Frequency-Agile FM Surveillance Bug |
Overview
This is a simple little project to turn a "car MP3 player" into a frequency-agile audio surveillance bug.
The car MP3 player used for this project was listed as "Portable Car FM Transmitter MP3 Player SD TF USB LCD Display Small sports Auto" on eBay and appears to be sold by various users for around $4 each. Like most of these Chinese toys, there is no documentation or even manufacture info. Oh, and no FCC ID either...
These MP3 players have an internal FM transmitter based on the Beken BK1085, which is in a 16-pin SSOP package with pins 7 and 8 appearing to be the left/right analog audio inputs. I wasn't able to find a datasheet for this chip, so that was mostly determined by poking around a bit with an oscilloscope and signal generator.
The MP3 player itself is designed to plug into a vehicle's cigarette lighter (+12 VDC). It will transmit audio from either an USB connection or SD card to the vehicle's FM radio via the internal transmitter. The transmitter tunes over the standard FM broadcast frequency range of 87.5 MHz to 108 MHz in 100 kHz steps. The RF output power is around 2 milliwatts (+3 dBm). The stock antenna is just a short wire coupled to the +12 VDC input line via a 0.01 µF capacitor.
On the DC input power cable, the red wire is +5 VDC, the green wire is ground, and the copper-colored wire is the antenna. There is a 78M05 regulator on the +12 VDC input to supply the circuit with clean +5 VDC. The current draw is around 45 milliamps.
The LCD displays the current transmitting frequency and has a white LED backlight, both of these may be removed to reduce power consumption and to save space.
To turn the device into a surveillance bug, an electret microphone and a small audio amplifier will be used to directly drive one of the analog audio input pins on the Beken BK1085.

Pictures & Construction Notes

Internal overview.
The 78M05 voltage regulator board is off to the left.
The red and black wires going to the LCD display are for its backlight.
The little black rectangle below the LCD display is the infrared sensor for the accompanying remote control.

Closeup of the rear of the MP3 player showing the different connectors for the digital file/audio inputs.
These can be removed to save space.
The Beken BK1085 FM transmitter is the 16-pin IC on the lower-left.

Closeup of the 78M05 voltage regulator board.
The red wire is the +5 VDC output.
The green wire is ground.
The copper-colored wire is the antenna which is AC-coupled to the incoming +12 VDC line via a 0.1 µF capacitor.

Removing some of the unnecessary connectors.
The analog audio inputs to the Beken BK1085 appear to be pins 7 and 8.

Removing the LCD display to save some space.
You'll need to double-check the output frequency using a frequency counter.
The two outside buttons along the bottom are "Frequency Down" (left) and "Frequency Up" (right).
The three connections along the top are (from left-to-right): ground, +5 VDC input, and the antenna.

Adding a simple 2N5210-based transistor microphone amplifier circuit to the transmitter.
This was just an experimental circuit and using all surface-mount components would significantly reduce the size.
The antenna is just a 24-inch long piece of #30 gauge wire.
A Motorola MWA130, or similar MMIC amplifier, can be used to increase the RF output power. Note that this will further increase the drain on the batteries. It's usually more efficient to increase the overall gain of your receive station.
The audio inputs on the Beken BK1085 appear to be low-pass filtered, so ultrasonic subcarrier generators won't work well with this transmitter.

Example of some different DC power options.
The MP3 player and microphone amplifier are designed for +5 VDC, but will work down to around +3.5 VDC, with reduced RF power output.
On the left, is a standard four "AA" battery holder from Radio Shack.
In the middle, is a salvaged +3.6 VDC, 1900 mA/h rechargable battery pack.
On the right, is a +3.6 VDC, 190 mA/h rechargable battery pack from one of those LCD "picture display" keychains. These can be recharged using an USB connection. You may want to try two of these in series.
