:: FM Trancieve Device Help ::

[suby786]

I have found a module that is a tranciever, that takes SERIAL INPUT and outputs SERIAL data on the outputs. It includes all decoding ONBOARD which converts the data using manchester coding, so i just supply it with SERIAL DATA... which is EXACTLY what i need in one package, BUT

the price in the UK seems a bit pricey, £18 per module, i need three @ the same frequency (1 x master and 2 x slaves)...flow of data is controlled by the MASTER hence i shouldnt get conflict as im using same frequency..

The modules i have looked around for, alot do not state SERIAL DATA interface, if i was to purchase an encoder/decoder chip, what could i then interface too? any FM module? or does it still need to be able to accept "UART/SERIAL data"?

The data sheet of the 1 i want to use is attached, as i say £18 is the cheapest, if i can get sub-modules that come up to £10 then i will probably look into that, if not much less that £18 i might aswell get this all in 1 package

 

[evandude]

I don't think you're going to do too much better than that price for transcievers. The cheapest ones I've ever seen are in about the same price range. The FM TX/RX part of the thing is the hard part, so using some encoder/decoder chips and trying to build the rest isn't going to make your life much easier. implementing that kind of FM TX/RX circuitry isn't exactly trivial; it only looks easy because these modules use pre-made circuits that are mounted inside neat little metal cans, but as far as I know these little metal can circuits are proprietary and NOT something you can buy separately. If you were to see all the same circuitry laid out with discrete components on a PCB, you'd probably realize why they cost what they do.

I have a similar FM data link (it's one-way, but looks to be the same basic design) and it's the cheapest construction I've ever seen... the board is crappy paper phenolic, and the pins on the board are not much better than strips of tin foil, they're so flimsy... so I definitely get the impression that they're already producing them for about the lowest price they can manage...

 

[suby786]

so at the end of the day, the £18 all in one modules seem to be good value for money,

ideally i planned to make a serial-dongle... with a DB9 connector on it, a mAX233A chip and 1 FM TRANCIEVE device and this would be on a small PCB... like a USB dongle but for SERIAL.. but where would i get the POWER from lol.

Im using a USB->SERIAL dongle... and as USB has +5 and 0V i was thinking of tapping off this but i dont think i can.. the 9 pins of a DB9 connector doesnt include +V lol

if any1 has any other ideas of creating my FM trancieve link, please let me know

 

[tunedwolf]

Given that a simple radio telemetry set-up is only on one frequency, no matter what you send to the module, the data has to be sent as a serial bit stream, encoded or otherwise. I have never used any simple radio module that had a parallel to serial converter onboard, but I dare say they exist. £18 is about the going rate for a module of this nature, but then it does have all the goodies built in. If you were to use ordinary pairs of receiver and transmitter modules, and handle the level shifting and manchester encoding and decoding yourself, you could probably save yourself a few quid, but if this is for a one off product, I would just spend the £18 and save myself the hassle.

 

[suby786]

Hmm you've given me the ammunition to go to my lecturer and propose he gives me the budget to buy three of these modules lol..

You are right thought it is less hassle, more likely to work. I will still need the level shifter on the PC <-> FM side of it... as the PC only accepst RS232 voltages but on EACH slave, i wont need ANY level shifters? as DATA OUT will be TTL SERIAL DATA?

 

[tunedwolf]

Correct, sort of, the radio data module is sending and receiving data at 3V3 levels, because of it having it's own onboard 3V3 regulator, so you will need to take care of proper logic levels. Perhaps adding a pull-up resistor is all that's needed. Only the PC end will require level shifting. I wouldn't bother using the standard 232 chips, unless you use a variant that no longer uses the external capacitors, it's just a waste of board space in my opinion.

 

[suby786]

im using the MAX233A which requires no external caps. The PC MASTER device should be REALLY SMALL, DB9 connector, MAX233A directly connecting to this module.. thats all... aiming it to look like a small USB dongle in a sense... except i have NO IDEA where im going to get +5v from

any ideas?

thank u for reminding me about the 3v3, i would have been baffled, a pull up to 5v should be ok on the DATA OUT on ALL SLAVES/MASTER? or could that not work?

 

[tunedwolf]

A pull-up should be ok on the data out of the radio module, assuming that it is only an open collector output, most of these small modules are o/c outputs.

The 5V line can come from an old USB A-B cable with the B end cut off...just insulate the Green/White wires (data +/-) and use the red/black pair (+V/Gnd) that way you have access at the PC to a nice clean 5V supply without having to hack anything together...If you can't spare a USB port on the PC, steal it from the Keyboard port using a hacked 5pin/PS2 adapter cable. Make sure you have good filtering/decoupling on your board to avoid spurious emf/rf being sent back into the PC.