I had the idea last night to reverse engineer an existing transistor radio in order to transmit signals to other radios around the home such as my voice (like an intercom system) or perhaps hook up my little nextbook with an .mp3 library and broadcast the contents on an unused frequency. Has anyone else tried this and if so, would it be simple to do or very complex? I'm not sure what the FCC regulations are but I'm just wanting to putz around within the confines of my property... not become a nuisance to others or violate codes of any kind!

 

Nope, that doesn't readily work.
A radio takes a weak RF signal and turns it into a more powerful audio signal.
A transmitter takes a weak audio signal and turns into into a more powerful RF signal.
The circuitry to do each of those functions is quite different.

You can buy kits that convert an audio signal into a low power FM signal for broadcasting in your house such as here.

 

Nope, that doesn't readily work.
A radio takes a weak RF signal and turns it into a more powerful audio signal.
A transmitter takes a weak audio signal and turns into into a more powerful RF signal.
The circuitry to do each of those functions is quite different.

You can buy kits that convert an audio signal into a low power FM signal for broadcasting in your house such as here.

I searched a bit and found this but I'm not sure how he did it because he never posted a tutorial:

 

An ordinary radio has an RF oscillator built in. You can turn that into a transmitter. But is it worth the effort?

 

A ordinary radio has an RF oscillator built in. You can turn that into a transmitter. But is it worth the effort?

I don't know as I haven't yet expended energy in order to try it out!

 

I'd recommend delving into some of the many tutorials and guides on the internet about making radio transmitters and receivers. The FCC rules permit making unlicensed transmitters as long as they meet certain guidelines. If you stay very, very low power then you are unlikely to attract unwanted attention, but it would be best to look up the rules so that you can be sure to adhere to the intent.

 

I'd recommend delving into some of the many tutorials and guides on the internet about making radio transmitters and receivers. The FCC rules permit making unlicensed transmitters as long as they meet certain guidelines. If you stay very, very low power then you are unlikely to attract unwanted attention, but it would be best to look up the rules so that you can be sure to adhere to the intent.

Good point. I wouldn't want to find a way to travel down to Colorado Springs and create an artificial Aurora Borealis or something!

 

Good point. I wouldn't want to find a way to travel down to Colorado Springs and create an artificial Aurora Borealis or something!

I have had some experience doing just that. I don't even think about using an AM receiver, as the biggest problem is the transmitting antenna size. Granted the receivers mostly use a ferrite rod for the antenna, it works fine for receiving because the AM stations generally radiate a lot of power. But to use a ferrite rod for transmitter is not very efficient, in my opinion.
FM band transmitters use much smaller antenna's and as a result they are more efficient and unlicensed operation power will generally cover a typical home and yard. The FM receiver has an oscillator in which the frequency is near the standard US 88-108Mhz frequency. To change the frequency is generally very easy. Modulating the oscillator or adding a RF amplifier and then modulating the amplifier to create the transmitted frequency can be a little difficult, but not impossible. There are number of Kits available to build FM transmitters. In my opinion some are good and some not so good.

 

Its a lot of effort to make a stereo FM transmitter, especially when you want to abuse a receiver.
Most are IC based, and to large degree the circuit inside the IC cant just be reversed.

There is a FM transmitter IC also the BA1404 from Rohm.

 

I have had some experience doing just that. I don't even think about using an AM receiver, as the biggest problem is the transmitting antenna size. Granted the receivers mostly use a ferrite rod for the antenna, it works fine for receiving because the AM stations generally radiate a lot of power. But to use a ferrite rod for transmitter is not very efficient, in my opinion.
FM band transmitters use much smaller antenna's and as a result they are more efficient and unlicensed operation power will generally cover a typical home and yard. The FM receiver has an oscillator in which the frequency is near the standard US 88-108Mhz frequency. To change the frequency is generally very easy. Modulating the oscillator or adding a RF amplifier and then modulating the amplifier to create the transmitted frequency can be a little difficult, but not impossible. There are number of Kits available to build FM transmitters. In my opinion some are good and some not so good.

Do you have any good recommendations on books or websites that will walk a person up from scratch through making several flavors of AM and FM receivers and transmitters with a focus on understanding the circuits involved (and I'm talking about discrete circuits, not IC-based circuits)?

 

most of the am or fm radios available today use integrated circuits, which are unfortunatly for you not usable for transmitting. older tube or completely transistorized radios might be converted using some of the parts.

 

most of the am or fm radios available today use integrated circuits, which are unfortunatly for you not usable for transmitting. older tube or completely transistorized radios might be converted using some of the parts.

Its not making much sense even for transistorized AM radio since you need particular parts, if you want to keep up with the intended mode of operation.

These still available actually contain a few small IF transformers, and a ferrrite bar which isnt a particulary good sending antenna.

discrete FM radios? Besides as DIY kit speciality, long gone from the market.

Schematics on the internet for radio circuits follow the convention to contain actual errors, and often are finicky, so if you use a different transistor, you need to change the circuit, or some voltage just will be wrong.
Or they use parts such as coils which they dont describe fully so you are also left in the dark.
A parts number doesnt help if these are long gone from the market.

 

Google FM transmitter circuit.

Here is one of many:

http://www.sentex.ca/~mec1995/circ/fmt2.htm

(Note that drawing shows the battery symbol upside down even though the polarity sign is correct.)

 

I built one. I could listen to my boys from my car. It took a while making the air core inductor.
I got the plan from http://www.talkingelectronics.com/projects/FM-BugsIntro/FM-BugsIntro.html

 

In answer to WBhan I can only say an older Amateur Radio Hand Book, Published by the ARRL. Alfacliff and takao21203 are also correct. I had forgotten what age is the present. I forget the year in which commerical transistors were first available, but I was in about the 8th grade and spent about $4.00 for a CK721, made by Raytheon. I built a audio oscillator with it.
The project posted by KLillie is good as the oscillator has a buffer amp. If the Vcc were regulated it would be great.
I appologize for any misconceived thoughts I may have generated.
Ned

 

the use of single chip am fm reciever I/c's has killed the idea of building a transmitter from a radio the parts from an older transistor radio could be used to make a modulated oscilatoreither am or fm. the newest radios dont even have a variable cap for tuning, just a built in pll controlled by the chip. back in the real old days, you culd get most parts out of a tv to build a low power transmitter, horizontal output tube, chokes, power supply and such, but now?????

 

I think the video is fake. His contraption has no antenna that I could see yet it transmits quite a distance with full quieting and low distortion and even seems to have proper eq. He just tuned to an FM station, me thinks.

 

Hello k7elp60
When I was about 12 I received a CK722 for my birthday.
I think it died when I took it out of the package and looked at it

 

I had one of those too, the leads were very brittle and broke off before I could do anything with it.

 

I had the idea last night to reverse engineer an existing transistor radio in order to transmit signals to other radios around the home such as my voice (like an intercom system) or perhaps hook up my little nextbook with an .mp3 library and broadcast the contents on an unused frequency. Has anyone else tried this and if so, would it be simple to do or very complex? I'm not sure what the FCC regulations are but I'm just wanting to putz around within the confines of my property... not become a nuisance to others or violate codes of any kind!

You should study and take the Tech ham radio license exam. If you are trying to get a signal to go across a room you could use a signal generator to do this. Just look at the FCC band plan and watch your power level. If it is music, be careful because that might get you in trouble on shortwave if it spills over onto other frequencies. But study, experiment and continue learning.