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Garage Door Indicator

Occasionally we discover that we had left the garage door open all night. This is a security problem and in the winter a waste of heat energy. I wanted to solve this problem by creating an indicator that lights up in our bedroom when the garage door is open.

My main goal was to make this project as simple as possible. There was already a doorbell transformer in the garage, so I used that for the power supply. I used some 24 gauge speaker wire to hook it up. A bi-color LED (Radio Shack 2760012) serves as the indicator. The trickiest thing was how to detect the state of the garage door. I wanted to use a microswitch to avoid corrosion issues with the detector switch. (An optical detector would be an alternative, but that's way too complex.)

Directly detecting the garage door position is problematic. The door moves over a large distance, and it's bulky and doesn't seem to move exactly the same every time. What I wanted was something that would reduce the range of motion to a smaller distance. Then the microswitch could be directly connected to this something. A gear reduction, or pulleys, or a lever arrangement would be rather complex. But there's a better way.

The weight of a garage door is counter-balanced by a spring. The spring is connected to the door with a cable and pulley. If you watch the spring, you'll see it's stretched out while the door is closed and it's under no tension while the door is open.

At the point where the spring is attached to the frame, there is no motion. But if you observe the spring's coils one or two loops away from the attachment point you'll see that it moves a small distance. Eureka! This is the motion reduction I need. All we need to do is attach one of the spring's coils to the microswitch.

Door Open

The chain is slack and the microswitch pulls itself to its inactive position. By using its normally closed contact, we can close the circuit when the door is open.

Door Closed

The chain is taught and activates the microswitch. The normally closed contact is now open.

Detector

The microswitch is mounted on the other side of the frame and the chain routed through a hole in the frame. I used a piece of aluminum that's intended for reinforcing the edge of plywood. The aluminum was mounted to the garage door frame with a 1/4-20 machine screw, 1/4x1 fender washer, and a stop nut. The microswitch was mounted with #2-56 machine screws and nuts. I used a little nail polish on the threads to assure vibration would not loosen the nuts. A piece of copper wire was used to connect the chain to the microswitch. I put a zigzag in the wire to allow the spring to pull it out as far as it needed to go when the chain was taught. If temperature changes cause the spring to pull more, the copper zigzag will simply be bent out a bit more. The slack in the chain assures that the microswitch's lever arm doesn't get bent when the door opens.

The rest of the project is pretty straightforward. We have two doors to be detected. The switches are simply connected in parallel so that if either or both doors are open, the circuit is closed and the indicator lights up. I put the current limiting resistor right at the power supply so if anything shorts out, nothing gets fried. That lets me skip using a fuse. Here's a schematic of what I did.

Schematic

Improvements

It's fairly simple to have the indicator light tell you which door is open. The addition of a couple of 1N914 diodes lets you make the indicator be red for one door, green for the other, and yellow for both doors open. This could help resolve who needs to go downstairs to close the door. (Or you could keep an extra remote control on your nightstand.)

Schematic 2

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