The saga of an old Sears Craftsman garage door opener

This isn't exactly about curiouser code; rather, it's about curiouser electronics and troubleshooting.

Prologue

We bought a house with a built-in garage with an old (very very old, pre-1997) Sears Craftsman electric garage door opener, which the old owners claimed was defunct but never bothered to replace. Rather than just go and replace it myself, I tried my luck playing with the bundle of low-voltage wires dangling from it - and found that it did work if you connect the right wires.

Better than nothing.

Next, I downloaded the manual and tried searching Amazon for a possible replacement remote. And was lucky again: after a few unsuccessful tries, this little dongle successfully paired with the opener and worked.

Much better than nothing!

However, my curiosity was aroused at this point. The manual mentioned that there was a possibility to attach a pair of optional optical-beam safety sensors to prevent the door from closing if something was in the way, or to reverse the door closing if someone crossed the beam. Since the garage is very tight and I don't want the closing door to crash into my rear bumper (or, heaven forbid, to injure someone), this was definitely a feature I wanted.

Part 1. Research

The easiest and most obvious solution -- to order the safety sensors set as an official accessory from Sears -- was quickly put to an untimely end with a "No longer available" message on the official spare parts website.

The next one to try, less obvious but still easy, would be to order a set of 3rd-party sensors on Amazon (in the same way as I did with the remote) and hope that it works. Alas, no luck this time -- everything I could see on the aftermarket was for newer models, and would not be compatible with my opener.

The real snag was that my opener was "old but not too old" -- recent enough so that optical safety sensors were offered as an optional feature, but manufactured before these sensors were made mandatory. And while this was precisely what made it possible to get it running without the safety beam at all (I might have given up trying had I not been so lucky in the beginning), this was what rendered all newer safety sensors completely incompatible.

The reason? As explained nicely in this video, all newer models are designed in such a way that unless they get a "safe" signal from working sensors, the door won't close; just so that those pesky users won't bypass their faulty (or out-of-alignment or not-having-survived-mediocre-parking) sensors with a piece of jumper wire, the "safe" signal is a pulse train (with about 150 Hz frequency and some 10% duty cycle), which the receiver generates in presence of the sensor signal.

The manner my opener functioned was quite the opposite: it would work in all cases except when the sensor terminals are shorted (presumably by the sensors detecting the beam interruption), in which case the closing would not commence or, if already closing, would reverse.

Finally, I just decided to blindly try my luck with this set hoping I might still get the old kind, but alas, sadly but unsurprisingly, they were of the "pulse train" kind, nicely confirmed with an oscilloscope

Part 2. Simulation

So my job was to somehow translate between the two protocols. Some device (labeled "???" in the diagrams below) had to listen to the sensor output, decide if there was a "safe" pulse train, and then short the opener's sensor terminals if there were no pulses.

  

At this point, many would scream Arduino or even Raspberry Pi ("and your door will be able to post its status on Facebook"), but I quickly decided this would be too difficult, and a massive overkill to boot. I didn't want my door to post on Facebook. I wanted it to stop closing when the beam was crossed. Even this sequel video using an integrated-circuit (555 timer) square wave generator as a "pretty damn complicated piece of jumper wire to bypass the sensor" seemed a bit of an overkill to me (and as it were, resulted in at least one real kill...)

On the face of it, I thought a simple transistor switch might do the trick - if I could have the transistor normally open via some pull-down resistors, and rectify the pulse train in such a way that the signal would offset the opening potential and close the transistor (opening the switch) if pulses are present, at the same time retaining some power on the sensors so that they would resume normal operation if the beam was restored... not obvious. After a few evenings spent in a circuit simulator I found that a single transistor was a bit too unreliable; however a MOSFET or a Darlington transistor pair might do the trick, like so:

 

The one on the right is what I chose to implement as a final circuit. The two terminals on the right connect to the opener in parallel to the sensors: the upper right (positive) to the "white" screw and the white wire from the sensors; the lower right (ground) to the "black" screw and the striped wire from the sensors.

Part 3. Implementation

With a few of Amazon "shipped from China / Hong Kong and arriving several months later" orders, I finally assembled the circuit on a breadboard...

I scrapped the current limiting resistor in the output circuit because the short-circuit current was measured to be about 40 mA anyways, and put two 1M resistors in parallel to achieve 500k. The rest is as shown.
The real wonder? It worked out of the box.
And works still.

Epilogue

The opener is still in operation, sensors included, with the original breadboard circuit loosely hanging off the drywall. A few times it would refuse to close because there was something left in the doorway (I guess that may have saved those items from being crushed).

It isn't seamless, however: after I insulated the door with some styrofoam (making it quite a bit heavier), the gears in the opener did kick the bucket. However I was able to replace them relatively cheaply and quickly.

I still have plans to solder the circuit into something more permanent (and even bought all the necessary supplies for this), but these remain low priority (ain't broken? won't fix it, at least for now).

I still wonder whether I can somehow reverse engineer the operation of "Lock" and "Light" features from the wall control unit. If you happen to have this one lying around, please drop me a line!