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 Moderated by: sparks, mikesndbs, Keith Clark, joecioppi, Jeff, fratka, EVPDave, Dr. Heinz Doofenshmirtz, Dmaumau, Bruce, ArizonaEvp, takne_007
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Cheap FM Radio kit  Rate Topic 
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 Posted: Jun 3rd, 2014 04:51 PM
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sparks
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Hi,
Yes, in most instances, the control of the varicap voltage can be done by an external circuit, where an introduced voltage waveform makes the radio scan up and down the band.
A resistor / capacitor network will not be ideal, as it is an exponentially shaped wave, meaning the scan is non linear in nature. This means the speed of the scan is fast at one end of the band and slow at the oither.
The best sort of waveform is either a triangle or sawtooth, Both are linear, so constant speed. The triangle wave means the scan goes up and down the band, and the sawtooth makes it scan to the end and then reset and start again.
The stepped waveform diagram you mentioned will replicate the stepping of hacked radios, but the stepping itself isnt really necessary.
There are function generator kits that can be adapted to interface with such a radio. One that has been used by others is made by kitsrus.com...
Kit 23 Opamp Function Generator
The kit can be interfaced with a radio, but it is not a straight forward procedure, as the waveform will need to match the unique scan voltage limits of the radio, and thus will require customisation of components in the kit in order to work properly.
I hope this helps.
regards,
JEFF

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 Posted: Jun 4th, 2014 02:42 PM
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sparks
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A 555 Timer is not suitable, as it is essentially a resistor / capacitor with level switching, hence producing an expontential waveform that I mentioned.
The 555 can be adapted to produce a sawtooth by adding extra components, but for the trouble this causes, it would be easier to buy the kit I mentioned, which will produce a nice triangle wave, and hence produce a nice sweep.

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 Posted: Jun 4th, 2014 03:37 PM
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sparks
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Ok, youre looking at a scan period of 8 seconds or so (8 sec = 8,000 milliseconds or 0.125Hz). It depends on personal choice how much of the radio snippets you want to hear as it scans past. The rule of thumb is only a small fragment, so the scan has to be quite fast.
That function generator on ebay will produce the waveforms, but unless the unit has dc offset and amplitude controls, the waveform will need to go though some sort of level shifter in order to be compatible with the radio's varicap voltage requirements for scanning the band correctly. Its one of the reasons that kit23 is useful as it can be tweaked to produce the correct amplitude and dc offset depending on the radio in use.



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 Posted: Jun 4th, 2014 05:19 PM
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sparks
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Ok, its perhaps got the ability to be compatible, but it wont go below 1Hz, so its therefore not suitable for the job.
Kit23 is able to go lower in freq by changing some component values. It is used for radio sweep by others.

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 Posted: Jun 4th, 2014 06:00 PM
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sparks
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Yes, the waveform is all above zero, and its center point may be something like 4.5V, so it might start at 1v, going up through 4.5v and up to the limit say at 8v, then starts the reverse of this, going downwards to 1v, then upwards to 8v, repeating (this describes triangle wave). In regards the tuning, 1v corresponds to the bottom of the FM band, and 8v the top. The figures I used are examples only. The real values need to be measured on the tune pot of the radio, and kits23, etc, then modified to match.


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