Wireless link at 433Mhz doesn't work

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BlackBishop64
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Posted by BlackBishop64: Thu. Jan 29, 2009 - 09:35 AM
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Hi,

recently I've bought a pair TX/RX radio link from Quasar (QAM-TX1 & QAM-RX4), I was prettey excited but my happiness quickly go away when I checked that the receiver gets nothing.

If I connect the datainput of the transmitter with the dataoutput of the receiver with a cable, the PC receives data, so I am sure that all the system works properly but the radio link.

If I check the datainput of the transmitter with a logic probe (low/high) it seems ok.

If I check the dataoutput of the receiver with a logic probe is is always high, and keeps always at this state.

I don't know how to progress :( :(

Thank you for your help,
BlackBishop

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Learning and Information, General Electronics
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Plons
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Posted by Plons: Thu. Jan 29, 2009 - 11:02 AM
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Sounds like a dead receiver. Pictures ? Datasheets ? Schematics ?

But even with a working receiver you will need to do some extra software-efforts as the receiver outputs noise as well. It *can* be done, but it's not simple.

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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newbie123
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Posted by newbie123: Thu. Jan 29, 2009 - 12:10 PM
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Are you measuring the receiver module without being hooked to the MCU just by itself, maybe on a breadboard or something? What MCU are you using? Can you post a snippet of the receive routine from your code?

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BlackBishop64
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Posted by BlackBishop64: Thu. Jan 29, 2009 - 05:40 PM
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Hi,

it seems that I need to use a Manchester coder/decoder. I didn't know that and I've conected the TX directly to the AVR and the RX directly to a TTL/RS-232 converter.

There are ICs for doing that but I imagine that it is also possible to implement al MCU level, but I don't know how.

Things are more complicated than I though :(

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Plons
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Posted by Plons: Thu. Jan 29, 2009 - 06:17 PM
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Google around a bit. Barnacle (fellow freak) wrote a thread about Manchester coding.

But it's still a mystery why your receiver doesn't give a signal when the transmitter is sending. For the time you don't have Manchester going, let the transmitter send ASCII hex 55 ("u"). The receiver must give an output !

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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leon_heller
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Posted by leon_heller: Thu. Jan 29, 2009 - 06:18 PM
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Manchester coding/decoding can be done in software.

Leon

Leon Heller G1HSM

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BlackBishop64
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Posted by BlackBishop64: Thu. Jan 29, 2009 - 09:39 PM
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Hi, I've sent some pictures of my project:

Attachment(s): 

Image icon A AT90S2313 at the right, on top of it the TX and a Voltage regulator of 3,3v. Antenna of 15 cm is close to the transmitter. At the left a TTL/RS-232 converter for direct connection to the PC, not used for the moment.
Image icon The receiver directly attached to a TTL/RS-232 converter. Now I know that I need a Manchester decoder so I have to remake this board.
Image icon Another view of the uC and the transmitter
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bobgardner
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Posted by bobgardner: Thu. Jan 29, 2009 - 09:51 PM
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I still dont understand why manchester. I hear its about symmetrical waveform with steady DC level. Why this matters I dont know. You should be able to make it work by sending chars like 0x55 0xaa 0x33 0xcc. They have symmetrical waveform with no dc drift.

Imagecraft compiler user

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Plons
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Posted by Plons: Thu. Jan 29, 2009 - 10:02 PM
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Bob wrote:

Quote:
I still dont understand why manchester.

OP wants a wireless communication. TxData from the Usart can contain characters that have subsequent 0's or 1's. And the transmitter doesn't like that. Think of it as if it's AC-coupled. And same story at the orher end, the Receiver.
One more problem occurs when the transmitter stops sending: the receiver will crank up it's AVC, and noise is the result (on the output)
I am waiting for CC2500-modules to come in. Hope those make my life easier :)

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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BlackBishop64
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Posted by BlackBishop64: Thu. Jan 29, 2009 - 10:07 PM
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I've checked again and again power and the pcb and everything is right.

I've written a small program sending the "U" (0x55) continuously and I've made some test with my logic probe, even without manchester decoder the receiver should show something! but It seems to me that something is wrong with it.

Attachment(s): 

Image icon Logic test at the input of the transmitter, logic probe shows both 0 and 1
Image icon Logic test at the output of the receiver, logic probe shows nothing at all!
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Plons
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Posted by Plons: Thu. Jan 29, 2009 - 10:16 PM
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Most likely your receiver broken. Like I wrote in my first response.

Does the receiver datasheet mention the need for a pull-up on the receiver output ? Mine DOES need it, and I need to buffer it before it can drive LEDs (which are great for these checkings btw ;) )

You're right with "U" instead of "u" ... my mistake.

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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newbie123
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Posted by newbie123: Fri. Jan 30, 2009 - 03:47 AM
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Is the pin on the avr set as an input or output? (that is, if you are using bit-bang code for reception)

There are two data pins on some of the receiver modules (depending on which one you have.)

The ones from Laipac have a digital output pin and a Linear Output pin. Be sure you have the digital one connected and the other one not connected to anything. This is according to their datasheet. (yours may be different.

Something else you can try:
Remove your MCU from the socket and then check for activity on the data line. These things will always be picking up some junk data.

How are you powering the module?

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ka7ehk
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Posted by ka7ehk: Fri. Jan 30, 2009 - 05:33 PM
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Some of these low cost receivers have a MINIMUM data rate. They won't respond to a steady carrier or if the bit rate is too low. If the bit rate is not high enough, they may not work properly with a long string of 1's or 0's (which you can get with certain characters). Manchester helps with THIS case but there are other encoding schemes that have less of a performance hit (for a manchester BIT rate of X, the data rate is X/2).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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BlackBishop64
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Posted by BlackBishop64: Wed. Jul 15, 2009 - 01:46 PM
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ka7ehk, you are right again!, after some investigations (now with an oscilloscope), I discovered that my receiver is very sensitive to the baudrate, it won't work if it is too high or too low!

Now, my problem is other, in normal conditions the signal at the receiver is pretty unstable but, when I touch the receiver's antenna, signal is clear and works like a charm, What is wrong with this? Is my antenna not adequate?

I send 2 pictures touching and not touching the antenna with my fingers. Upper signal is the input of the transmitter, bottom signal is the output at the receiver.

Thank you very much!

Attachment(s): 

Image icon Upper signal is the input of the transmitter, bottom signal is the output at the receiver touching the antenna with my fingers.
Image icon Upper signal is the input of the transmitter, bottom signal is the output at the receiver in normal conditions
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theusch
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Posted by theusch: Wed. Jul 15, 2009 - 02:29 PM
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You simply need to ship a finger with each unit. ;)

Quote:

Is my antenna not adequate?

The people that really know will have to comment. With the finger-touch you are adding more antenna through a high-impedance link. You also add capacitance at the antenna which will dampen reflections?

I guess the first thing is making sure that the antenna is the correct length for the frequency. (from the pictures I cannot make any judgments)

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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BlackBishop64
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Posted by BlackBishop64: Wed. Jul 15, 2009 - 03:00 PM
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jajaja, I knew someone were going to say that joke ;-)

The antenna is 15,5 cm (around 6 inches) for 433Mhz transmission, directly soldered to the PCB, there is a picture at the 7 post of this thread (the second one)

Thank you!

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davef
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Posted by davef: Wed. Jul 15, 2009 - 09:01 PM
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The proper length is one issue. Whether or not the antenna "works" properly with whatever groundplane that is around, is another thing. Then we could look at how well the antenna is matched to the output of our transmitter.

What about the antenna on the RX, ditto to all the above.

I'd suggest the ARRL Antenna handbook or even the ARRL Handbook as a good starting place for some basic antenna theory.

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