Tuesday, February 26, 2008

A great low power FM transmitter

FMTransmitter.jpgSaw this little FM transmitter for the broadcast band on the Make Blog.

I often want to listen to the Sunday ham radio broadcast but it would be great to not have to sit by the rig for the whole duration so I've used an iPod FM transmitter to relay the audio but they have very short range, even with an antenna added.

Turned out I had all the required components in my junk box. Built the oscillator first and it just worked right off, then built the buffer stage.

This circuit has an RF buffer stage so touching the antenna won't pull it off frequency. Running on a 9V battery it puts out enough power to cover the house. It's getting hard to find a spare frequency but I put it down near 88Mhz and it works very nicely.

I really love the aesthetic of "ugly" construction and it's so quick to throw together these little projects. I did release the smoke on the buffer transistor by accidentally using a 1K bias resistor in place of the 100K but after a small explosion that was fixed.

This design is to be recommended, it worked without any problems (except those of my own making) for me other than a bit of stretching of the oscillator coil to get it on band and it seems exceptionally stable given the construction.

If you tap the board it transmits a lovely metallic "clang" sound from the coil but that's to be expected.

I remember many years ago building JostyKit transmitters which had the coil etched onto the circuit board.


Mal said...

Hi Peter, Love the blog.

I am also in Sydney and have too many hobbies.

See a4x4kiwi.blogspot.com and http://members.optusnet.com.au/a4x4kiwi/scooter for a couple of my projects.

BTW, I am also a make subscriber since before issue 1 :) and am planning to go to the maker faire in May. (I went to Austin last year and had a hoot)


Peter Marks said...

Thanks Mal,

hey, great self balancing scooter project!

We should have a maker faire out here some time.



Anonymous said...

Hi Peter,

Great article. I love dead-bug construction!

Got the link to the circuit diagram - did you add a mic to it?


Peter Marks said...

Thanks Willem,

The circuit link is in the opening paragraph.

No mic, I feed it line level audio.

Best wishes,