# 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$, and my project requires at least 2 modules, so, i'm looking for a cheaper alternative: building my own. i'm looking for a freq that is not in the AM, FM range and different from 27 and 49Mhz (since most RC toys are 27 or 49) as for range, i'm hoping for 30 meters or more no need to mention, but the simpler the circuit , the better thanks alot #### Chippie ##### Member I'd still go with the orig recommendation...Go buy one(some) #### blueroomelectronics ##### Well-Known Member Since the popular units are 433MHz it would cost more to DIY than buy a premade unit. #### RadioRon ##### Well-Known Member Hello, I want to build my own RF module; i did a little search on your forum and most advice were about going and buying a commercial one Here, in Lebanon, these things cost at least 16$, and my project requires at least 2 modules,
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?

#### 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)??

#### colin55

##### Well-Known Member
The signal will be above 700mV. Just ampplify it with a transistor.

#### 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

#### trennonix

##### New Member
here's an ILLUSTRATION of the transistor circuit and the circuit diagram, plz do check them

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• schem.JPG
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• transistor.JPG
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#### 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.

#### Attachments

• trans.JPG
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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

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?

#### Attachments

• opamp.JPG
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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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