Dial-a-Door Remembered
Ward Cunningham
October 2008

It must have been 1968 or so. Roommate Rick Wartzok and I had better than average audio/visual and beverage capability in our Owen Hall dorm room. We shared happily with fellow residents who sometimes came and went without us. But what about keys? We wanted some kind of combination lock that had a shared code that could be selectively enabled, and a longer, master code for our private use. Our solution was Dial-a-Door.

Now its 2008, forty years later, when our old dorm room neighbor Chuck Stewart comes across these slides and is kind enough to scan them for us. My contemporaries are most impressed with the glasses we wore back then. But I'm thinking about the door, the lock, and the decoding mechanism.

This device, called a SECODER, mechanically decodes the pulse stream from the conventional telephone dial. This unit was originally used for selective calling in mobile radio applications for the railroad. I can't remember how it was that Rick happened to have one.

The device is particularly interesting in that it used a single-coil electro-magnet to activate two different ratchets. Differences in the flux path made for different relaxation time constants as required to count pulses (fast) and then check the resulting digit (slower).

The SECODER's nameplate cites three patents, all of which are available online.

Patents issued October 3, 1944 and November 13, 1951 are for inventions by Harold A. Stickel of San Francisco. His 1951 drawings resemble our device the most.

The third patent, issued January 15, 1963 to inventor Herbert M. Penningroth of Hales Corners, Wisconsin, seems to date the manufacture of our device to 1963 or later. That would make it six years old or less when we put it into service. Hardly old junk then.

The SECODER's mechanical behavior delights. I built this model of the device from memory before I located the real thing. The model is animated with Quartz Composer which can be viewed with Apple's Safari browser. The closeup photo is animated with javaScript which should be compatible with any modern browser.

My mechanically and electrically inclined friends have pitched in to help me get the SECODER ready for display.

Wayne Downer tweeked the mechanical components that had been bent or otherwise abused over the years in the bottom of the junk box. We hooked it up with the original dial and fed it serious power from a deep cycle battery. It worked, sorta. Another hour or two of fiddling got it working 90% of the time.

I built a stand for the assembly out of foam-core board and included a Radio Shack 12-volt indicator lamp to be lit when a number decodes successfully.

John Providenza helped me diagnose the lamp switching. Surface corrosion was introducing megaohms of resistance in the ground circuit. More fiddling got this down to below a 100 ohms. Let there be light.

We're always happy to have blogers write about our work. 40-year old work is no exception. Here are blogs we've noticed that notice us.

• Mark Dilley at AboutUs Weblog
• Amber Case at Hazelnut Tech Talk