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ThijsBeckers

Tapir - Ultrasensitive wideband magnetic/electromagnetic field detector

Status: Finished
3043
6
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April 15, 2016
tapir.jpg
Why build an electrosmog (‘E-smog’) detector, which the TAPIR basically is? The answer is quite simple. More and more of our everyday objects are based on some sort of electrical ‘core’: your toothbrush, camera, cellphone, TV set, and so on. Each and every one of these devices generates electrical radiation in some way. There are of course rules manufacturers should abide by, but that doesn’t mean that devices are completely free from electrosmog. In fact, even with the widespread ‘CE’ certification stamped onto your device, it is not certain that a device complies with all the rules and doesn’t interfere with other electronic devices. Ever tried calling someone (or, when receiving an incoming call) holding your mobile phone close to a cheap alarm clock radio, or a set of low-end PC speakers? (your guitar amp probably loves those cellphones too…)

An E-smog detector is designed to detect the ‘radiant misbehaviour’ of nearby electronics. The TAPIR — short for Totally Archaic but Practical Interceptor of Radiation — is a simple design capable of detecting, and audibly pinpoint, any source of electric or — with the appropriate antenna — magnetic field. Its application area extends from home use (“Where can I sit without being microwaved?”), to practical use (“Where’s that Wi-Fi-antenna aimed at?”), to professional use (“Who the devil is thwarting me?! I’m trying to do some sensitive measurements here!”). It’s even suitable as a first SMD soldering project — with your (grand)child — since it’s so easy (and fun!) to assemble.

Different fields
TAPIR is able to detect electric as well as magnetic fields of high frequencies. Magnetic fields are mostly generated by transformers and loop antennas, while electric fields are emitted by high voltage transmission lines, EL backlights and old mopeds passing by. Electromagnetic fields are a combination of both fields, mostly occurring in the ‘far field’ at a larger distance from the generating object.

Two different antennas can be connected to TAPIR, each optimised for one type of field. Magnetic fields are detected with a ferrite cored coil, while electric fields are detected with a rod antenna, which can be constructed very easy from a piece of installation wire.

How does it work?
The schematics show the simplicity of the design and are very similar to an Elektor circuit published in 2005. Basically it consists of a three-stage low-frequency amplifier with high gain. There is no low-pass filter in the circuit, consequently high frequencies are passed on to the gain stages. This way the non-linear characteristics of the transistors have a demodulating effect on these high frequency signals, so they can also be heard via headphones.

The bias point of the gain stages is automatically adjusted via a DC feedback path from the output through R4 and R1. To suppress the AC component C3 is added, which shorts this part of the signal to ground.
The output voltage level has an offset of about 0.7 volts, hence C4 is added to remove this offset and protect any connected headphones (or other devices).

The total gain is high enough to be able to ‘hear’ the intrinsic noise of transistor T1, so it’s best to pick a low noise transistor for this. We went for the BSR17A, which has far better noise figures at high frequencies than a BC847B. Signals of mere microvolts are audible via headphones connected to the output.

The whole circuit starts to operate from 1.2 to 1.5 V, so a single AAA cell can be used as its power source. The low supply voltage also acts as a kind of limiter; even if strong signals drive the amplifier into saturation, the output levels and thus the headphone levels never become excessive.

Don’t flee 4 the SM-Dee
This E-smog detector is available as a low-cost kit from our Elektor Store with the PCB and all the components included (except the batteries). Worried about soldering those tiny SMD components? No need! Even though there are some components with the ‘0805’ shape, it can all be done with your standard tools, provided you have a reasonably small soldering tip at hand for your soldering iron, and a pair of precision tweezers.

An extensive construction manual is available for (free) download. Follow its instructions and you'll find this project is an fun build with a fun end result!

TAPIR in use
Using the TAPIR is dead easy. Connect the headphones and an antenna and switch it on. Move it around any electrical device and you’ll hear different noises with each device, depending on the type and frequency of the emitted field. Give these a try: a TFT PC display, a cellphone, an iPad or e-reader, a fluorescent tube lamp or any energy saving lamp, a fridge, a microwave oven, a light dimmer, a PC, a laptop, a (switch-mode) wall wart, a (wireless) router or access point, a Wi-Fi hotspot, et cetera (and then test them all again using the other antenna with different results). Don’t be surprised to find your battery charger sounding like someone blowing a whistle, or your telephone tap dancing through TAPIR. LC displays in particular (actually the circuits controlling them) produce interesting sounds. There’s a video available on Elektor’s YouTube channel where we demonstrate various noises (fields) generated by devices used daily.

Take a stroll down the High Street and marvel at the levels of e-smog present there. Switch-mode power supplies, neon lighting, routers, repeaters, GSM/3G/4G antennas, police officers, automated ticket dispensers and vending machines all emit their own characteristic bleeps, buzzes and whistles. You can also use TAPIR for listening in on the inductive loop transmission system frequently present in museums and other public places.

It’s actually quite fun to have access to a sixth and seventh set of senses. But it also makes one aware of a world our own senses cannot detect. And what goes on in this world might not be as nice as you’d hope it would be.
We would like to thank all contributing partners for making this project possible:
  • Design: Museum Jan Corver [4] and YiG Engineering;
  • Original design: Burkhard Kainka.
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