433.92 MHz transmitters

Discussion in 'Wireless & RF Design' started by rjjenkins, May 18, 2014.

  1. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Hello

    I am hoping someone can help me with a basic problem. For a while I have been using these cheap 433.92MHz transmitters and receivers

    http://www.uctronics.com/433mhz-rf-transmitter-and-receiver-kit-arduino-project-p-381.html

    (Not necessarily these specific ones but the generic ones which are sold everywhere.) I've used them with Arduinos and also with the ATTINY85 and generally there has been no difficulty. But I want to use a smaller transmitter and found these ones which are tiny

    http://www.mouser.com/ds/2/238/RCT-433-UTRDatasheetrev1_0_0-8601.pdf

    It is RCT-433-UTR

    I thought I would just be able to substitute one for the other, but I am getting no response from the receiver when I put in the new transmitter. I have the right voltage (3.5V) from a battery and the right length antenna and I have double checked the pinouts. If I simply take the data input high on the tx, there is no obvious response on the rx data output. It makes me think perhaps there is some fundamental incompatibility with something like modulation system, but I am very ignorant of such things.

    Any help appreciated.
     
    Last edited: May 22, 2014
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the mkodulation schemes are different. one is afsk and the other a orm of am.
     
  3. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Thanks - I am still puzzled though. The first one (the cheap ones sold everywhere) use ASK modulation. The Radiotronix one uses OOK which I thought was the same as ASK (or at least that ASK is a kind of OOK). I don't see any reference to AFSK.

    Might this be relevant? "Data should be inverted when using the transmitter with a UART". I am using an ATTINY85 with the virtual.wire library. The ATTINY doesn't have a UART but should the data still be inverted?
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    From my experience with the cheap ones, they use on-off keying. At least with a 0-5v digital input to the transmitter they do.

    You might get some ideas from this page;
    http://www.romanblack.com/RF/cheapRFmodules.htm
    :)
     
  5. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    OK, I've read your page and I think I understand the principles. Looking at the Radiotronix datasheet, their module seems to operate exactly the same way. i.e. the transmitter simply generates a carrier wave at 433.92 MHz. When the data input is high, the wave is transmitted; when the data input is low, there is no carrier. Everything else is done in software?

    So, if I get an scope and attach it to the antenna output pin, I should be able to detect a 433.92 MHz signal when the data input pin is high?
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    use a germanium diode conneced to your scope probes. that way it will show the detected waveform and you wont need a 433 mhz rated scope. just hook the diode to your scope probe and ground and hold it near the antenna.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Correct. As the serial USART is normally HI when there is no data, it is a good idea to turn the USART off when not transmitting and manually set that output pin to 0v. Then you turn the USART on, (pin goes HI) and wait a few mS before popping data bytes into the USART.

    Yes, but not many people have a 500 MHz capable scope! Alfacliff suggested a great way to test the transmitter.

    I preferred to test both the transmitter and receiver together, by putting a 1kHz squarewave from a 555 timer (or signal generator) into the transmitter, and the same square wave should appear on the receiver output when you scope it. Any old scope will do.

    The benefit of testing both together is you can also be testing range, antenna lengths/angles, and even tweak the tuning coil on the receiver if needed (although I found them to be already tuned pretty good on the 5 TX/RX pairs I got from ebay).
    :)
     
  8. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Yes, I hadn't thought about the scope capabilities. I don't own a scope but have access to one - don't know its capabilities.
    The germanium diode idea is basically the old crystal set thing, isn't it? It will show if there's a signal but it won't show the frequency. But it would be a start. I wonder whether the current would be enough to pick up on a multimeter.
    Yes I will try the square wave idea.
     
  9. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    OK, with the germanium diode method, I can pick up a quite reasonable signal using the multimeter on the cheapo transmitter, but nothing whatsoever on the little one. I guess the datasheet pinout diagram could be wrong but it seems unlikely. Stumped.
     
  10. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Yes! The pinout diagram is wrong! Or at least inconsistent. In case anyone stumbles across this one day, the antenna is pin 3 and the data is pin 1
     
    THE_RB likes this.
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