RF transmitter/receiver help

[ronjodu]

Just received one of these.
http://cgi.ebay.com/RF-Radio-Wirele...6006750524QQcategoryZ1200QQrdZ1QQcmdZViewItem

No datasheet included. Any help would be apreciated.

I have a project with a pic 16f876 displaying the time of day on an LCD @9600baud. I'd like to send the time to the LCD wirelessly. I'm using Pic Basic Pro with a Melabs bootloader.

Thanks in advance for any help.

 

[audioguru]

They look awfully simple and cheap. I don't think their frequency will be stable. It will probably change with the temperature, supply voltage and if anything gets near the transmitter or near its antenna. :cry:

 

[zachtheterrible]

I beg to differ with you audio :lol: . Despite their simplicity, these little RF modules frequency is rock solid and they have good range. They use a SAW resonator. I have a couple of them and have used 'em in one of my projects with great results, never failed me once.

They are made for transmitting a digital signal only. I have to wonder though if there is any way of modifying them to transmit analog. They would be the perfect thing to do so with. Good range and stability. I'm going to have to see if I can do it with my 433mhz RF modules. It would make the ultimate bug:lol:

Ronjodu, if you could post your circuit that would be very helpful. I don't know much about microcontrollers yet, but I would imagine that you have a couple outputs of the PIC wired to the LCD module, is this correct? If so you will need a transmitter and receiver for every output of the PIC, which obviously complicates things a good bit.

Like I said I don't know much about microcontrollers, so i'm sure someone else might have a better wway of doing it than I.

 

[audioguru]

Hi Zach,
The transmitter and receiver have only 2 transistors. The transmitter also has that metal thing which might be a saw filter but the ad doesn't say.
The receiver might be a super-regen.
They are similar to the circuits used in cheap RC toys.

 

[Nigel Goodwin]

They look awfully simple and cheap. I don't think their frequency will be stable. It will probably change with the temperature, supply voltage and if anything gets near the transmitter or near its antenna. :cry:

They are licence free modules, and as such have to comply with the fairly strict requirements of the licence free status.

The transmitter is probably actually entirely inside the round can?, it's quite commonly done like that - and the 'round can' is small enough to fit inside a key remote.

The receiver might well be a super-regen, these are often used on the cheaper units, although for slightly more money you can get supethet ones. I 'think' the one I've got in my hand at this second is a superhet?, at least it has a fairly large surfacemount IC (30 odd pins?, too small to count!) and a crystal or filter on board.

Stability of the modules is improved by being built on a ceramic substrate (like mine), but I notice that the ones in the link are just PCB.

But they should be fine, superhet's give better range, but they are only intended for short range use anyway.

ronjodu:

You can't just send RS232 type serial data over the link, because they are AC coupled - if you invert the data at each end (so resting state is LOW - not HIGH), then it can work. But you're better off using a proper encoding scheme such as Manchester coding.

I'm currently working on just such a tutorial, based on the routines from http://jap.hu/electronic/codec.html.

If you try the BASIC from http://www.mikroelektronika.co.yu/english/product/compilers/mikrobasic/index.htm I understand it includes Manchester routines?.

 

[ronjodu]

LCD hookup

Thanks for the replies so far. The LCD I'm using is a serial LCD from Imagesco.com.
http://www.imagesco.com/articles/lcd/03.html
It works great wired to one output pin.

Hope this helps.

 

[ronjodu]

progress

Thanks for the help guys. Your tutorials are a big help Nigel, looking forward to the RF tutorial.
I've made some progress. This thing is pretty much a byte in is a byte out.
It appears as my baud rate was too high(9600). I dropped it to 300 and I get most of the characters I expect out of it. There is some garbage as well. I think the antenna may be to blame. Is there a way to tune an antenna to the transmitter?( I'll try a google.)
I've tried varying lengths of copper with with some luck.

Thanks again for the help.

 

[zachtheterrible]

I wouldn't think the antenna would be to blame, unless your receiver is really far from the transmitter. If it's just a couple of feet it will work even without an antenna.