hey guys, a while back I posted a thread about sending an audio signal across my dorm. In that project, I used a IR LED to send the signal and it was received by a IR photo-resister. It worked, but with very limited range (no more than 5 feet). I'd like to revisit this project but I want to stay away from IR tech as the range is short and the receiver must be within the line-of-site or the transceiver.

Now, the area of circuitry that I really haven't explored yet is wireless technology. I can figure out the circuitry (I believe) as far as converting the signal from an audio source and interpreting it on the receiver's end, but as far as the transmission in concerned, I need some help.

The whole premise is simply to take a single from an audio jack (whether it be from a PC or stereo or whatever) and send it across a room (range should be minimum of 20 feet, but 30-50 would be nice) to a receiver and play it through a small speaker.

I have this link:
http://www.hobbyprojects.com/T/transmitter_circuits.html
but I really just don't know what I need for this application. I'm thinking cheaper the better (as this won't be a one-time unit) with a minimum range of 20 feet.

I'd appreciate any advice!! Thanks guys!! -Joe

 

If you Google for "simple AM radio", or FM, you'll get several options. Note that you're edging towards a meeting with the FCC if you start stepping on signals used by your neighbors.

 

wayneh: I think I remember reading something about that in "101 spy gadgets for the evil genius". I'll reference back to that, but how would I avoid trouble?? Thanks!!

 

Hey guys, been doing some research and found a few schematics that I will build and play around with and see if it works for my needs.

Transmitter: http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html

seems simple enough. But doesn't have room for change/fine-tuning (variable cap or variable inductor) I believe tunes to about 1000 kHz

Reciver: http://www2.eng.cam.ac.uk/~dmh/ptialcd/trf/trf.htm

Also seems pretty simple, and has room for fine tuning (both a variable inductor and variable capacitor. Has a range from 400 to 1075 kHz

Does anyone think that these are NOT compatible?? That's really what I don't know about this sort of thing. Can anyone verify that this all checks out and will work for about 20-50 feet WITHOUT interference??? THANKS! -Joe

 

I've been looking to build that same transmitter (you should correct your post, it's not a transceiver), but haven't tried it yet. I've asked here if the transformer might be replaced with a capacitor to couple in the audio signal. An obvious problem with the design is that it puts a square wave onto the antenna. So it should be generating infinite harmonics all over the place. No idea how big a problem that might be.

 

oh wow, how would you combat that potential problem?? I guess you'd use like a 200pF cap in parallel with the transformer (on the crystal-side)??

Also, on the receiver...where does the Antenna go???? (i'm looking at the resonant LC circuit) What about the power? That circuit looks incomplete.

 

I'll be "combatting" it by ignoring it! I'm hoping for a range of only a few feet on the main carrier. In fact I'll have to "turn it down" if I get more than that, since that would defeat my purpose (finding a buried wire). So I'm thinking that if the carrier itself is weak, the harmonics will be even weaker. And, I'm hoping I won't need to use it more than a few minutes. They can't catch me that fast.

 

well if the range is only a few feet, nobody could catch you. Do you think that transmitter could be used for decent sound quality for wireless speakers???

But what about the receiver...where does the Antenna go???? (i'm looking at the resonant LC circuit) What about the power? That circuit looks incomplete. Do you have a link to a decent receiver circuit?? How's the circuit at the bottom of this link???
http://www.mikroe.com/old/books/keu/02.htm

 

Do you have a link to a decent receiver circuit??

No, but I have a drawer full of old AM radios. I'm only sending a tone, maybe two, so I'm optimistic it will work. I doubt it would give good fidelity for wireless speakers. My wireless headphones use a hugely more complicated circuit. The audio transformer in the design is only rated down to 300 Hz. And I doubt that modulating the power to the oscillator will give a linear response at the output, since it was never designed to be an amplifier. But as I said, I haven't tried it.

 

well then what do you think it would take to make decent wireless speakers?? Whenever I search, I find these really ghetto circuits, like people taking apart wireless headphones and adding a amp-circuit and whala. But I need to make my own circuit from scratch. Any direction you could point me in? I feel all the research I did with these simple radios that past day has been a complete waste now...

 

well then what do you think it would take to make decent wireless speakers?? Whenever I search, I find these really ghetto circuits, like people taking apart wireless headphones and adding a amp-circuit and whala. But I need to make my own circuit from scratch. Any direction you could point me in? I feel all the research I did with these simple radios that past day has been a complete waste now...

The ghetto approach makes good sense. Building a high (or even medium) fidelity RF transmitter and matching receiver is not trivial. Since these devices already exist and can be purchased for far less than it will cost to DIY, the make-or-buy decision leans heavily towards buy. You say you need to start from scratch. To me, that constraint is what "makes no sense" without more explanation. If you want a project and want to learn, that's a perfectly good explanation. But many that post here just want the shortest path to the destination.

 

If you want a project and want to learn, that's a perfectly good explanation. But many that post here just want the shortest path to the destination.

I'm all about learning my friend Believe me, these skills pay off