EM detector - understanding a circuit

Thread Starter


Joined Nov 10, 2019
Hi everyone,

I would like to build an EM detector and I found the following circuit on this YouTube video.
I don't feel like just doing it without understanding the way it works so I tried to figure out what the different parts of the circuit were doing. In that case I think it's just a bunch of different filters, so I tried calculating the cut-off frequencies and it didn't really add up. So I left a comment on the video but never got any reply... So I'm turning to you :)

I'm a little confused by the 10nF capacitor. Wouldn't this capacitor filter almost any frequency above 160KHz? At 160KHz the capacitive reactance is at 100 Ohms. And the RC filter that follows only filters everything under 15Hz right? So what's the point? The potentiometer after the antenna is a high pass filter right? So it can filter out very low frequencies, is that it?

Every insight into the circuit is welcome (no need to explain the part with the LM3914 though). Thanks!!



Joined Jun 4, 2014
The LM386 is an audio amplifier. It isn't going to anything much above 160kHz.
The pot is a sensitivity control.
There is no obvious RF detector. I think they must be using the LM386 input transistors as a detector.
Don't expect too much from this circuit.


Joined Mar 10, 2018
Pin 5 is output, why that is shunted to ground with the 10 nF is....?
Unless the design is relying on the small signal Zo of the LM386 to
form a LPF with the cap, which would not be a well defined way of
creating a filter that way. Device to device.....

The 10nF creates a LPF then followed by a HPF, the 4.7 uF. Not sure
what that is doing....in light of the fact the LPF filter is so badly
determined by Zo of the LM386, whatever that is....

Regards, Dana.
Last edited:


Joined Aug 21, 2008
An engineer I once worked with would sometimes comment "This circuit wasn't designed. It was discovered!" Clearly this is one if those that were discovered.

As said earlier, this is an audio circuit that happens to respond to electric fields. What is it that you would like to detect?

There is another recent thread on a similar topic that might have some circuits that would interest you.


Audioguru again

Joined Oct 21, 2019
The symbol of the LM386 amplifier is wrong and its missing 10 ohms resistor is also wrong.
The author also wrongly has the LM3914 intensity control on the wrong pin and his 1 ohm current-limiting resistor values are also wrong and not needed since the LM3914 regulates the LED current.
Also missing is a very important supply bypass capacitor.