How do garage door sensors work?

MrChips

Joined Oct 2, 2009
26,084
How do the garage door sensors work? Mine have stopped working and I want to fix the problem.
Let us begin by asking what is the make and model of the garage door opener.

I am going to assume that by garage door sensors you mean the light beams at the bottom region of the door that detects when an object is in the path of the door?
 

crutschow

Joined Mar 14, 2008
29,776
The ones I have use a photo sensor.
There is a light transmitter on one side of the door near the floor and a light sensor on the other side.
It stops the door moving if anything interrupts the beam.

If the two get out of alignment, then it won't work.
There may be an indicator light on the receiver that goes on when the alignment is correct.
 

Thread Starter

Jack_K

Joined May 13, 2009
143
The ones I have use a photo sensor.
There is a light transmitter on one side of the door near the floor and a light sensor on the other side.
It stops the door moving if anything interrupts the beam.

If the two get out of alignment, then it won't work.
There may be an indicator light on the receiver that goes on when the alignment is correct.
I know WHAT the sensors do, but not HOW they do it. From another thread it seems there's 11 volts on the sensors at times and a 330 HZ square wave at other times. So my question is -- how do the sensors tell the opener whether the light beams are blocked or not? I suppose there's information on that square wave?
 

MaxHeadRoom

Joined Jul 18, 2013
25,196
The output of the transmitter is a square wave and the circuit in the door reads the loss of this frequency rather than just being a simple loss of light source.
Might need a new transmitter ?
 

crutschow

Joined Mar 14, 2008
29,776
how do the sensors tell the opener whether the light beams are blocked or not? I suppose there's information on that square wave?
It just detects for the presence or absence of the square wave.
But unless you are knowledgeable about electronics, I don't see how that will help you solve your problem.
 

Thread Starter

Jack_K

Joined May 13, 2009
143
It just detects for the presence or absence of the square wave.
But unless you are knowledgeable about electronics, I don't see how that will help you solve your problem.
I disconnected the wires from the opener and measured 11 volts at its terminals. I measured between the wires and they are not shorted. I replaced the wires on the opener and measured 0 volts on the sensors. It would seem that there's a broken wire somewhere in the run. My meter leads are not long enough to measure from the opener to the sensors, but I suppose I could use some hookup wire to extend the leads.
 

MaxHeadRoom

Joined Jul 18, 2013
25,196
You can get a new set of transmitter receiver, but there is also possibility of loss in the main unit, a 'scope would be an advantage.
The transmitter is usually a amber light and is lit all the time, the reciever usually green, will go out when blocked or misaligned.
 
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