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RF rx\tx module

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trennonix

New Member
Hello,
I want to build my own RF module; i did a little search on your forum and most advice were about going and buying a commercial one

Here, in Lebanon, these things cost at least 16$, and my project requires at least 2 modules, :mad:
so, i'm looking for a cheaper alternative: building my own.

i'm looking for a freq that is not in the AM, FM range and different from 27 and 49Mhz (since most RC toys are 27 or 49)

as for range, i'm hoping for 30 meters or more

no need to mention, but the simpler the circuit , the better

thanks alot:)
 

Chippie

Member
I'd still go with the orig recommendation...Go buy one(some)
 

RadioRon

Well-Known Member
Hello,
I want to build my own RF module; i did a little search on your forum and most advice were about going and buying a commercial one

Here, in Lebanon, these things cost at least 16$, and my project requires at least 2 modules, :mad:
so, i'm looking for a cheaper alternative: building my own.

i'm looking for a freq that is not in the AM, FM range and different from 27 and 49Mhz (since most RC toys are 27 or 49)

as for range, i'm hoping for 30 meters or more

no need to mention, but the simpler the circuit , the better

thanks alot:)
How much data do you want to transfer? I'm wondering if you can make do with fairly slow data, or do you want to pass a lot of data quickly. If you are just using the RF to act like a serial port, what baud rate do you want to support and what is the lowest baud rate you could tolerate?

If $16 is too much for a pre-made one, then obviously we have to find an idea that costs almost nothing. A very simple transmitter is not too hard, you can make something with a few capacitors and a transistor oscillating at around 430MHz to form a simple pulse position modulated device much like garage door operators. Of course, we would also need a receiver and this is where it gets more difficult as the receiver is likely to require several transistors or one IC and can be tricky to build. Can you find some scrap garage door openers anywhere? Broken ones can be used to supply the receiver. This would be a lot easier than trying to build a receiver from scratch.

The range will depend mostly on what antenna you can use. At 430MHz it is best to use a full size half wave dipole or quarter wave monopole.
 

trennonix

New Member
i'm looking to send 23bits in no more than 200ms

i sent them using an RC toy, but i had to slow down the rate till it reached 4sec!
can i build my receiver out of an existing toy, and simply change the value of the inductor or the capacitor \ crystal??
and to achieve greater speeds, remove the existing IC (encoder\decoder), and send\receive my signals directly?

RC toys are cheaper than rx\tx modules :S and i got many of those just laying around

thanks johnsmith but it's not the transmitter that i'm most worried about
 

colin55

Well-Known Member
Buy a $10.00 RC car from a junk toy shop. I has up to 5 channels and outputs to drive a motor in both forward and reverse direction and a solenoid in forward and reverse polarity as well as "turbo." You can change the freq to anything you want.
 

trennonix

New Member
i don't want to use the encoder\decoder of the toy, because it gives really slow baudrates, i want to send and receive directly from the RF part of the circuitry.
i found the signal pin on both the encoder and the decoder (datasheet), but i can't seem to get any readings from the receiver

so, how do i amplify this signal (it can't even turn an LED on, transistors didn't work or i set them up wrong)??
 

trennonix

New Member
if i connect the signal pin of the encoder on the transmitter to the positive lead, should i be able to get a signal ?

on the receiver side, my highest reading under normal transmission was 20mV, but the IC detected it fine
i amplified it with 2 transistors (2n3904 to be precise) and still, it couldn't light up an LED

with the 2 transistor, without any signal being sent, the reading was at 4.5 V
with the signal, there was hardly any difference
and yes, my transistor circuit is correct, i tried it with some infrared receivers and a TV remote
 

colin55

Well-Known Member
You can light a LED with 1mV and 2 transistors. . . . . don't say your transistor circuit is correct.
 
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trennonix

New Member
if i connect the signal pin of the encoder on the transmitter to the positive lead, should i be able to get a signal ?

and fine i'll double check :p
 

s3c

New Member
Your transistor circuit will only work if the input voltage is above 3.4v (2 times 0.7v for the transistors and 2v for the led) and even then you'll kill the led instantly since you have no current limiting. Basically, you're doing it wrong, transistors can be used to amplify incredibly small signals to as high as you want.

[EDIT]

I seriously doubt that your signal level is that low and it sounds like you'll probably be better off with an op-amp or comparator but here's something you can play around with.
 

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trennonix

New Member
thanks for the advice, this my new circuit using an op-amp from the AN1358 IC

is it supposed to work? because, well, it didn't :(

Vout= Vin(1+R2/R1) -> Vout=Vin(1+100K/10K) -> Vout= 11Vin

it wasn't able to click an 8ohm speaker :confused:

now i don't know much, and i hope that i won't feel stupid after asking this
but the signal that i get from the receiver, at the input pin of the decoder is connected to a breadboard via a 20cm or ~8inche long telephone wire
could this be too much for the signal?
 

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s3c

New Member
I doubt that that's your problem. It feels like we're kinda stumbling around in the dark right now, what do you wish to accomplish by connecting a LED, are you simply trying to get a feel for the output signal?

If you are sure you have the correct signal from the board try to find some specifics, what is the minimum and maximum signal levels? Or better yet, if it's coming from an IC what is the Part code?
 

trennonix

New Member
for now, yes, all i want is the feel of the signal, but once detected i want to connect it to my own decoder (a PIC micro), like i said, i want to use it as an Rx\Tx module, i'm only intereste in the RF reception part of the circuitry.

The IC is a decoder, so i'm not getting anything from the chip, i'm high-jacking the wire that goes to the input pin of the decoder.

The datasheet is in chinese :S, but gives the pinouts so i was able to identify the signal pin, and it gives an example of an RF receiver in which the chip can be implemented.

But, the circuit that i submited earlier (op-amp), is it correct?
 

s3c

New Member
If the voltage you measured is 0.2V it's either an analog signal or fast switching digital signal, either way the LED isn't going to tell you anything, I'm assuming you're doing this on a budget so try the following, strip an old mic and add some wire clips where the mic part use to be and download the following:

Visual Analyser 2009 HR

It turns your sound card into a makeshift oscilloscope, the line in port works better than the mic port and try not to over voltage it, 5v should be no problem but you don't want to blow your sound card, mine is fine but it's always possible. Hook up your circuit and you'll be able to see what's going on inside :)
 

trennonix

New Member
SWEET! well now it's almost midnight, i'll try it tomorrow morning

but you didn't say if my op-amp circuit was correct, i might need it some other time
 
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