Posted by admin on December 14, 2011
Last year, my garage doors started opening and closing randomly. At first, I thought one of the neighbors had changed to use the same code, so I changed my code. But, the random openings continued, sometimes I would come home and find the left door open, sometimes it would be the right.
After some research, I came to suspect that the nearby Air Force Academy had switched to using Land Mobile Radio System (LMRS) radios, which operate on the same frequency, 390 MHz, and are known to cause interference. My opener, a Chamberlain 700WHC, has 9 dip switches with + 0 and – positions, allowing 19,683 possible combinations. It seems, though, that the new LMRS transmit 25,000 bits per second, and, sooner or later, the opener senses the right combination of bits and opens. My fix was to retrofit the opener with a new receiver with billions of possible combinations. Unfortunately, the old receiver continued to work; installing the new receiver did not stop the random openings and closings. With online searching revealing that there was no easy way to disable the old receiver, I resorted to good old fashion, tried and true methods, attacking the circuit board with a soldering iron!
Conveniently, the circuit board is manufactured with a test point between the radio receiver and the open/close logic. Just above the down force adjustment variable resister, labeled “DNF” (click on the photo to see the large version), you can see where I unsoldered one end of a wire to disconnect the receiver from the remainder of the circuit board. Disconnecting this jumper wire disables the radio receiver. If you don’t want to unsolder it, you can cut it.
The new receiver kit uses rolling codes and many more symbols to avoid interference. Installation is simple following the supplied instructions.
The new kit comes with a receiver, remote. “wall wart”-style power supply, and wire. You fasten the receiver to the ceiling near the existing opener, connect the wires, plug into power, and train it to the remote.
This change solved my random garage door opening problem. This is a much simpler, less expensive, and less time-consuming solution than replace the whole garage door opener just to get around a radio interference problem!
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