Circuit : Andy Collinson
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Description
An electromagnetic field probe designed to detect changing electric and magnetic fields. The probe has a meter output and headphone socket as well. Circuit Notes
This tester is designed to locate stray electromagnetic (EM) fields. It will easily detect both audio and RF signals up to frequencies of around 100kHz.Note, however that this circuit is NOT a metal detector, but will detect metal wiring if it conducting ac current. Frequency response is from 50Hz to about 10kHz gain being rolled off by the 150p capacitor, the gain of the op-amp and input capacitance of the probe cable. Stereo headphones may be used to monitor audio frequencies at the socket, SK1.

Probe Construction
I used a radial type 1mH inductor with 50cm of screened cable threaded through a pen tube. The cable may be used with a plug and socket if desired. The value is not critical and 1.5mH or 3.5mH will also work well. The Multicomp MCSCH895 Series sold by CPC Farnell will work well. My original probe is shown below:

The handle is from on old ball point pen, and the inductor is glued or taped to the pen handle. If you cant find a suitable inductor then wind about 50 turns of 0.5mm insulated copper wire around a 3mm drill bit. Remove the drill bit and secure the turns with candle wax. This will make the probe.

Meter Circuit
The output signal from the op-amp is an ac voltage at the frequency of the electro-magnetic field. This voltage is further amplified by the BC109C transistor, before being full wave rectified and fed to the meter circuit. The meter is a small dc panel meter with a FSD of 250uA. Rectification takes place via the diodes, meter and capacitor.

The meter used in the circuit was from Maplin LB80B

Testing
If you have access to an audio signal generator you can apply an audio signal to the windings of a small transformer. This will set up an electromagnetic field which will be easily detected by the probe. Without a signal generator, just place the probe near a power supply, mains wiring or other electrical device. There will be a deflection on the meter and sound in the headphones if the frequency is below 15KHz.

In Use
Switch on, plug in headphones (optional) and move the probe around. The LF351 op-amp can drive low medium to high impedance headphones 32 Ohm and higher, but they will shunt the meter circuit and reduce its sensitivity. Any electrical equipment should produce a hum and indicate on the meter. I remember once building a high gain preamp (for audio use). I made a power supply in the same enclosure. The preamp worked, but suffered from an awful mains hum. This was not directly from ripple on the power supply as it was regulated and well smoothed. What I had done was built the audio circuit on a small piece of veroboard, and placed it within a distance that was less than the diameter of the transformer. The transformers own electromagnetic field was responsible for the induced noise and hum. I should however note, that this was when I was new to electronics with very little practical experience. You can now buy toroidal transformers which have a much reduced hum field.