The circuit is a modification of one made by VK2ZAY, all credit goes to him for the excellent design:
Schematic:
Final assembly as hand-held tool:
Some notes on assembly:
I used metal can transistors (to reduce noise) since I had them on hand, but you can use any NPN with decent gain.
The original circuit used a 9V battery, but they're expensive, not rechargeable, and have pretty poor capacity. I replaced this with an 8.4V rechargeable pack made from a couple Li-ion cells from an old laptop.
I used a 33mH inductor instead of the original circuits' 5mH; this seems to increase range/sensitivity considerably. If you have an even higher value one available, it would certainly be worth trying.
I added a simple bargraph VU meter IC to give some visual indication of signal strength. This lets you see signals that are above the range of hearing. This can be any bargraph IC, does not need to be BA6144.
The variable resistor is used to set the bargraph meter sensitivity. You can use this to zero the meter in high noise environments.
Uses:
One major use of this circuit is in tracing AC wires behind walls. It helps to have something noisy, like a CFL bulb on the other end of the circuit.
It can also be used to detect EMI sources, both at a large and a small scale. For instance, finding which power adapter on a circuit is generating radio interference, or determining which components on a circuit board need shielding.
You can also detect fixed magnets. They will not produce a tone, but if you swipe the sensor passed them, you'll hear a blip of static.
Improvements:
There is no reverse polarity/short circuit/low voltage protection for the battery back. Since the Li-ion cells can easily deliver 10's of amps and destroy transistors, there should really be a fuse or current limiting circuit. The cells themselves can be damaged be running them down below 3V each, so there should be something like a TL431 low battery indicator.
High frequency signals are inaudible over the piezo speaker. One solution to this would be to build in a variable local oscillator/mixer before the audio amp.
