I want to make a small bug with a range of approx 20-50m
The way i figured is to take an RF 2400bp transmitter hook up an analog to digital converter to a microphone and the digital output to the transmitter and on the reciever just the Digital to analog converter and from the analog to the headphones.
Am i correct here?
The A/D converter requires 20Mhz clock so how can i generate that without a Microcontroller?
The general idea is do-able, but you need quite a bit more bandwidth than 2400 bps if you want to send intelligible speech.
If you assume that you use an 8-bit A/D and D/A, and you add a start and stop bit to each byte, then you will be sending only 240 bytes per second. The Nyquist Theorem says that you cannot send a signal of more than 120 Hz at that rate (and that’s the theoretical best-case).
Realistically, you need a data rate that is 10 to 50 times faster than 2400 bps.
Also, you cannot connect a microphone directly to an A/D - you would need a pre-amp in between.
An example of something that already does what you describe is a wireless Bluetooth headset for a cell phone - it sends decent audio over a relatively simple digital link.
A more practical approach for a “bug” is to send an analog audio, rather than digital. An interesting experiment using those little digital xmtrs would be this:
Tie the data line to +5V all the time.
Take a microphone, a small audio pre-amp, and a couple of misc. parts and use the audio signal to modulate the power going to the xmtr module.
At the receiver side, connect the RSSI (analog) output thru a capacitor to a small audio headphone amplifier.
Pete
what range would that have? would it work? I am gathering the parts to purchase and if you know any links to an analog transmitter then please let me know. I understand that it would be something on the FM ?
saipan59:
A more practical approach for a “bug” is to send an analog audio, rather than digital. An interesting experiment using those little digital xmtrs would be this:
Tie the data line to +5V all the time.
Take a microphone, a small audio pre-amp, and a couple of misc. parts and use the audio signal to modulate the power going to the xmtr module.
At the receiver side, connect the RSSI (analog) output thru a capacitor to a small audio headphone amplifier.
Pete
Yeah, nice one!
Kind of waste of technology 
but it’s a thing that could work.
The thing is that FM transmitters are illegal in most countries so i would want something small that could run on 3V on a distance of 50m(maybie a bit more) so digital would be the way to go
would the Nordic 2.4 G tranciever do the job? th speed on that is 250 kbps
Here's an example:any links to an analog transmitter then please let me know
http://www.jameco.com/webapp/wcs/stores … tId=117604
Another approach is to go to Wal-Mart and buy the cheapest baby monitor. Older ones operate at 49 Mhz - not sure about newer ones. It will come with a matching receiver…
There are also various things in the toy department, including little “walkie-talkie” type gadgets (also 49 Mhz).
The Nordic type of thing could certainly do it (something very similar is inside a wireless Bluetooth headset). But as I mentioned before, there’s work to do to add a mic preamp, an A/D, a D/A, a microprocessor, etc. It isn’t worth the trouble unless you need encrypted transmissions.
Pete
these FM transmitters operate on atleast 9V batteries and are quite big, i need something with a size of a coin. so SMD components must be used and a small Clock battery 3V
So to make it more complete, i would need a A/D Autio converter 24 bit a microcontroller and a RF tranciever . Can i do it without the Microcontroller to save space?
Am i correct here?
also if i would need the microcontroller should it be also 24 bit? of could i use something like pic 16F88?
i need something with a size of a coin …
So to make it more complete, i would need a A/D Audio converter 24 bit a microcontroller and a RF tranciever . Can i do it without the Microcontroller to save space?
Am i correct here?
also if i would need the microcontroller should it be also 24 bit? or could i use something like pic 16F88?
Re: Coin-sized bug. A ‘security company’ can sell you a ready-made bug that is small and effective (but expensive). Building a device that small using digital technology yourself would be very challenging, even for an expert. An analog solution is your only reasonable option.
Re: 24-bit. You don’t need 24-bit audio. 8-bit audio would be adequate for just listening to voices, if you have a reasonable sampling rate (say 6-8 Khz).
Re: Microcontroller. Yes, I’d say you need an MCU if you’re doing digital audio. A non-MCU approach is possible, but probably harder to do.
Search the web for something like “FM wireless mic schematic” - I bet you can find something that can be built very small and cheap.
Pete
i’m willing to try and do it , but just to make sure, i’m going to use the 2.4G tranciever with an A/D audio tranciever and a small pic, that should do it, yes?