TX/RX modification, Strange behaviour

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TX/RX modification, Strange behaviour

Postby Nelix » Tue Sep 05, 2017 7:44 am

Hi All

I hope I'm posting this in the correct forum, it's RF related so I believe it's correct.

I have a transmitter and receiver combo, basic stuff really, the transmitter is a single button key fob (Similar to Garage Door opener) and the receiver basically has a relay that is activated when the transmitter is pressed.

The problem I had was that this combo was using 315Mhz, which is illegal to use here in the UK as it is a reserved frequency. My thinking was to swap the small capsule on the transmitter for a 433Mhz capsule and swap the RX board on the receiver, again for a 433Mhz board. This was simple enough to accomplish and appeared to work fine, however after putting the TX board back in its case, it stopped working so I removed it again and it was working again....... anyway, to cut a long story short, I discovered that the TX side would only successfully fire the relay if my finger was touching a small surface mount resistor on the back of the TX board.

My question is how do I fix this so that it works without my finger being there.

If you need more info or images of the boards in question then please let me know, I do not have a circuit diagram or a schematic.

Cheers
Nelix
 
Posts: 3
Joined: Tue Jul 25, 2017 3:58 am

Re: TX/RX modification, Strange behaviour

Postby Valen » Thu Sep 07, 2017 3:31 am

Pictures are probably the best. Since you are using it at a different frequency it could be that the present geometry/part distribution has detuned the antenna circuit. Or your finger on that resistor makes you a better antenna than what it previously had. I don't know much about RF either. But others may have better responses if they see the details.

Also make sure the antenna doesn't touch any metal parts while you have everything closed up. And receiver and transmitter too close together is usually worse than a few feet apart.
Valen
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Re: TX/RX modification, Strange behaviour

Postby Nelix » Thu Sep 07, 2017 9:43 am

This is the transmitter:

http://partyknights.org.uk/Random/RX-TX/122444493.jpg

this is the rear of the board, the resistor in question is circled:

http://partyknights.org.uk/Random/RX-TX/122452172.jpg

And this is the receiver unit:

http://partyknights.org.uk/Random/RX-TX/154418393.jpg

This works on 315Mhz so I thought I would buy some of these:

http://partyknights.org.uk/Random/RX-TX/RX-TX.jpg

I replaced the SAW resonator, shown on the small board, with the resonator on in the transmitter. The larger board I removed the one on the receiver (it's the board that stands vertically) and replaced it with the 433Mhz board in the last picture.

I have had a play and discovered that if I put a 220K Ohm resistor in parallel with the 270K resistor in the picture then I did not have to touch the board and everything works well. with this in mind, I purchased some surface mount 120K resistors and replaced the 270K and it works a treat, just got to find out if it is actually working on 433Mhz as an unmodified transmitter will energise the relay but will not de-energize it.

Hope this helps

*EDIT* For some reason the images would not display properly so I have just put a link it to them
Nelix
 
Posts: 3
Joined: Tue Jul 25, 2017 3:58 am


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