not exceeding 30 mA.
Author: John Mosley