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Wednesday 19 March 2003

FM transmitter project [lurks]

My latest plan for world domination is the construction of a 50mw FM Stereo transmitter. Basically you use this thing to send audio from your PC or your HiFi into any FM receiver anywhere in your house, kitchen, garden, neigbors of whatever. Strictly speaking it's illegal of course, 50mw of power is quite capable of blotting out an FM radio station for your neigbors if you tune the thing wrong. Practically speaking, so long as you don't do that - no one will care.
Costs me about 50 odd quid worth of parts to make with the internal transmitter circuit, proper steel box, PSU and detachable telescopic aerial. I've ordered up all the parts. I'll build early in the week and then flog on ebay to see what I can get for it. With luck it'll go for a ton or a bit less. If so, I'll make a few of them. Obviously I'll do cheap cheap for the EED posse :)


  1. It seems I've drummed up a fair bit of interest in this, so far 4-5 people are interested. Two definite sales, assuming works as advertised. A few more on the fence. One thing you bastards may be able to help me on is the aerial configuration.Currently I'm putting a panel mount metalic phono connector on the top of the steel case. Then I'm soldering the bottom of a 7 segment (nice 1/4 wavelength size) telescopic aerial onto a phono plug. So the telescopic aerial can be detatched for transit.The other option was to put an RF coax socket on the back and ship with one of those ribbon balun units you get with your tuner. You know, a T-shaped ribbon that you need to tack up on the wall. Potentially it's lame because it looks bad and needs fixing to the wall but on the up side, mounted correctly it'd be a better aerial. I still think the telescopic aerial will be more than good enough to get suitable range. You guys got an opinions on the matter?Another issue is that pumping audio that's too loud into the box will result in over modulation. Ideally one should fit an audio compressor but that's another £20 of parts - far too expensive. Perhaps a small LED level meter would suffice? The first ones I do wont have any level indications. LED level indicators would be about £10 in parts. I think people would expect a level control if given that too. That's overkill. I'll look at a simple 'overload' LED.I got the last of the parts this morning so I set aboutconstruction of the prototype. Building the FM transmitter circuititself was time consuming but went without hitch. Fired it up and itworked first time! All that remained was putting the circuit board ina steel case and wiring up the back panel phonos and power supply,front panel switch and LED and the aerial. What should have been aquick job took much longer because steel is such a bastard to drill.Anyhow, the prototype is finished and it works beutifully. Soundquality is immaculate, radiated power at 12V seems to be around 40mwwith the telescopic aerial. I've yet to test the range out, it's a bitdifficult in my location but when I find my old portable World RadioI'll give it a try. Pretty good result all around! I don't have adigicam but I did find a crap webcam and take anXvid AVI capture of meshowing off the box.Right, so who wants to buy one then? :)Here's the best eBay listing evar!It seems I got it all wrong concerning this FM transmitter project.I think it's the best thing since sliced bread and I've decided Ican't do without one myself. Yet I've failed to convince anyone elsethat they're cool besides a couple of friends (who may well have beenhumoring me) and my listing on eBay has yet to attract a singlenon-EED bid. Seems I didn't have my finger on the pulse of this one,I'm quite surprised really. I thought being able to cast PC/digitalradio audio anywhere in your house/garden etc is absolutely tops.Maybe in summer? Maybe people think this sort of thing should cost £20tops? It can't be the device, it's first rate. It can't be my listingeither, I reckon. Mad, these things go like hotcakes in the US andreally crappy spec ones with 1/5th the power of this.Just as well I only built 3 so far. I'll keep one, one went to my bestmate and the last is on eBay and will hopefully get a bid over £50 inthe last day so I break even on it. However on the whole it's been anexpensive excersize I can ill afford. Bummer.Update 19th-March-2003: The saga continutes. My first listing of the first transmitter I built went for a good £20 under what I wanted to some (0) rated yank guy that hasn't been in touch. Suffice to say I was not pleased. Given I just sold two copies of Edge Retro for £50, I refuse to believe I can't get a decent price for one of these! So I've just rejigged my listing quite a lot and made it quite obvious, tabloid-style if you like. Also trying buy-it-now to sort of give an indication of worth and putting it in different categories. This is my new listing, after this I'll give up. :)

  2. fm transmiter circuit and project