Ciprian "YO6DXE" Popica's "Spy Bug" Is a Compact, One-Transistor FM Baby Monitor Build
Designed for installation in a battery-powered lamp, this low-power transmitter is a lesson in compact simplicity.
Radio ham Ciprian "YO6DXE" Popica has designed an ultra-compact PCB hosting a single-transistor FM transmitter — a "spy bug," as he puts it, or a "baby monitor" if you're looking to stay out of trouble.
"I tried building it as a small as possible, because I was told I have to install it in a desk lamp, the one that sits next to the baby," Popica explains of his creation. "I think you can build it even smaller if you're trying, but it works fine [like this]. I'm going to power it from the batteries that are already inside that desk lamp."
The device itself is a simple FM transmitter with an on-board microphone: audio is captured, modulated, and transmitted through a simple antenna to a receiver tuned to the matching frequency — with a range, as designed, of anything from a few feet to a few hundred feet depending on how much power the radio's given.
As with Popica's previous builds, there's a minimum of components: the transmitter works with just one transistor, a cheap 2N3904 NPN bipolar junction transistor serving as an amplifier, an electret microphone, a Zener diode, and a handful of passives, plus a length of wire as an antenna and a hand-wound coil inductor.
"The power consumption is somewhere around 14, 15 milliamps," Popica notes, "so the [lamp] battery should last quite a bit, it shouldn't drain. It's a very simple schematic: one microphone, one transistor, you have four capacitors — three if you don't want to use the 100nF one on the power rail — and two resistors and a small inductor."
The project is documented in the video embedded above and on the DX Explorer YouTube channel; Popica has also made project files available for download under the reciprocal Creative Commons Attribution-NonCommercial-ShareAlike 3.0 license, and has uploaded the board design to PCBWay under the same license.