Sunday 26 May 2019


Here is an easy to build EM Field detector with 4 stage LED strength indication and which has a wide range of applications. This circuit will detect electromagnetic fields and also static electricity. It detects the mains hum on a 240 Volt (or 110V) wall socket or cable without having to touch the object. It is enormously sensitive to any changes of the EM field surrounding it so it could be used to detect lightning (proof is in the video below) or maybe even ghosts. (No video proof of that alas! At least not yet  ^__^ )

Here is the circuit (click on image to display full screen):

(Last revised: 02-june-2020: Changed 1M potmeter for 20 to 50K potmeter.)

Parts list:

8 x BC547 

1 x 680 Ohm 
4 x 470 Ohm
1 x 220 Ohm 
1 x 4K7 
1 x 3K3 
2 x 2K2 
1 x 100K
1 x 1M 

1 x 20K or 50K potmeter (use either a trimpotmeter or a panel potmeter if you're building this into a case.) 

LEDs (3mm):
3 x green, 1 x yellow, 1 x red

1 x 1N4148

9V battery clip, 1 switch (SPDT Toggle Switch ON-ON), 1 Bullet conncector for antenna. (optional)

Before I go on with the rest of the explanation, here's a video showing this EMF Detector in action in a lightning storm. In the background audio you can hear the crackle of the lightning on an AM radio I had switched on, and you can see that the meter lights up as the radio crackles and lightning occurs. Sometimes it even detects the build up of the electric field in the air before lightning happens. I'm not influencing the meter in any way. I'm just holding it by the 9 volt battery underneath. Here it is:

I designed this because I always found it a shame that these "everything detectors" or 8 Million times amplifiers never had a strength indicator so you could actually see if and how it's working. So I tried combining two pre-existing circuits and see if I could make them work together and it turned out to work very well. The first of these circuits is this 4 LED signal strength indicator

and the second is this circuit which is the actual detector stage, consisting of the 8 million times amplifier.

You can easily build this on a piece of stripboard.
The circuit needs only 8 transistors (BC547 or 2N3904), 5 LEDs and 11 resistors. The extra (5th) LED is there simply to function as a on/off indicator and could be left out if you so wish. I used 3mm LEDs on this project but 5mm will work too. Don't use LEDs that draw a lot of current though like bright white LEDs or blue LEDs. The circuit is fed from a normal 9 Volt battery.

The sensitivity of the circuit can be changed with the 20K or 50K potmeter. If you're using it like me, without a case, you can use a trim-potmeter. If you're building this into a little case then use a panel potmeter for sensitivity. Make sure there's a grounding point when you build it into a case. Some connector from where you can ground it.

The circuit is very sensitive and it reacts to all sorts of things. If you hold this EMF Detector  near any mains cables it will instantly detect the voltage, I noticed that if you hold it near metal it will detect that too and even in an open space it will sometimes indicate a field even if there's nothing visible there but it's not a malfunction because it will keep indicating on the same spot in the room. 
This meter works best if it is grounded properly, either by connecting minus to a metal case in which you build the meter and then holding it in your hand  or by  grounding it to some metal item (do NOT connect to ground of mains power supply!!!)
Here are some pictures of the detector I build:

Enjoy building this awesome little "everything detector" ^____^ oh and hey, while you're here, please leave a comment! That'll be cool! :-)