Cheap RF modules.

[Nigel Goodwin]

I've recently started playing with cheap RF modules again, having failed miserably with them before

I'm still failing, but I've put more effort in to it this time.

This is the type of receiver:

And they 'fail' because as soon as the key is released, or it reaches the inter-frame gap, it starts sending out random rubbish'. For a transmitter I'm using a number of different PT2622 based key fobs, all of which work perfectly with the cheap Chinese relay remote modules, like this one:

So I wired the radio module on the relay module to the PIC, where it works fine - it still outputs random rubbish without any signal (as expected), but not so quickly as to happen in the inter-frame gap, and stop the decoding process.

I tried adding a wire aerial to the receive module (I hadn't bothered originally, as I was only a couple of inches away during testing), and if I get close enough it will work - but more than an inch or two, and it stops working again.

I get the same results using them with an Arduino Uno, and the usual libraries.

I'm presuming the modules are 433MHz and not 315MHz?, they were ordered as 433MHz, along with the respective transmitter modules (which are clearly marked as 433MHz), but the receive modules aren't marked in any way. I've taken the keyfobs apart just to check, and those are 433MHz as well. I seem to remember I might have a 315MHz key fob somewhere?, I'll have to try and find that, see if it's any better.

So has anyone had any luck using the cheap RF modules?

 

[rjenkinsgb]

That style receiver appears to be more commonly available as an "XY-MK-5V" (or MX-05) and there is quite a bit of data on those.

It's a minimal AM RX, so noise passes through the data slicer when there is no signal.

RF mx05v etc notes

tlfong016715Sat 16:30And I googled the schematics of the Xmit and Recv modules. I found them simple electronic circuits, without any intelligence, just blindly sends/receives high low signals. So I…

tlfong01.blog

I've never used AM ones, only the type approved FM ones by RF Solutions.

With those we sent a few 0xAA bytes to allow the data separator to settle mid-range, then if I remember right an 0xFF, 0x00 as a framing code to get the UART start bit in sync, then address, data & error checking bytes.
Any received frame of the wrong length or with any errors was just discarded.

We also used a multi repeat in the transmitters, with a kind of randomised exponential delay, two fairly quick, ~100mS then longer intervals; something like five frames in total over possibly two seconds, with the data rate 9600 Baud.

They were being used with anything up to around 30 in the same area & that seemed to work very well, no reports of failures. It was only switch states for anything from one to six switches on each transmitter so no complex data.

 

[schmitt trigger]

Nigel, a while back I experimented a little with those receivers. Fortunately for me, at the time I had access to a lab which possessed an RF generator and spectrum analyzers. What I found back then was how easily one could detune those tuned-radio-frequency (TRF) receivers. Those unshielded trimmer inductors are undiluted POS.

Try to obtain a superhet one, and one which has a crystal. like the one shown below.

 

[dougy83]

Some of the sellers of the cheap receivers claim to receive both 315 and 433MHz; the tuning is "selected" with the antenna length.

 

[rjenkinsgb]

Some of the sellers of the cheap receivers claim to receive both 315 and 433MHz; the tuning is "selected" with the antenna length.

Some of the stuff I found says it's selectable by adjusting the coil in the centre.

 

[dougy83]

Some of the stuff I found says it's selectable by adjusting the coil in the centre.

Well, that would make more sense. It could be tuned without special equipment by maximising the amplitude of the analog output (the input to the bit-slicer opamp), while the transmitter is pulsed.
If the tuning-by-antenna-length statement were true, then the receiver selectivity is garbage.

 

[Nigel Goodwin]

Well, that would make more sense. It could be tuned without special equipment by maximising the amplitude of the analog output (the input to the bit-slicer opamp), while the transmitter is pulsed.
If the tuning-by-antenna-length statement were true, then the receiver selectivity is garbage.

Well selectivity is going to be crap, it's a crap circuit - but it is dirt cheap. Check the circuit above.

Basically, I'm just playing with these cheap modules, because I've got a few of them - no particular purpose in mind. At the moment, I'm trying to use them to read from standard PT2622 key fobs, as I have a few of those.

If I wanted a specific data transfer system, I'd use HC-12 modules - and I do Much greater range, much more reliable, and a simple RS232 interface.

I would agree that adjust the coil should adjust the frequency of the receiver.

I've managed to find my 315MHz key fobs, so I'll give those a try later.

 

[Nigel Goodwin]

Nigel, a while back I experimented a little with those receivers. Fortunately for me, at the time I had access to a lab which possessed an RF generator and spectrum analyzers. What I found back then was how easily one could detune those tuned-radio-frequency (TRF) receivers. Those unshielded trimmer inductors are undiluted POS.

Try to obtain a superhet one, and one which has a crystal. like the one shown below.

As I mentioned, I'm only 'playing' with the modules as I've got some - but as I was doing an order from AliExpress today, I stuck a few superhet modules on to give a try.

I did find the 315MHz key fobs I'd got, but they didn't work at any range - I might take them to bits and check the batteries