Having tried many of the Mains Detectors on the market, to find they are not sensitive enough, this project was designed with a number of improvements.

The sensitivity is very good due to the long probe and the output is a buzzer, giving a "chirping sound" to coincide with the 50 or 60Hz mains.
It is much better to have a sound output so you can keep your eyes on the job.
The buzzer is NOT a piezo diaphragm but a mechanical buzzer with internal coil and transistor, to produce a self-contained oscillator. It just needs DC to activate.
This buzzer is very loud on 9v.
A MAINS TRACER is essential when trying to find a cable.
It works on cables in ceilings and walls. It will chirp when the light-switch is turned-on and pick up the AC in a power cable at all times. The probe must run parallel to the cable to pick up the electromagnetic radiation and this allows you to accurately detect the location of the cable.

CONSTRUCTION
All the parts fit neatly on the PC board.
The IC fits into a socket with the cut-out shown on the overlay.
The placement of the transistor is also shown.
The buzzer must be connected with the positive on the buzzer going to the hole with the "+" sign. The diode must be fitted with the line on the body down the correct hole and the LED has a shorter lead (cathode) down the hole marked with the arrow-head on the symbol. 

The most crucial item to connect is the battery snap. The "cup" on a 9v battery is the negative, but we are making a RECEIVING snap, and the "cup" is the positive. You must look at the diagram to see how this is connected. 

MODIFICATION
200Meg resistors are not very common and it is necessary to make a value as close as possible to this value by using a number of 10 Meg surface mount resistors.

Here is the way to place 17 x 10Meg surface mount resistors on the board:

The land on the top of the board in the position shown in the diagram above must be totally removed and any traces of the plate-through hole material must also be removed from the top surface so the surface-mount resistors do not contact the 0v rail at this location.
Start at one end of the board and place a resistor. "Butt-joint" the next resistor to it so that as many resistors as possible can be placed along the board. We are aiming for 200Meg or as near as possible.
The value of this resistor is very important as the input impedance of the chip is very high and anything less than 100M will load the input and reduce the sensitivity of the probe.
The 470R resistor for the LED can be increased to 1k as we are supplying a high brightness LED in the kit.

14-5-2014