If you’ve got a garage with an automatic door opener, I’ll bet that at one time or another, you’ve left it open by accident. No? Well, whenever I come home and find my garage wide open to the world, my heart races. Thankfully, the few times someone in my family has done this, nothing has been stolen. But according to the makers of the “Protectrix” automatic garage door closer, more than 40% of all burglars enter through an open garage. Their system, which is similar to the “Garage Butler” and several others on the market, automatically closes your garage door after an adjustable amount of time. But what if you want to do some work in the garage, or outside, and you’d like to leave the door open while you’re home? In that case, you’ll need to remember to hit a special button on most of these automatic systems which tells the door to stay open despite the elapsed time.
Here’s a thought though: why not have the closer sense the presence of your vehicle and then only close the door when it leaves? Then you won’t have to worry about overriding the automatic closing feature (and forgetting to re-enable it later). In my case, I rarely leave home without my car, so this would work perfectly. But how might this sensing be accomplished? My first thought was to use a couple of these wireless XBee modules:These are great little devices, and I’ve used them in a number of projects over the years. They may be a little expensive for some applications though; the basic models cost about $20 while the mile-range “Pro” versions go for around $35. But this isn’t bad for an FCC approved 115kbps wireless link. Plus all models also come with a bunch of I/O (digital and analog) and a number of sophisticated features including encryption and mesh networking.
So my idea is to use a pair of these devices as proximity sensors. One of them would hang out with the garage door opener and the other would plug into your car’s cigarette lighter. Both would need additional power regulation circuitry to provide the necessary 3.3V supply. The one connected to the opener would likely be controlled by a cheap microcontroller (MCU) of some kind. The MCU could monitor the status of the garage door and send “pings” to the car-mounted XBee. If the car leaves and the door remains open, the MCU would then send a signal to the opener to close the garage door. And if for some reason you still wanted to door to remain open, well you’d still need that override button. But still, I think this approach greatly simplifies things.
Oh, and if you’re worried about the car-mounted device draining the car’s battery, don’t. While the “Pro” device (whose extended range is probably unnecessary in this case) draws 1W during transmission, we only need to transmit for a split second every 10 seconds or so for our sensing ping. So let’s say the transmission takes half a second, and we ping every 10 seconds – that’s an energy draw of 1.2Wh/day. Let’s say your 12V car battery is rated for 40Ah. Power equals voltage times current, so your battery stores 12*40 = 480Wh. Thus, the XBee could run on battery power for up to 400 days. No problem.
I may try this out in the next week or two. We’ll see what kind of modifications are necessary to the garage opener… Any suggestions, feel free to comment! Thanks.