Why does the gate open on its own during the day with nobody home?
There are several ways a gate will open. You need to check each of them but the most common issue we find when repairing gates is the exiting vehicle detection device is opening the gate.
There are two main types. The best type will be a loop embedded in the driveway. These normally open the gate by driving over them. A sensor in the operator sends an electrical wave through the loop. A vehicle affects this wave and the sensor detects it, flips a switch and opens the gate.
If the installer used standard PVC wire, and the driveway cracks to let water sit on the wires, they could change the wave occasionally and open the gate. Water can take days to soak through and remain around for days. So while you do not have rain or sprinklers going the day the gate opens, it could still be the cause.
The sensor inside the operator could also be going out. Usually, they have lights showing when they are active. If you have one, and the detection light is on, turn off the machine, disconnect and reconnect the sensor and see if the detection light comes back on.
Next check the loop wires. Physically, look at the wires wherever you can. Check the driveway for exposed wires. If you find them, call a pro to replace them. Once this is done, you need an ohmmeter. Plunge a screwdriver deep in the ground. Set the dial on the ohmmeter to measure how much resistance there is between one wire in the loop and the screwdriver in the ground. Make sure you are not touching the leads or wires with both hands since you will be measuring your bodies resistance and not the short between the ground and the wires you are looking for.
If you find anything but an open circuit, there is a problem with the loop wires and you will need to replace them. If you have a completely open circuit, then change the detector. Ideally, you have another detector for the safety loops and can switch them to see if the problem moves to the safety loops. If it does, replace the detector.
The second type of exit device is called an exit wand or exit probe.
These are overly sensitive and don't last as long as the loop detector. Your dogs collar can set it off as can a bicycle or even a workers steel-toed boots. Add to this, the propensity to fail, and if you have one of these, you have likely found the problem.
The fix for these is to pull the probe up, look for nicks or cuts in the wire and fix them, dig a deeper trench, cover the wire with PVC pipe to prevent more cuts and finally bury the wand itself at least six inches deep. By the way, if the probe cable runs next to other electrical lines, they can make the probe react too. I have heard this can be prevented by using copper pipe instead of PVC pipe.
Other causes could be a remote in someones purse or pocket going off as it bumps around, a switch used to open the gate shorting out, a call box getting signals and in general anything used to open the gate breaking down or being set off inadvertently.
Rarely, and I mean very rarely, a control board, the big circuit board controlling the gate motor itself, will go out and cause this sort of problem. Out of everything, this is the least likely.
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