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I've encountered a strange problem with a 12V key-fob device. We use a device to remote control a relay via a key-fob transmitter. It's installed in a plastic box that is mounted at an operator station. It works quite well until it was requested that an LED be added to show that power was on. Easy to do, or so I thought.

I installed an LED (plus the appropriate resistor), and connected it to a couple of available pads on the PCB (the same location as the DC supply connections). This is where it gets interesting.

With the LED connected, the signal drops and will not trigger the remote device. I can see the signal level decrease on a scope (albeit, not a very good one). Disconnect it and the transmitted signal returns to it's proper level. But wait, there's more...

If I connect the LED at the DC input (about 24" away), it works fine. Connected inside the box on the PCB, it kills the signal. Examining the PCB there is some trace length matching on the positive side of the DC input (using the typical PCB trace weave pattern). It comes off of the connection points, weaves, then ties into the main circuit. This is the first time I've seen matching on a supply connection.

So what am I missing here? Am I somehow affecting the impedance of the transmitter by adding the LED circuit?

Additional Information: I don't have much to go on. It's a mass produced remote relay like this:

Wireless Remote Relay

According to the spec sheet it operates at 433Mhz using ASK Super-Heterodyne Wireless Reception. Sorry, I just don't have much to go on.

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  • $\begingroup$ Welcome. // I‘m afraid without knowing more about your specific set of black boxes we can flood you with speculations and little substantial information. $\endgroup$
    – MS-SPO
    May 25 at 5:18
  • $\begingroup$ LEDs consume energy. Unless there was excess where you attached it, you will need to provide it from somewhere with more. Look into buffering the attachment point from the power consumption. Even a simple mosfet fed from elsewhere should work. $\endgroup$
    – Abel
    May 25 at 12:46
  • $\begingroup$ Now mark where you connected the led on the circuits $\endgroup$
    – Abel
    May 26 at 1:38
  • $\begingroup$ I seem to have resolved the problem. When I initially installed the LED I created a small loop of wire from the connection points to the LED. This "service loop" was the cause of the signal loss as the ground wire was creating (for lack of a better term), a small faraday cage effect. I shortened the wires and routed them away from the antenna and now it's working properly. So obvious I can believe I missed it. $\endgroup$
    – Daniel Szalay
    May 26 at 13:11

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