A simple FM transmitter for the iPod & other devices
A friend wanted to connect her iPod to her home stereo system, but said stereo doesn't have any external inputs. The simplest solution, avoiding diving into the wiring in the stereo, is to transmit the audio from the iPod to the FM radio in the stereo system. This gave me the opportunity to play with Tetsuo Kogawa’s MicroFM transmitter design as featured recently on Mark VandeWettering's website.
The circuit really is about as simple as it gets, a single transistor is configured as a free-running L-C oscillator somewhere between 88 - 108 Mhz (the FM broadcast band) and signals on the transistor base changes - modulates - the oscillator frequency.
R4 and R5, the pair of 1k resistors in the schematic are just to passively mix the left and right audio channels from the iPod, producing a mono signal to drive the transmitter. In this application there is no major problem with messing with the audio in this way, this isn't supposed to be a high quality high fidelity device. A stereo FM transmitter is much more complicated.
I built the circuit on a scrap of FR4 PCB material cut to fit a case I planned to use. The parts are a mixture of 1206 size surface mount and standard pin-though-hole parts. I didn't get a photo of the board before I mounted it in the case. A strip of adhesive backed copper flashing tape stuck inside the case forms an antenna and increases the transmitter range from 50cm to around 20m.
The transmitter draws approx 10mA from a 9v supply, so should run for about 50h from a fresh Zinc Carbon or Alkaline battery. Audio quality is surprisingly good considering the simple nature of the circuit.
Looking at the output of the transmitter on a spectrum analyser only the large peak is due to the transmitter, the rest are usual broadcast stations. There is no evidence of any spurious transmissions or multiple outputs that Mark saw with his version of the circuit.