There’s something fascinating about electromagnetic fields. Thanks to the modern world and the prevalence of electronics and electricity, they’re all around us these days. But because of the extremely limited array of senses that we humans have, we spend most of the time completely oblivious of them. Wouldn’t it be cool to make something simple that could not just detect them, but would allow you look at the waveforms on an oscilloscope. An EMF probe in other words.
Some time ago as I was googling about EMF detection and the applications of such ventures, I came upon a link to a paper about “Contactless Sensing of Appliance State Transitions Through Variations in Electromagnetic Fields”. It was an interesting article and they used a pretty simple circuit based on an instrumentation amplifier. I decided to try something similar.
I already had some instrumentation amplifiers, some TI-manufactured INA122s. I get them as free samples from TI every once in a while. The inductor is just some random inductor I happened to have. It’s inductance is probably somewhere between 100uH and 1mH. I did one prototype on some veroboard by deadbugging the instrumentation amplifier, but I wanted something a bit more permanent. My university has a circuit mill that I can (ab)use, so I designed and made a simple board.
As you can see, the circuit is very simple. We have power input and a power switch (and the switch is on the low side because it was easier to route that way). The singlesided voltage supply is divided with a resistor divider to provide a virtual ground in the middle. Then there’s the instrumentation amplifier that amplifies the voltage across the sensing inductor. There’s also a RC low pass filter, but it’s probably useless. I just added it in there so I’d have some space on the PCB for a lowpass filter or a highpass filter if it seemed like I’d need it. Then the output is routed out to a BNC connector. All you need to do is attach a BNC cable between the EMF probe and an oscillocope, and you’re ready to do spend an evening waving that thing towards anything that includes electronics.
There’s a lot more stuff that I experimented with. CRT monitors were especially intriguing. The fields were strong and you could actually see the waveform change depending on what was on the screen in a predictable way.