Single chip FM transmitter

[joelby]

This is all kinds of awesome - an FM transmitter made using a single ATTiny45, a battery, and nothing else.

[HLA-27b]

wowzers 

[mobbarley]

Saw that on HAD. It's cool, but i'm more impressed by a good sounding rickroll fitting in the internal memory.. I've tried coding some audio before and failed pretty miserably.

[MikeK]

It's a neat trick, but the radio guys have been doing better transmitters with a transistor and a couple other discretes.  The problem with the AVR implementation is that it outputs a square wave, which contains the fundamental plus all of the odd harmonics (i.e. noise/interference).

[IanB]

It's a neat trick, but the radio guys have been doing better transmitters with a transistor and a couple other discretes.  The problem with the AVR implementation is that it outputs a square wave, which contains the fundamental plus all of the odd harmonics (i.e. noise/interference).

But can a transistor, a battery and a couple of discretes be programmed to broadcast a tune without any external inputs?

[MikeK]

But can a transistor, a battery and a couple of discretes be programmed to broadcast a tune without any external inputs?

Maybe that guy with the tricky serial circuit has such components.

[vk6zgo]

Sorry,but that thing is as close to a real FM transmitter as a sphere is to a cow!
MikeK put his finger on the problem,that it also transmits on every other harmonic of 24MHz.
Many very simple FM transmitters have been made,which,although all pretty useless, would out perform this one.

VK6ZGO

[joelby]

I think you're missing the point It isn't meant to be a high performance FM transmitter - it is just a wonderful hack.

[MikeK]

I do like the idea of modifying the tuning register to do the FM'ing.  Any way this could be used to alter the frequency of a true sine wave oscillator?  Can a microcontroller be used as the source for a tank circuit?

[vk6zgo]

I don't think the centre frequency is going to be very stable anyway,as the thing is self referencing.
I'm at a loss to see how the tuning register can stabilise this,as it must ultimately be dependent on the RC oscillator stability.
If we in turn mess with it to produce FM,any stabilising effect will be compromised.
To get a sine wave output,you would need a BPF,the bandwidth depending on how much you want to shift the frequency.

This kind of reminds me of something we discovered when we were using an "El Cheapo " video modulator kit back in the 70s.
 (We put a crosshatch signal in to produce a modulated signal to use in setting up convergence on a TV set.)
Even without power on,you could still see the crosshatch on the TV,as there were enough harmonics generated from the video to appear as RF.
We thought we had found a neat trick,& played about with it for a while,but it wasn't really useful,as the TV didn't lock very well,& it was quite noisy.
In the same way,the "FM transmitter" idea is interesting,but impractical.

VK6ZGO

[amyk]

On the topic of transmitters made from unusual devices... http://bellard.org/dvbt/

Also this, but probably won't work with LCDs now: http://www.erikyyy.de/tempest/