I live in a house where it is not easy to see if the garage door is open or closed. We have a button in the house, but the door is out of sight. The thought of engineering some sort of switch and power supply was undesirable because of the high probability of failure, being in a dirty and hot or freezing environment. Also I don't know an Arduino from an ATMega328, so fancy wi-fi solutions were out of the question.

It occurred to me that there must already be an open/closed switch within the opener to tell the unit where the door was positioned and when to stop. In my Liftmaster/Craftsman unit it is called a “limit switch”, and it uses 3.7 volts DC. Now I have a status indicator in my house for the price of two LEDs, a resistor and some wire (Some Chamberlain units have the same switch. Newer units are set programmably, but perhaps still have some sort of physical limit switch that you can use).

As the motor turns one way or the other an attached gear turns a 3 inch threaded rod containing the +3.7 volt electrical contact, which then travels one way or the other depending on which way the door is moving, contacting an “up” and a “down” electrical contact at either end. The voltage between the center and end contacts can be used to light the LEDs. When the center contact grounds out at one side or the other, the processor stops the door and one LED will extinguish, leaving the other one lit.

Warning! This hack works well but I am not responsible for any injury or damages you incur. Please only proceed of you are sure of what you are doing and at your own risk.

Supplies

two LEDs
resistor
wire

Step 1: Attach Wires

First, unplug the unit and remove the cover. I attached 50 feet of wire to the middle and each end contact and connected them to the LEDs inside the house.

There are a few things to consider:

Where to attach my wires to the sensors? I did not want to cut any existing wires, so I soldered mine to where the existing wires attached. In the photo the grey wires are the center travelling contact, and the yellow and brown wires are the up and down limit sensors. Make sure your middle wire is flexible and long enough to travel to each end. At this point it does not matter which contact is 'up' or 'down' - you can switch the leads at the LEDs if you want to change which LED represents up and down. I used thicker wire for the first 6 inches before soldering to the thinner wire so that I can easily disconnect if need be. You can see the big wide middle electrical contact, 'L' shaped end-contacts on their white threaded rods, as well as the gears that turn the middle rod.

I find this switch brilliant because it is so simple but robust, able to last 20 or 30 years in a harsh environment. It is also serviceable, replaceable and affordable, unlike new computer controlled units.

Step 2: Connect the LEDs and Resistor

What value for the LED resistor? I added the required current dropping resistor but found that enough current still flowed to stop the door in mid position. It took about 9000 ohms of resistance before the processor stopped seeing current, but the LEDs still lit up nicely. I positioned the resistor at the LED end so that it would be easier to change if I wanted a different value.

What kind of wire? We are talking about such a small amount of voltage and current I was able to use thin strands of CAT-5 network wire I had lying around.

I used different color LEDs to represent the up and down status (you could also use LEDs of the same color with labels however it would be difficult to discern up from down from across a room. You could also also use a two color LED with three leads). You may find that you need to use sensitive/bright LEDs since the resistor is of such a high value. I soldered the middle-contact +3.7 volt wire to the resistor and then to the two LED positive legs. Each end-sensor wire then attached to one LED negative leg.

Step 3: Install the LEDs

Lastly, I positioned the LEDs on an old unused alarm panel next to the opener button. If space is tight, you may wish to solder the resistor two or three inches away from the LEDs. The bulbs in the picture are hard to see, but the emitted light can be seen across the room. As an added bonus neither end-contact is touched as the door is traveling up or down so both LEDs will be lit.

Option: If you only want to know when the door is up (or down), you can use only two wires and one LED and use the middle and only one end-contact.