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Acme Traffic Light - Restoration Project

This project is a showcase of the restoration of an Acme traffic light that belongs to the Southern California Transportation Museum.

Parts list

Qty Product Part number
1 Raspberry Pi 3 Model B 896-8660
Am1 has not written a bio yet…


August 3, 2018 07:40

Robustly designed (for long operational life) components are common in safety-critical systems and industries. There are different types of longevity, though. Wear is affected by the number of operations. But other factors like corrosion are affected by the environmental, and others have to do with both. So the relay that was taken out of service a hundred years ago may have a reduced remaining life, even though it was physically idle while stored. I remember this and shake my head when movies have scenes where treasure is protected by elaborate booby-traps put in place in by ancient civilizations, and the mechanisms are in fine working order despite bad environmental conditions.

I'm glad to see you looked at potential problems when designing replacement circuitry. I once worked with a system for testing integrated circuits. Each pin could be connected to any of multiple power supply buses, ground, stimulus, and measurement circuits. The system had no interlocks protecting against conflicting configurations. The test programs had been debugged to make sure they were commanding such conflicts. But when building power glitched during thunderstorms, we'd dash to turn off the main power to the tester, because such glitches could scramble data in the registers controlling the relays, connecting multiple power supplies to the same pin at the same time, or worse.
It would have taken little additional logic on the pin driver cards to prevent such mishaps. The cost of doing so would have been small relative to the cost of repairs and down time due to lack of interlocks. Dealing with that oversight in someone else's design was an effective lesson in early in my career.

We added an interlock on the main power circuit that would shut the tester down if it detected a power brownout and require manual reset before restoring power. That greatly reduced the number of board future board repairs needed. I also wrote a "virtual tester" device driver that could be used during debugging, that kept track of what relays had been selected and would display the status on screen as well as give error messages when hazardous combinations were selected.

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August 2, 2018 09:22

I would look for US&S and old Westinghouse subsidiary for relays, used by the railroads. The Europeans had relays for HVAC that were reportedly rated for cycles in the millions, back in the 1990s. Telemechanique was the company that made the solid state relays we used in place of NEMA controllers which had cycle ratings in the tens of thousands, such as 50,000 cycles, not even close to ONE million, let alone ,2,3 or 5 million. I'd probably also add some limit switches, and some comparators to monitor motor current. A good motor winding shop, if any still exist, should be able to rewind the motors. They may have to make up custom skeins but with 3D printing that is pretty easy today.

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