FM Transmitters

Click here for an excellent introduction to the theory of 'simple' 2 and 3 stage FM transmitters (ZIP, 1.0MB).

K7 - 3V FM Transmitter Price Photo Docs
The most powerful 'bug' available for its size, 3V supply and number of components. Range up to 30 metres (100 feet). Easily tunable in the FM band. Greater range at higher voltage and better aerial.

K16 - FM Telephone Transmitter Price Photo Docs
Miniature transmitter which attaches in series with one of the two lines to your telephone. Transmits up to 30 metres (100 feet) to an ordinary FM receiver. Transmits further if the FM receiver is near the phone line. Tune with ceramic trim cap. Uses the phone line as an aerial and power source.

K18v2 - 9V FM Transmitter Price Photo Docs
Improved version of the original K18 kit. More powerful FM transmitter than K7. Tank oscillator coil built into the circuit board. Can be tuned anywhere in FM band. 9V battery operation. Over 30 metre (100 feet) range in the open depending on aerial used.

K28 - Voice Activated FM Transmitter Price Photo Schematic
A sound activated switch and an FM transmitter joined together to make a voice activated FM transmitter. Set the level of sound that it will respond to. Two stage transmitter needs tuning to get best results. Tuning Kit and instructions enclosed.

K32 - Two Stage FM Transmitter Price Docs
Our most powerful FM 'bug' to date. A two stage FM transmitter with an RF transistor (PN3563 or ZTX320) in the output stage. 9V operation. On/off switch mounted on the PCB

An excellent technical review of K32 (and other FM transmitter kits) can be found at this website.

K51 - Miniature FM Transmitter Price Docs
This FM transmitter is a miniature version of K18 using normal passive components. Components have been squashed together as much as possible while still allowing good access to the tuning capacitor. The PCB measures just 1" x 0.7" (25mm x 16mm). Any further reduction would require surface mount components!