SparkFun Forums

[dosman]

Hi, I know this is a popular topic . So I've been using two pairs of 434MHz TX and RX modules (WRL-08946 and WRL-08949) without any issues (I run them at 1200baud since that is more reliable and gets better distance). I've used the KLP Sparkfun tutorial and have moved on to using a small RFDriver library and it works great, I get reliable data transmitted across the link with zero noise.

So I ordered two sets of 315MHz modules (two TX/WRL-08945 and two RX/WRL-08947) a while back from Sparkfun thinking it would be an easy way to have another transmitter operate in the same vicinity without interference. I just tried them out in my circuit and I'm not receiving anything. The 315's and 434's are pin-compatible and I just swapped the boards on both sides (TX & RX), receiving side no longer gets data. Using a radio scanner I can clearly hear the transmitter signal at about 315MHz, but I'm not getting data into my Arduino on the receiving side. Connecting an oscilloscope up to the data output line on the receiver board proves that it's only outputting a flatline 5V, no data. Normally you get noise when there is no signal, but I'm not even getting that. Tried both sets of 315MHz boards, same thing with either set. I've tried running them at 4800, 2400, 1200, and 300 baud to no affect. The last thing I just tried was increasing voltage to the 315MHz TX from 5v to 9v thinking that perhaps these boards just require a stronger signal to get anything across. This has not changed anything, although my scanner can pick up the 315MHz TX further away at 9V.

Again, the 434's work fine in the exact same circuit, the evidence seems to lean towards both the 315MHz RX boards being toast. Anyone have something else I can try? Maby there's something I've still missed in this process.

Thanks!

[another-dave]

You know the antenna lengths will be different for 315 than 434MHz. The data sheet for the modules says to use 23 cm(about 9 inches) for 315 and 17 cm(about 6.7 inches) for 434. If both the TX and receive module antenna are incorrect in length then they will each contribute to reducing the range. Also make sure both antenna's are in the same polarity (both pointing up/down) rather than one up/down and one horizontal(cross polarized). Cross polarized antennas greatly decrease range.

[cpolley]

I encountered this problem a couple of years ago, and unfortunately I never managed to resolve it. You can check to see if the pinout has changed for some reason (it's happened before - pin assignments changing or RX-enable pins suddenly appearing), but I'm afraid you might be out of luck.

Also with getting a refund; SFE are pretty good but understandably don't like taking back merchandise that you've powered up.

I would be interested to hear from anybody else who has ordered these and whether they had any problems using them ... I wouldn't have much confidence trying to buy more.

[dosman]

Thanks for the replies. I talked to customer support briefly and they suggested a refund. But before I do that, I've got more 315 and 434 boards coming. If the additional 315's I've ordered still don't work I'll try adding the specific length antenna, but I wouldn't expect that to be required when the boards are 3" away from each other.

I will follow up with more information once the new parts I ordered come in.

[stevech]

You know the antenna lengths will be different for 315 than 434MHz.

Insignificant difference given there's no "real" dipole with careful attention to VSWR due to the small size.

[SFE-Nate]

I wouldn't expect that to be required when the boards are 3" away from each other..

You don't need an antenna for the 315MHz radios when transmitting at that short of a distance through air.

I think (don't quote me on this...or do I don't care) the 434 MHz receiver doesn't float at Vcc but has a logic one as Vcc and a logic zero/idle state as gnd. Except that wouldn't explain your problem as you said you peeked at the output of the 315MHz receiver using a scope, unless you had the time division is WAY off on your scope...

Also with getting a refund; SFE are pretty good but understandably don't like taking back merchandise that you've powered up.

yep

...the 434's work fine in the exact same circuit, the evidence seems to lean towards both the 315MHz RX boards being toast.

That is what I am leaning towards. We don't build these here at SparkFun. We buy them from a supplier. There is a chance you got some toasted modules.

[languer]

I saw a similar problem with some other modules sometime ago. The problem occurred when the manufacturer introduced a new receiver module. The older receiver module was of the super-regenerative type, and the newer module was PLL-based. Although the PLL-based provided much better performance, it had one caveat; the transmitter and receiver frequencies had to be spot-on. With the super-regen receiver, the capture bandwidth is much wider.

Perhaps you should try the WRL-08948; you will probably have better results.

Note that if this is your issue the fault is not with the receiver, but with the transmitter. Being on a PLL-based system, the receive frequency will be very stable and accurate. A free-running oscillator (i.e. the transmitter; SAW-based, LC-based, or otherwise) frequency will depend on the resonator device (in this case the SAW device), will have drift, and will be able to be pulled up or down in frequency (something not easily achievable on a PLL system).

[dosman]

Ok, so we got this figured out finally. It appears the 4800baud 315 AND 434MHz receivers don't have enough juice to drive an Arduino input directly, but the 2400baud receivers can (well, I can confirm the 2400baud 434MHz receivers can anyway). We found that disconnecting the receiver showed it was clearly outputting data on the scope, but when connecting it to the Arduino the output was pulled high (yes, the Arduino was configured with that pin as an input). We ran the receivers output through two CMOS inverter gates (4049) and that did the trick. Party on!!

If anyone is interested, we where using these to teach an Arduino RF workshop at our hackerspace. It went over very well, having more than one frequency to work with next time should make it even better .
http://www.bloominglabs.org/index.php/ArduinoRF_Class

[waltr]

Just a thought but is there an internal pull-up on the Arduino input?

Good you got it working.

[dosman]

I would say that I'm skeptical because I've found reports of people having the same issue with these boards when directly driving them from an ATMega rather than an Arduino. But having said that, looking at the schematic for the Duemilanove shows a 1k resistor tied to M8RXD/TXD, whatever those are:
http://arduino.cc/en/uploads/Main/ardui ... ematic.pdf

Assuming that is the FTDI chip (which should be tied to those pins anyway) it may be possible to add a "disconnect" jumper to the Arduino which unhooks the RX pin from any other input source except the input header. We where using some breadboarded Arduino's in the workshop and I know for a fact we did not have a pull-up resistor on the TX and RX pins, however we where using an FTDI breakout board to power the circuit and load code. I will try powering the circuit without the FTDI breakout attached and see where that leads in the next week or so.

[languer]

Very cool that you got it working. Looking at the datasheets again, after looking at strefli3's post, and the reviewing the plots (more than a couple of times); it is not at all unlikely. The older receiver module seems to be able to drive 3 times as much current as the new module; adding a buffer makes a lot of sense.

[antlaa]

Ah, thanks dosman! After few wasted evenings I got it working with 4069 buffer.