FM Transmitter Hack
(Apologies for the lack of photo documentation)
I was looking for ways to slightly boost the range of a simple Bluetooth FM Transmitter. Research online showed a number of YouTube videos of people removing resistors soldering on antennae to the RF output pin. A more general Instructable by KevinM183 was referenced when tinkering with my FM transmitter. In general, these were slightly larger boards with larger IC's; however, after probing the board I discovered simply jumping a resistor increased the range quite a bit. I do not have a way of quantifying the power output difference, but a simple ad-hoc experiment of jumping individual resistors with an operating car radio showed that I could bring the RF output of the transmitter in and out of range with the simple change.
This is a simple jumping of a single resistor and does not involve adding or modifying the antenna.
The Bluetooth FM transmitter is a Doosl FM Transmitter (based on DSER107) purchased from Amazon. (However, my board appears to be labeled DSER 106).
This FM Transmitter uses a QN8027 FM transmitter IC. The schematic on the datasheet shows pin 5 as the RF output pin. Using the physical location of this pin and the location of the attached antenna, I probed likely resistors. The resistor is on location R1 on the display side of the board. I believe the resistor is 100 Ohms. If I'm understanding the datasheet correctly the maximum RF output of this chip is 121 dBµV ("63dB Stereo SNR, 0.05% THD; Maximum 121 dBuVp RF output level with 42dB adjustable range"). It looks like some power settings can be coded. I cannot find clear information about the legal limitations of FM transmission with the FCC. Please comment if you are familiar with these limitations.
CAUTION: The transmitter does contain a lithium-polymer battery soldered directly to the board, which can fail catastrophically if damaged or improperly handled. Use proper safety precautions and procedures. It is advised to properly discharge the battery before removing the battery from the board before any work is done on the board.
Supplies
- Soldering Iron (fine-tipped, if available)
- Plastic Opening Picks
- Plastic Opening Tool
- Eye-glass screwdriver - Philips
Jump R1
This is a challenging and difficult item to solder.
Essentially disassemble the outer housing which is a combination of clips and adhesive on the ends. The adhesive gave way with sufficient force, but using heat to gently warm the bottom and top of the housing (the flat ends) may make disassembly easier.
CAUTION: DO NOT push any tool you may be using past a few millimeters, you may damage or puncture the Lithium-polymer battery resulting in an uncontrolled thermal event.
The battery is attached to the housing with double-sided tape and gentle force releases the battery from the housing. The circuit board is attached with four screws. These screws must be removed to access the side of the board that contains the resistor.
Jump R1 resistor. I soldered a very light gauge wire. This is quite difficult with a typical soldering iron. If such a thing exists, I would recommend a fine-tipped soldering iron.
Reassemble.