I wanted to see if I could modify one of those cheap car transmitters that people use to send music from an mp3 player to a car stereo and use it to send music from my computer to the kitchen radio.
The cheap transmitter: I picked one up that had both USB and line-in input possibilities.
These are really cheaply made, but they work well enough to transmit signals over short distances, so my initial idea was to open one up and tweak the antenna and antenna driver circuit. The first thing that I noticed when I opened it up, was that it didn’t really have an antenna… or… it had a wire that acted as the antenna… sort of. But it didn’t seem like it was designed for the kind of range that I wanted to get.
Things you'll need:
x1 cheap FM transmitter
x1 BJT transistor (2n2222 worked well)
x1 100 ohm resistor
x2 220 ohm resistors
x1 22pF capacitor
x1 10nF capacitor
x1 radio antenna
x1 good times
Step 1: Disassemble and Remove Voltage Regulator
The other thing that I noticed was that, even though the thing is packaged to work with 12 volts from a car cigarette lighter, the internal circuitry only needs 5 volts to operate. So we removed the internal 5-volt regulator.
Step 2: Extend Wires
After removing the voltage regulator, and the plug for the cigarette lighter, we were left with this, the case with the circuits and display, and 3 wires sticking out the top. The red wire is the positive power lead where we need to connect 5 volts, the black wire is ground, and the white wire is the built-in antenna that doesn’t really work very well.
Step 3: Build Circuit
I used a simple BJT amplifier circuit to boost the modulated audio signal, and drive a bigger antenna that I removed from an old radio.
Step 4: Good Times
Put it all on a breadboard temporarily, plugged in some Daft Punk, and ... Success!! The little amplifier circuit increased the range of the transmitter from about 1 meter to about 10 meters.