Cool EMF Detector

Introduction: Cool EMF Detector

About: Being an engineer, I just love to understand how things work. This website is a great opportunity for me to learn new stuff !

Wanna see what the eye can't see : live electricity ? Just try this !

This instructable gives you a way to detect if there is a live wire nearby without contact. I played with this because I want to be able to say if a wire is live without having to plug something on it (a little challenge as you will see).

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Step 1: EMF in a Pinch

EMF stands for ElectroMagnectic Field, I won't cover the topic of electromagnetism (the maths behind it is a pain in the neck), but just say that those are fields created by electrical charges (moving or not). Magnetic field is the one responsible for electric motors and aurora borealis. Its buddy is the electric field which makes dust stick to a balloon or hair standing up with static electricity, that's the one we are trying to catch !

This instructable is about a cool way to detect the low frequency electric field created by the live wires of your mains (50 or 60 Hz in general) even when no current is flowing. It won't work with static electricity or DC (like a battery). If that's what you were looking for, try one of the transistor variants listed at the end of this step.

A little hint about the physics behind : An electric field reaching a receiving antenna translates into voltage and voltage applied to an emitting antenna translate into an electric field (perfect symmetry). A live wire has an alternating voltage (even if the circuit is not closed !) so that the electric field will generate a similar alternating voltage on a receiving antenna which can be detected. Don't worry, those are really tiny fields, since a "poorly efficient" emitting antenna must have a length of at least tens of kilometers (the wavelength of a 50 Hz electric field is 6000 km). Actually, the USA (among others) used huge antennas to send messages to submarines because those fields can penetrate quite deeply into the water (look at this if you're curious).

Please note that the nice circuit used is NOT my invention and that I found it on the internet while looking for a way to detect the presence of live voltage on a wire without any contact (even if there is no current running).

Original websites :

Variants using NE555 or plain transistors :

Arduino variant :

Step 2: The Circuit

What you will need is :
1 CD4017 counter
1 100 ohms resistor (or bigger if voltage supply is over 5V)

1 0.1 uF capacitor (100 nF)

1 small wire with insulation (a few inches)
1 battery (3.7V Li ion or higher)

The CD4017 is a dirt cheap circuit that takes a clock input and gives out a high logical level on one of ten outputs pins (0 first, then 1, ..., 9, 0, ...).

What the circuit does is really amazingly simple : plug a short wire (antenna) on the CD4017 input and it will interpret the Electric Field variations as a clock. This means that if you get close to a live wire, you will detect a 50 (or 60) Hz AC voltage that will be interpreted as a ... clock input by the very sensitive CD4017. This will trigger, in a rolling sequence, the 10 outputs and let you count the cycles.

The other cool idea (not mine again !!) is to use only one input, it means that you will see only one out of 10 "clock signals" : the LED will blink at one tenth of your mains frequency : 5 or 6 Hz, slow enough to be seen by the eye.

Depending on your DC source remember to add a current limiting resistor (rule of thumb : 100 ohms for 5 V, 390 ohms for 9 V).

Step 3: The Results

Well, that circuit is really sensitive and for me it was more difficult to find places without signal than the opposite. The guys who played with it before say that you can adjust its sensitivity by using shorter or longer antenna wires.

You can easily detect a live wire 30 cm (1') away. I could tell if a DC power supply is a simple old fashioned transformer or a switching supply : I saw the LED blinking like mad which means that there is a way higher frequency (say 160 Hz since you can see a 16 Hz flicker). That's quite a mystery since I thought that those supplies had a much higher frequency, maybe that's just a parasitic harmonic ?

Step 4: Why Not Use a AC to DC Power Supply ?

Well, I like a little challenge, so I told myself : eh, why not just use that 9V power supply with a 5V breadboard converter and see what happens (noise I thought...).

Well, indeed lots of noise and not the kind that goes away with a capacitor between ground and +5V. It was just triggering all the time. I didn't lose hope and realized that the DC supply gave floating outputs and that somehow it might couples itself with the AC ambiant noise (ooops).

Well, just make sure that your supply is isolated (with a little transformer inside like mine) and then you can just connect your circuit ground to the mains earth and bingo works like a charm ! Isolated means that both outputs of your supply are not linked electrically to the mains, the voltage at both tips just "floats".

WARNING : If you are not sure, don't do it, you might just connect a live wire with earth, which will make a nice spark, burn your components and your power supply, hurt yourself and trigger your mains breaker.

OK, you think that's stupid because the circuit works on a battery and you don't need mains. Well, maybe for playing around and checking that the live wire is indeed the one controlled by your light switches (you might be surprised....), but I need it for a future project (approximately online here when my kids are 21 yo) for which I don't want a battery because it will be working all the time without interruption. My wife has indeed very demanding specs like : don't burn down the house, don't want to see it, don't want to know it exists, make it work and why are you not finished yet ? I should have read the small prints on the contracts, poor me (this is a joke, sorry sweetie, I'm so amazed you read this).

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    2 Discussions


    3 years ago

    Nice! Can this circuit detect lightening too?


    Reply 3 years ago

    I can't guarantee it, but it might. The lightning RF signal can be something like 300 kHz and it's as long as the lightning, so it's just a question of amplitude and interferences. Try for a specially designed circuit. If someone has the opportunity to try, let me know!