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

 

this link may give you some ideas.
SIMPLE FM STEREO MULTIPLEX ENOCDER CIRCUIT

 

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

__________________
Eccentric millionaire financed by 'er indoors

 

Since the popular units are 433MHz it would cost more to DIY than buy a premade unit.

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com/

 

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.

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RadioRon

 

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

 

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.

 

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)??

 

The signal will be above 700mV. Just ampplify it with a transistor.

 

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

 

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

 

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

 

here's an ILLUSTRATION of the transistor circuit :P and the circuit diagram, plz do check them

Attached Thumbnails

   

 

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.

Attached Thumbnails

 

 

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?

Attached Thumbnails