Did you close the garage door?

When I was growing up, my family had a light in our kitchen that would come on when the garage door was open. It was easy to see when the door had been left open. The light was probably sold as an optional accessory for the opener, because the opener unit had a pair of terminals on it that could be wired to the indoor indicator.

Some time later, the opener had to be replaced, and the replacement opener had no connection for the indoor door‐open indicator. They got used to not having the light, but I didn’t, even though I don’t live there any more. As we grow up, we expect that our parents’ houses shouldn’t ever change, don’t we?

For the stop light project I have been working on, I need the controller to know when the door is opened so that it can wake up go into its parking sequence mode. The door‐open sensor for the stop light would be nearly identical to what would be needed to replicate the old indicator that my parents used to have in their kitchen, so I bought enough components to build both projects. Not knowing whether they would want indicators, I included two indicator light assemblies in their kit.

The key component is a C&K MPS80WGW magnetic proximity sensor. This sensor comes in two parts, a magnet and a switch. The magnet is mounted to the moving part of the garage door, and as it closes, it moves into the switch’s range. The switch closes, and turns off the indoor indicator light. At 2″, this sensor has the largest active region of the ones available at Digikey. This is handy, because the sensor will be forgiving of misalignment, and it will tolerate the door being left slightly open, as is sometimes done in hot places like Texas in order to allow some air to circulate into and out of the garage. The magnet and switch are mounted on simple steel brackets and secured to the door and the track.

The indicator lights themselves are nice big 10mm SSILXH1090SRD LEDs from Lumex Opto. They come installed in bezels that fit neatly in a single gang plate cover drilled with a 1/2″ hole (bored out just a bit with a Dremel tool). The forward voltage of these LEDs is 1.7V, so a 160Ω resistor limits current to 20mA. This is bright enough to be noticed, but I think not so much so that it is distracting.

A simple driver assembly has connections to the indicator LEDs and to the proximity sensor. I used a spare 5V wall transformer from an old cell phone to supply power through a thermal fuse to the LED driver, a common 2N3904 transistor. The driver assembly fits inside a single gang side mount electrical box, intended to be installed in the attic between the garage and the other rooms where the LED indicators would go. I drilled a small hole in the cover plate so that an internal status LED can be seen, as an installation aid.

The LEDs and sensor operate at 5V, so simple phone cord wiring and low voltage wall boxes are sufficient. The inside of the driver is shown below.

A wall box with the LED installed is shown below.

The following schematic shows the components and their connections. (PDF) The components in the shaded box are in the single gang wall box. Having some perf board and screw terminal connectors on hand was helpful to hold everything together and simplify installation. I included these installation suggestions with the kit.

Some shots of the components as installed follow.

This project was nothing fancy, but it was satisfying to put together some simple parts to restore a nice feature to my parents’ house. It was also a nice way to add a fun feature to the stop light controller with some similar components.

After I had wrapped up the project, I found another garage door sensor project that was way more involved, and pretty neat: Ultimate Garage Door Monitor.

Lessons from tire shopping

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In other car‐repair news, I learned a couple things about tire shopping recently. Two weeks ago, I ordered a set of new tires for the Volkswagen. I did my research and placed an order online at Sam’s Club for a set of Michelin MXV4 S8 tires at $112 each. Curiously, the same tire model — dimensions, ratings, and all — was available under two different SKU numbers. One SKU was $112 and the other one was $143 — 28% higher! Naturally, I ordered the less expensive ones.

When the tires arrived and I had them installed five days later, they charged the $143 price. Checking online, the exact tires, down to the SKU number, that I had ordered were indeed now available only at the higher price. I provided proof of my original order, and they adjusted the price without a fight.

Lesson 1: There are a zillion combinations of tire models and ratings, and it’s probably a nightmare for retailers to keep their databases current. If you are ordering tires, check thoroughly for similar models, because you may find an unexpectedly good deal.

Lesson 2: Tire prices apparently can fluctuate wildly in a short amount of time. So, if you know you’re going to need new tires soon, select a tire model, and then monitor the price for a little while. If you see the price go down, jump on it!

Car maintenance is back

kdk_0263AutoZone’s net income is up 8.6%, pleasantly surprising investors. It’s no surprise that people are trying to make their cars last longer in favor of buying new cars. But it turns out that last year, even the stingiest among us were postponing major repairs because gas prices were so high. Now that the prices of gas is coming back down, shade‐tree mechanics now have a little extra cash to fix their clunkers.

Via the WSJhttp://online.wsj.com/article/SB123608361570718211.html

In its annual April auto issue, Consumer Reports estimates the expenses of owning a car over its first five years: