Introduction: Electromagnetic Interference Detector #phablabs

About: PHABLABS 4.0 is based on combining the World of Photonics (Science of light and light technologies) with the growing creative ecosystem of existing Fab Labs. The “Photonics Workshops” introduce a variety of fu…

This is a DIY electronics skill-builder with a practical application—finding hidden smart phones and checking your microwave oven for leakages. Detecting microwaves from unknown sources.

Properties of this workshop:

Timeplanning: Total: 2,5h

Target audience: Students (15-18 years old)

Estimated cost: € 10

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Step 1: Electromagnetic Waves and Interference

Photo 1: Electromagnetic waves cover many orders of magnitude in wavelength, from a 108 m at a few Hertz (resonances of the Earth – ionospheric resonant cavity, the so-called Schumann resonances) to less than a femtometer at >1024 Hz (gamma rays, produced in radioactive decay, black holes, neutron stars and so on). In space and Earth’s atmosphere, all electromagnetic waves propagate at the same speed; 299,792,458 m/s, the highest possible speed for information. In media such glass, light speed diminishes by a factor given by their refractive index n (about 1.5 for glass, 1.33 for water).

In this workshop, we will build a little device which picks up electromagnetic waves in the GHz regime (1 billion oscillations per second). Here, the wavelengths range from 30 cm at 1 GHz to 3 cm at 10 GHz (note the inverse relationship: the faster a wave oscillates while propagating at light speed, the smaller its wavelength becomes).

Photo 2: Have you ever wondered why soap bubble shine in various colors? Why oil on the road has all the colors even though oil itself is colorless? Or why butterflies have all these beautiful shining colors that changes when you look at butterfly at different angle? Well, now you can find out.

First of all you have to know that white light consist of main seven colors: red, orange, yellow, green, blue, indigo and violet. We can break light by using triangular prism (see Picture B) – since every color is a different length electromagnetics wave it breaks in different angles and that’s why we can distinguish colors.

So if we want to found out why soap bubbles are changing colors, we have to find out some properties of waves.

Diffraction is a process that takes place when any wave on its way meets any obstacle. It is defined as the bending of light around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle. These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength.

Diffraction can be used to separate different wavelengths of light using a diffraction grating. A diffraction grating can be a series of closely-spaced slits or a mirror with a series of small grooves. Diffraction gratings work because different wavelengths of light will constructively interfere at different angles. Diffraction gratings are used in many analytical chemistry tools, such as a spectrometer.

If we have single slit, the light waves reaching a given point on the screen each arrive from a different part of the slit, so their amplitudes must be added, and an interference pattern results. Interference is a phenomenon that occurs when one wave comes into contact with another wave and they interact. Interference can be either constructive or destructive. For two waves of equal amplitude interfering constructively, the resulting amplitude is twice as large as the amplitude of an individual wave. When interference is destructive, the intensity will decrease, sometimes to a point where it is completely destroyed.

Waves passing through one of two long, narrow slits will diffract in passing through each slit as described above, but in addition there will be interference with the waves from the other slit. According to geometry, shown in Picture D, here is constructive interference causing intensity maxima at points on the screen for which

sinθ = nλ/d , n=0,1,2…, d – the distance, that slits are separated.

So how about the soap bubble – you ask?

The colors seen in a soap bubble arise from interference of light reflecting off the front and back surfaces of the thin soap film. Reflected light is coherent so the interference phenomenon occurs. Depending on the thickness of the film, different colors interfere constructively and destructively in different places. That’s way we see all these different colors by viewing in different angles. The same explanation can by applied for oil spill and etc.

Photo 3: All these light wave principles apply for other waves – sound, waves found at sea and etc., and can be used for many experiments and technologies. Earlier was mentioned that every color is a different length electromagnetic wave, but you should know that electromagnetic waves are not only visible light. Exist thing, called electromagnetic spectrum (see Picture J) which shows the variety of electromagnetic waves. From spectrum you can see that visible light is very narrow part of spectrum but wave principles (like diffraction, interference and etc.) are suitable for all electromagnetic waves.

Step 2: Part List

Photonics parts:

*2 green, blue or violet LED (photo 1)

Other parts:

*Breadboard 17x10 pins photo 2
*Capacitor 0,1 micro-Farad photo 3
*Resistors 0,25 W: 1x 47kOhm, 2x 100 kOhm and 1x 1MOhm photo 4
*Electrolytic capacitors: 1x2,2 micro-Farad, 1x4,7 micro-Farad, 1x22 micro-Farad and 1x100 micro-Farad photo 5
*Silver wire 0,2mm² photo 6
*Bell wire, 0,2mm², not braided photo 7
*LM358 (dual operational amplifier) photo 8
*9V battery + clip photo 9
*Loudspeaker, 45 Ohms photo 10
*switch photo 11

Tools (for example in Fab Labs):

*Laser cutter

Don't find the material you are looking for? Via this link you could buy all the photonics material needed for this workshop.

Step 3: How Does the EMI Detector Work?

Photo 1 is the circuit diagram for the device. You can skip this part if you like. The microwaves are caught by the connecting wires, they act as an antenna. The waves also travel towards the OP amp LM358, which contains a lot of transistors and therefore diodes to “rectify” those waves. This sounds complicated but basically means that the extremely fast oscillating microwaves are translated into something “slow” which can be measured by regular electronic components. The first OP amp amplifies the tiny signal between pins 2 and 3 by a factor of about a million and outputs it at pin 1. The amplification is determined by the resistor between pins 1 and 2. The “blue” circuit grabs that signal and feeds it to transistor BC547 which is amplifies the current for the loudspeaker. The BC547 Is one of the most wide spread multipurpose transistors of the past decades and can basically amplify anything for a just couple of cents. The second OP between pins 5,6,7 does the same as the transistor for the LEDs, which need less current than the loudspeaker. The green circuit shows this little circuit. Can you explain why we better use 2 antiparallel LEDs than a single LED? Find the solution in the last step.

Some more technical explanations:

The electrolytic capacitors at pins 2 and 3 stabilize the amplifier and suppress noise. The two 100 kΩ resistors together with the electrolytic cap on the lower right provide low noise, half the battery voltage for the lower OP, which is necessary because we use a single battery (for details check Wikipedia). The resistor defining the amplification is replaced by a wire, which means that the amplification is 1: the output voltage at pin 7 equals the input voltage at pin 5. But, now have much more current at our disposal (40 mA max.) which we use to power two LEDs.

We built this receiver on a breadboard of a size of at least 170 points (170 contacts where you plug a wire in). In Photo 2 you see the breadboard schematics indicating where to put the components.

The breadboard has letters and numbers on it which denote rows and columns. In each column, points A-E and also F-J are internally connected! This is indicated by the green pale circles shown in those columns which connect to a component.

Start with the LM358 (its notch has to be on the left), then continue with silver wire (shown in grey), the green and blue wires (it can be any color, whatever you have). All wires should be as short and as close to the breadboard surface as possible, except for the arced wire on the right, which connects the OP amps negative voltage supply input to that of the battery. Make it a bit longer than the other wires since it also acts as an antenna.

Then add those resistors (shown in red), their resistance is shown next to them; 47k = 47 kiloohms, 100k = 100 kiloohms, 1M = 1 Megohm and so on. Next, plug in the 0,1µF and then the electrolytic caps which look like cylinders. It’s important to put them in the right way, “minus” is the filled electrode, “plus” that without filling. All caps are drawn in blue. If a cap’s terminals are too short, insert it anyway and connect it to its destination point with a silver wire (keep in mind the connecting rails inside the breadboard!). Insert the transistor BC547, which is the amplifier powering the loudspeaker, and then the loudspeaker wires.

Now, take a quick break and drink some coffee. When you get back, carefully doublecheck your setup, especially the polarities of IC, transistor and electrolytic caps. When everything looks right, connect the battery. You should hear a muffled static noise which tells you the device is picking up electrons hitting metal cores in the wires and random noise from ambient electronics and radiation from space. If not, immediately disconnect the battery and search for the mistake. Otherwise, continue.

Connect the antiparallel connected LEDs with long enough insulated wires to the breadboard as shown.

Step 4: Housing

The enclosure for the electronics is made with a laser cutter from wood (files attached), but you can also make your own box. The material must be non-conductive though or the microwaves won’t reach the receiver inside.

Photo 1: The individual parts and how to assemble them is pretty much selfexplanatory. The ring holding the loudspeaker needs to be glued to the ground right below the sound outlet in the cover lid. The LEDs are glued to the Ø5mm holes (lower right) and the switch mounted into its place in the lid as well (upper right). Make the wires long enough so the lid opens easily. The breadboard has an adhesive tape on its back and is permanently placed with it.

Photo 2: Glue the back of the loudspeaker into the ring and connect it and the LEDs to the breadboard as before. Close the lid. Done!

Testing of the device

Grab you smartphone.

In ‘settings’, go to the SIM section and change mode to the old, but still widespread standard “2G”. Then hold your smartphone close to your EMI-receiver and call your own number. You should now hear your phone signal loud and clear in the speaker, and the LED should be flashing.

3G and 4G are much more difficult to detect because they are very efficient and broadband (rectified/demodulated signal not in the audible range). Nevertheless, you can still hear your smartphone make some grunting microwave noise here and there, even though it’s much less talkative than with 2G.

You can also check your microwave oven for leaks. Put a glass of water in it and turn it on full blast, so it’s on continuously. Active your EMI receiver and move its antenna close to the edges of the oven. When it’s an old cheap oven, you’ll hear a buzzing sound in your speaker and the LEDs flash. This signal changes with the plate rotating inside.

Step 5: End Result and Conclusions

Trouble shooting:

Should the receiver maks a buzzing noise and the LEDs are on all the time, the circuit oscillates which is bad. Make sure the caps are put in correctly, and repalce the 100µF with a larger cap it that helps. You can also email me (Frank) with your question.


Solution to the question in step 3 on why we use 2 antiparallel LED rather than a single LED: If we used just one LED, the capacitor would charge up without being able to discharge later one. This would basically be like a one-way-street in a dead end.

PHABLABS 4.0 is a European project where two major trends are combined into one powerful and ambitious innovation pathway for digitization of European industry: On the one hand the growing awareness of photonics as an important innovation driver and a key enabling technology towards a better society, and on the other hand the exploding network of vibrant Fab Labs where next-generation practical skills-based learning using KETs is core but where photonics is currently lacking.

This workshop was set up by the Joanneum Research in close collaboration with Fab Lab Graz.