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:
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.