14th April 2011 12:28 PM
another FM transmitter project
This looked like a nice transmitter to build when I came across it on the web. But only later did I see it uses a negative -3 V power supply, and a hard-to-get ferrite core coil.
All I want to know is how to create a +4 and a -3 v supply that I can use, and how could I substitute the ferrite beads with 6 holes.
14th April 2011 12:30 PM
the ferrite beads are RFC1; 2; and 3
14th April 2011 01:36 PM
It doesn't specify the voltage rail (there's only one!) on that circuit - those are just pin numbers, notice 1, 2, 5 and 6 are there as well.
14th April 2011 04:47 PM
Nigel goodwyn, are you saying that the bottom rail is in fact ground, and that "line" 4 goes to an unknown + voltage that is not given on the schematic?
If so, that solves the supply V problem.
14th April 2011 09:22 PM
Yes, the diagram makes no claim that those are voltages (and would make no sense if they were), they are obviously simple pin numbers for connecting off the board
Originally Posted by email@example.com
16th April 2011 01:06 PM
The extremely old circuit is a Smartkit from Greece and is posted at Electronics-Lab 4 Watt FM Transmitter
Its single-polarity supply is 12V to 18V.
It is designed to use a very old "crystalic microphone" and needs a modification to use a modern electret mic.
It does not have pre-emphasis like all FM radio stations have so it will sound muffled without high audio frequencies on any FM radio.
16th April 2011 01:13 PM
would it work if I used the mic input to feed an audio signal into directly, or would I have to match the impedance to a crystallic mic. If so, HOW.
16th April 2011 02:39 PM
An audio signal has a much higher level than a mic signal so an attenuator is needed.
A 4W transmitter is illegal in most countries without having a licence to operate a radio station and that poor quality old circuit is not good enough to be a radio station anyway.
16th April 2011 05:13 PM
Thank you, the circuit is only as an intro into RF. And besides, I'd rather be a guy that prefers science to abiding the law.
I'll google a suitable attenuator. BTW you specifically have been helpfull at this site, since when have you been interested in this wonderfull world (hobby?) of electronics?
16th April 2011 06:42 PM
You must not break the law with a poor quality high power transmitter because it causes all kinds of interference to aircraft communications and guidance signals, FM radio, TV, police, ambulance and fire department radios.
Make a radio instead of a transmitter as an intro to RF.
I began playing with electronics about 55 years ago. I built an AM radio about 51 years ago. I built an FM tuner and amplifier about 49 years ago and I built an oscilloscope about 48 years ago. I built a low power FM transmitter about 47 years ago and built a much better one 6 years ago. My entire career was in electronics, mostly sound systems.
Last edited by audioguru; 16th April 2011 at 06:43 PM.
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