EMI problems can be a real headache to debug. If you need to prove what’s causing your WiFi to slow down or your digital TV signal to drop, the ability to measure electromagnetic fields (EMF) can be a big help. Professional equipment is often very expensive, but building an EMF detector yourself is not even that difficult: just take a look at the Arduino expert [Mirko Pavleski]Handy portable electromagnetic field detector.
The basic idea is quite simple: connect an antenna directly to the analog input of an Arduino and visualize the signal it measures. Since the input of an ADC is high impedance, it is very sensitive to stray currents picked up by the antenna. So sensitive in fact, that a few mega-Ohms resistance to ground is needed to prevent the sensor from triggering on any type of random noise. [Mirko] made this resistor adjustable with a few knobs and switches so the detector can be used in both quiet and noisy environments.
Getting the whole device to work reliably was an interesting exercise in electromagnetic engineering: in early iterations, the detector triggered its own LEDs and buzzer, trapping itself in an endless loop. [Mirko] solved this problem by enclosing the Arduino in a closed, grounded metal box with only the required wires sticking out. The antenna design was largely based on trial and error; the current setup with a piece of 7cm x 3cm sheet aluminum seemed to work well.
While not a professional-grade calibrated instrument, it should be useful for finding sources of interference, or even just locating hidden power cables. You can view this as a more advanced version of [Mirko]Junk Box EMF Detector; if you have a second Arduino lying around, you can use that one to generate interference instead.