This simple 2 transistor audio transmitter will send the sounds picked up in a room to any FM radio tuned to the same frequency as the transmitter, somewhere between 80 and 100 Megahertz. The expected range will be at least 100 feet and could be substantially longer depending on the parts used and the quality of your final product. This circuit is based on one that has been around since the 1960s and published thousands of times, so it is tried, tested and guaranteed to work if you follow the instructions. Performance is “OK”, but since this is the one of the most basic transmitter circuits possible, don’t expect high quality or rock solid performance.

via LucidScience – Build the BASIC SPY TRANSMITTER – Page 1 of 16.