Dorkbot Awesomeness: DIY FM Transmitter

DIY FM Transmitter

Dorkbot rocked last night.  Big shout out to Brett Ian Balogh who shared this sweet FM transmitter and provided us the opportunity to build it.  This transmitter consists of only one transistor, three resistors, four caps, a homemade inductor coil, a variable capacitor, a 9V battery and some wires.  For $15 dollars and about 1 hour of your time, you can transmit whatever you please some decent distances, like 50 feet or so.  With a little modification and a little more power, you could broadcast a good distance.  Say two 9V batteries and a higher power transistor, maybe beefier resistors and you’re on your way.  It’s tuneable through a good bit of the FM band.  I can easily swamp any station.  In Chicago, we have some pretty lame radio.  So, I’m thinking that this could provide a rolling cone of silence for your least favorite radio station.  Not that I’m advocating any unseemly FCC violating behavior (wink wink).

Click through for talk about how it works and an image of the schematic.

Not too complicated, n’est-ce pas?  This circuit centers around the npn transistor T1.  Here we’re using a 45V, 800mA npn.  That’s more than enough for this application.  It will never see anything close to 45V or 800mA.  So, you could sub another npn, but the important part is that it isn’t too capacitive and switches quickly.  Look for a switching npn.  Resistors R2 and R3 bias, provide a dc operating point, for T1.  C2 and C3 are bypass capacitors, filtering high frequency content from the nodes to which they are connected.  C1 and the inductor form a resonant oscillator. They provide the frequency of transmission.  The frequency can be adjusted by messing with C1 or toying with the inductor.  I’m not exactly sure about C4, but it’s adding capacitance to T1, so I’m guessing that it helps stabilize the oscillator and provides a shunt path around T1 for high frequencies to get to the antenna.  Audio comes in through C5, which stops DC from the audio source from messing up the bias created by R2 and R3.

We built it on a copper/fiberglass sheet with little pieces of copper/fiberglass sheet cut out to act as pads to solder components to.  The copper sheet acts as a ground plane to avoid spurious capacitances and other high frequency weirdness.

Enjoy transmitting!  Here’s a link to a parts list.

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