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

 

Perhaps there's a poor solder joint at that resistor, or the resistor is cracked? Try re-soldering it.

 

I'll give that a try but this was working fine prior to the modification and I never when near this particular component whilst doing the work, I'll report back after I have tried this.

Any other ideas anyone?

Cheers

 

Does that resistor connect to the antenna by chance? Is the antenna still connected?

 

Does that resistor connect to the antenna by chance? Is the antenna still connected?

I've attached an image showing the resistor in question on the board.

 

Oops. I misread. You said Tx and I was thinking Rx.

So is the antenna properly connected at the Rx end?

Oh, and are you sure the battery is good in the transmitter?

 

Batteries are new in the Transmitter and receiver. Antenna is attached as well, it's just really strange the transmitter only works when the resistor is touched while pressing the button, the little light on the transmitter still lights but the relay on the receiver does not activate. I'll try re-soldering it in a little while and report back.

 

I doubt that it is a soldering problem. I think you are increasing the antenna size when you touch the resistor and, therefore, increasing signal strength.

My guess is that you can improve the operation by touching a lot of different points in the transmitter... If so then the problem might be at the receiver.

 

This is why I started with:

https://m.banggood.com/DC-12V-Singl...tch-Self-Locking-p-997084.html?rmmds=myorders

However it works on 315 MHz so I bought these:

https://m.banggood.com/433Mhz-RF-Tr...no-ARM-MCU-Wireless-p-74102.html?rmmds=search

I removed the small circular canister from the TX unit and the separate TX board and swapped them and the removed the board that sits vertices on the RX unit and replaced it with the separate one in the second link above.

I realise this is probably a tuned circuit for 315 but was hoping that as I don't need a big range for what I'm using it for, a few metres at most, it would work.

 

Are the transmitter frequencies crystal (Or SAW) controlled or are they just using capacitor and inductor to set the frequency ? If they are not crystal controlled the capacitance of your hand could be changing the frequency.

Les.

 

Are the transmitter frequencies crystal (Or SAW) controlled or are they just using capacitor and inductor to set the frequency ? If they are not crystal controlled the capacitance of your hand could be changing the frequency.

Les.

I have no idea which it is, I'm only an amateur with basic electronics knowledge who likes to tinker, some would say that's the worst kind lol. What your saying sounds very possible regarding the capacitance of my hand, it there anything I can do to fix this?

This is the other side of the transmitter it it is any help. The circular canister at the top right is what I changed.

 

I wrote my post be I saw the links to the pictures of the devices. (I forgot to click the post button. Which I pressed later when I returned to my computer.) From the picture of the transmitter I think it's frequency is controlled by a SAW (Surface acoustic wave.) device so it would not change frequency. So my theory is wrong.

Les.

 

That's a shame as I thought we might be on to something.

I wonder what's causing this strange behaviour then!

 

Morning all.

I've been doing some reading in to SAW I can now say that the "small circular canister" is actually the SAW resonator.

So in theory, as I have changed this to one running 433 MHz, it's working within a UK freq that is not illegal, unlike it was prior to modification when it was working on 315 MHz. So all I have to do now it work out why it only works 100% when the resistor is touched and what I need to do so tat I can put the board back in its case and not have to worry about touching anything other than the button on the front.

Any SAW experts out there?

 

I suspect that the problem has something to do with you putting a 433 MHz resonator in a transmitter designed to operate at 315 MHz.

The resistor you indicated appears to be connected to the RF oscillator (transmitter) transistor, so apparently your finger is retuning the circuit.

Most likely the transmitter circuit is similar to one of the two shown below, though there are other topologies in use.

In both circuits there are two capacitors in series from the collectors to ground, with the emitter connected to the junction of those two capacitors. You might find that reducing the values of those capacitors to about 75% of their present value will cause the circuit to work again.

There is a chance that the 37% increase in frequency is too high for that transistor to oscillate. You might want to switch to a transistor more suitable for that frequency. As an alternative, you can try to find the emitter resistor (is that where your finger was?) and decrease it some because this might increase the transistor's high frequency gain.

Be careful, I don't know much about UK radio laws but in many countries such modifications are illegal, and ironically that includes your changing the SAW so the transmitter would be legal.

 

I suspect that the problem has something to do with you putting a 433 MHz resonator in a transmitter designed to operate at 315 MHz.

The resistor you indicated appears to be connected to the RF oscillator (transmitter) transistor, so apparently your finger is retuning the circuit.

Most likely the transmitter circuit is similar to one of the two shown below, though there are other topologies in use.
View attachment 134407View attachment 134408
In both circuits there are two capacitors in series from the collectors to ground, with the emitter connected to the junction of those two capacitors. You might find that reducing the values of those capacitors to about 75% of their present value will cause the circuit to work again.

There is a chance that the 37% increase in frequency is too high for that transistor to oscillate. You might want to switch to a transistor more suitable for that frequency. As an alternative, you can try to find the emitter resistor (is that where your finger was?) and decrease it some because this might increase the transistor's high frequency gain.

Be careful, I don't know much about UK radio laws but in many countries such modifications are illegal, and ironically that includes your changing the SAW so the transmitter would be legal.

Thanks Dick, very informative.

I've been having a play and the resistor in question is 270k, I have added a 220k (that's all I had in my spares box) resistor in parallel with it and it works a treat, I'm thinking if I get a 487k surface mount resistor and swap it I might have cracked it.

I'll take you advice regarding legality and look into it.

Thanks again and I'll update when I have more news.

 

This sounds like it is the base resistor.

Did you put the 220k resistor in series with the 270k?

If you put it in parallel you need to replace it with about 120k.

 

My mistake, thanks for pointing that out. They were in parallel so I need 120k.

I've tacked the extra resistor on and I'm getting a reading of about 23k, I'm assuming that's because it's still on the board and other components may be affecting the reading, would this make sense?

 

Yes that makes sense.

 

Well the 120K surface mount resistors arrived today. I've fitted one and all appears fine. Still looking into the legality side of things, it's not as straight forward as it sounds

I also need to check it is actually working on 433Mhz