I am very interested in spy earpieces. They work on 1.5 V and amplify EMF signals generated by a nearby device. I wanted to get a circuit and re-create the spy earpiece but was not able to find circuit online. After digging a bit more I found an image showing the internals of a spy earpiece. By looking at that picture, I guess it is run by op-amps but I was not able to find the used op-amp or circuit. It looks like an op-amp is used and no other components are used.

Can someone here help me reverse engineer the circuit or tell me what the circuit actually is and what components are used?

Photos of internals of spy earpiece

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    \$\begingroup\$ Could you add some information of the work you have done so far to design/reverse engineer the circuit? What research have you done into how these circuits work? Generally people on here aren't willing to do all the work for someone but to help them once they can see what they've already tried :) Also please add a clear and answerable question to your post! \$\endgroup\$
    – Entropy
    May 26 at 7:23
  • \$\begingroup\$ I am sorry about that i am new to electronic just started 3 week back and also new to this form that is why I needed help even for this tiny circuit i works by amplifying the ac signal induced in its coil by transmitter divice it works on 1.5 v ...i.imgur.com/lEcGRKQ.jpg \$\endgroup\$ May 26 at 7:42
  • 2
    \$\begingroup\$ It isn't possible to reverse-engineer something based on blurry pictures of PCBs taken from afar. It might not even be possible to reverse-engineer it without measurements on a physical device. \$\endgroup\$
    – Lundin
    May 26 at 7:49
  • \$\begingroup\$ Reverse engineering takes years of experience. You basically need to know how to design it in the first place. Start with the basics first and watch some bigclivedotcom videos on reverse engineering. \$\endgroup\$
    – winny
    May 26 at 7:59

1 Answer 1


What you are calling a "spy earpiece" is nothing more than a regular hearing aid in which the microphone has been replaced with a coil.

A simple hearing aid has a microphone, a small amplifier, a single button cell, and a small speaker. The button cell provides power to the amplifier. The amplifier takes the signal from the microphone and drives the speaker.

Such a simple circuit might look like this:

enter image description here


Replace the microphone in that circuit with a coil and you've got your "spy earpiece."

In a modern, good hearing aid, the op-amps are replaced by a specially designed IC that includes a digital signal processor and some stuff that allows it to be programmed from outside - the one IC handles amplification, automatic gain control, equalization (it can amplify specified frequency ranges to compensate for the loss of hearing in specific ranges,) and communication with the programming system.

The "transmitter" for such "spy earpieces" is also a simple coil, though much larger. It is usually driven by the earphone output of a hidden radio. The transmitter coil may be in a small box hidden under the clothing, or it may be around the neck disguised as a necklace or other jewelry or decoration.

  • \$\begingroup\$ Thank you for your valuable answer i just started learning electronics . Could you please tell me what op amp can I use in such a size of small circuit most of op amps are too big while others have higher oprating voltage while i need something that works on 1.5v.. \$\endgroup\$ May 26 at 10:19
  • \$\begingroup\$ @CrazyHacks: Have a look here. That (should) lead to a selection of op-amps that will operate down to 1.5V. You may find it easier to build a circuit that operates on 3V (two button cells in series) rather than jumping straigh to low voltage. Even better would be to work with some normal, simple amplifier circuits before you start trying to miniaturize things. \$\endgroup\$
    – JRE
    May 26 at 10:29
  • \$\begingroup\$ thanks for all your help i will follow your advice.. \$\endgroup\$ May 26 at 14:30
  • \$\begingroup\$ @JRE You probably want to use a link more like this, because a min supply voltage less than 1.5 V is what you want. These are probably going to be powered by a button battery, and it'll drop in voltage over its life. \$\endgroup\$
    – Hearth
    May 26 at 15:04
  • \$\begingroup\$ Maybe use a lithium cell with about 3-4V available. Could be rechargeable or a primary cell. \$\endgroup\$
    – PStechPaul
    May 26 at 21:00

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