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Old 6th Sep 2006, 12:59 pm   #1
adibrook
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Default 555 am tx project

I'v been looking at this project again, and i think now i understand how it works i may try to build it.

http://www.williamson-labs.com/images/555-am-xmitt.gif

But, i think i will need to wind my own coil. I have a ferrite rod from an old portable tv/radio, but the tap for the 555's output will have to be added.

On a normal diameter ferrite rod like used in most radios, does anyone know how many turns the main winding shoudl be? And how many turns the low impeadence tap?

Also, i have a wierd chip here called an RS Timer A. I think i herd somewhere that thats just a rebadged 555. Is this correct? Thanks.
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Old 8th Sep 2006, 10:36 am   #2
boiss
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Default Re: 555 am tx project

Hi Adi,
If I remember correctly from the old crystal set/single transistor reflex days, It was always 60 turns and a 500pF tuning cap. I'd love to see what the O/P looks like on a spectrum analyser. Always fancied knocking up a circuit with a 555 and ferrite rod etc, running at about 1kHz to zap those walkmen on public transport.
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Old 9th Sep 2006, 6:09 am   #3
adibrook
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Default Re: 555 am tx project

I think i found a suitable coil inside a little portable tv. It refused to work so i tool it appart. I think i can use the FM winding for the 555's output and the AM winding as the main tank circuit.

Unfortunatly i only seem to have 2 555's left. One is a really old LM555 and one is an RS Timer A. I dunno what that is but it seems to be pin and function compatible to the 555.

Anyway, theyre both very old chips, and i just cant push them high enough for AM. Ifter about maybe 200-300 KHz the output waveform begins to go wierd, and if you go higher it just doesnt work.

Next time i'm in maplin i'll get some modern 555's. They can be pushed to about 1MHz as far as i remember.

I'm pretty sure the output of this transmitter will look horrible on a spectrum analyser. But then again the power levels involved are so tiny that i dont think it will bother anyone. With the ariel i'll put on it as well i doubt if it will radiate over 10 meters. All i need it for is to transmitt music to my radio.
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Old 9th Sep 2006, 8:38 am   #4
Mike Phelan
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Default Re: 555 am tx project

I did make a similar device years ago, as my ears were continually assaulted with junk radio on campsites.
A scrap transistor set with ferrite rod and tuning gang intact, BF195 and a few components, and bingo - the OPs AVC was tuned to silence!

A similar idea occurred when I ran a video repair and training department where the installers loaded their vehicles outside; one of them insisted on his fix of Radio 1 with the wick turned up. On a good day I can stand about 30 seconds of that.
There were bars on my windows (for security pusposes, you must understand! ) - this, connected to a length of coax and the full output on my Advance E2 worked a treat; sadly, he went to cassettes.
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Old 9th Sep 2006, 11:28 am   #5
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Default Re: 555 am tx project

I know that this is a bit OT but I'll risk the wrath of the Mods.

I once heard that a guy in Australia was fed up with intrusive rubbish music that he had to put up with on the beach so he built a small AM Tx as a jamming device. When the big day came he switched it on & all the surrounding radios squawked like demented cats. Did they switch them off? No, they seemed quite happy to leave them squawking rather than have silence.

Joe:
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Old 9th Sep 2006, 1:11 pm   #6
adibrook
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Default Re: 555 am tx project

Umm, i dont wanna jam anything. I just want to build a micropower TX to transmitt music from my laptop to my radio.
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