FM radio converted to Transmitt

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unix60959

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Hey guys, I new to electronics and have no formal electronics backround other then my own experiments and the internet :grin:. Anyway, I read something online about how you can modify FM radios to transmitt from by splicing into one of the coils near the tuning capacitor. So I tryed it out and it worked all though there are two things I was wondering. One, how does this work, and two, if you watch the video, why does the way I splice the input signal on this radio work?


Thank you,

unix60959
 

More than one tv episode has the hero saving the day, when he takes a radio receiver, and quickly rewires it to transmit a call which brings the cavalry to the rescue.

I saw a techie at a message board state this is impossible. The circuitry that transmits is not at all like circuitry that detects.

And the here does the impossible all the time on tv.

But just to be open-minded...

There's a chance that you can clip a wire to some stage in a receiver, so that if you put a radio a few inches away, it will have a sympathetic response in a corresponding stage.

It would need to be very close to the front-end circuitry. Or it would need to be spillover on a radio frequency where it's unintended. A receiver does not have the signal-carrier-modulating circuit necessary to generate a normal broadcast.
 

I'm not sure I understand your response, are you saying that you think my video is fake? Otherwise, can an FM demodulator be 'flipped' around and there for used to transmit? obviously not very far since the 'flipped' circuit isn't amped, since the amp would be on the 'other side' of the modulator?
 

Personally I could not see how it is possible either, just by rearranging a few wires

firstly the receiver doesnt even have a modulator. and the demodulator is normally an FM IF chip of some sort
that can not be used as a modulator
Thats the the first of many reasons why not

Dave
 

I know enough about radio circuitry to be skeptical about your claim.

I would need to know a lot more about radio, in order for me to say what you achieved is possible or impossible.

I responded because I believe your claim to be intriguing enough that it should get some kind of response at an electronics message board.

You see, I once made a simple fm transmitter from a $7 kit. It's similar to the circuit below (found on the internet, electronics-diy.com/1.5v-fm-broadcast-transmitter.php).



As you can see there's a simple coil/cap/transistor/cap arrangement as the scene where the action occurs. The incoming audio causes the frequency of oscillation to change.

Since fm modulation can be done that simply, I guess there might be a conceivable way to do it by rearranging things inside a receiver. Say, tap somewhere on the tuning coil, then tie in an amplification stage (elsewhere in the radio) to generate oscillations, and modulate it with audio from your mp3 player. I think it would also require disconnecting a few components as well.

So if it can be done then it's news to me after all these years.

Again it will require someone with more knowledge than I have, to say whether it can or can't be done.
 
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It's surely possible to turn a radio frontend into a crude FM transmitter.

I can't decide from the shown video, if the author actually did it. To make it yourself, you need sufficient knowledge of RF circuits and preferably some equipment.
 

Yes, YouTube… :lol:
I have seen this video and it isn't proof of anything. It’s only a edited video and today we know that's almost certainly we have the ability to edit any video… So in theory all it’s possible, but in practice we need lot of work for doing that for real. ;-)
 

A ordinary old fashion FM radio is often a also a transmitter. When I was young and did travel with my parents for camping in tent was my favorite game as follow:
In the evening, when you can see the shadows of the people inside other tents, were they did listen on a FM radio, did I turn on my own FM-radio, tuned 10.7 MHz away from the channel they were listening at. Leakage from my LO did very effective disturb the other FM radio. As I could see the shadows could I see when someone did try to adjust the disturbed radio. Exactly when they did reach it ,did I turn off my own radio. When it had been repeated 50 times, was the radio owner rather upset. If they were listening at a football game, did I wait until it was a tactical right moment for making them as mad as possible. In at least one case did he throw the radio long outside of the tent, yelling some words that I not was allowed to use as I only was about 10-12 years old.

As it was long time ago was my FM radio most likely tuned by a big tuning cap, which controlled LO frequency. A bit newer FM radio and an electric controlled varicap can be expected. If Internal speaker now is rewired as microphone, and its LF voltage is connected to the tuning potentiometer would it then be a FM transmitter. Some other tricks including improve antenna connection, and we have done a pretty good MacGyver transmitter.
 

:-D
Yes… the spirit of MacGyver is an inspiration to each and every one of us, but the question remains. Can we provide the most efficient beginner's guide how to modify a simple receiver for a transmitter kit?
 

FvM said:
It's surely possible to turn a radio frontend into a crude FM transmitter.

I can't decide from the shown video, if the author actually did it. To make it yourself, you need sufficient knowledge of RF circuits and preferably some equipment.
Click to expand...

I've done this on multiple radios, all you have to do is locate the coils near the tuning cap and carefully unwind, cut, and then wire you input signal. it just so happens that with this particular radio, which was an old model (atleast 80's) that one input lead was attached to one side of the coild and the other attached to part of the tuning cap.


i guess ill have to upload a better/more descriptive video :shock:



I think this is how my radio is working, somehow the method i used allows you to input your own signal on that leakage as if it were a carrier wave?
 

hey guys this is the first transmitter i build from a radio. maybe it will give me more credability? haha

 

To keep the explanation simple, the superheterodyne receivers uses a local oscillator for tuning and uses the difference between the local oscillator frequency and the radio station frequency to create an intermediate frequency that is lower than the input frequency. The IF frequency mathematically is the frequency of the local oscillator subtracted by the radio station frequency. That’s why the FM radio receivers can cause interference to other nearby FM radios. The source of this interference is the local oscillator used in the receiver. (And by the way, same idea can apply to the radar detectors = superheterodyne radar receivers, using a local oscillator makes your radar detector detectable by the police ). So the local oscillator is tuned such that the IF frequency is always equal to 10.7 MHz, and by modifying the local oscillator frequency we can adjust that oscillator to be a low power transmitter that is transmitting an un-modulated RF signal. Any nearby receiver can pick up this signal although no sound will be heard on the receiver, and to complete the transmitter design we need only to apply some external audio signal to the transistor-based oscillator.

 
THANK YOU! that helped a lot mister_rf
 

jpanhalt said:
Are these experiments with making a receiver into a transmitter connected in any way to these other experiments?

https://www.edaboard.com/threads/238991/#post1022287
https://www.edaboard.com/threads/239241/#post1023541

John
Click to expand...

No not at all. The transmitter was a tutorial I saw on youtube so I followed it to see if it worked. Then recently I became interested why it worked so I asked. I guess the perpetual motion and the hydrogen fuel cell are related in the fact that they both deal with energy and such... I'm always looking for ways of improving alternate energy resources and things of that nature...
 

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