Acme Traffic Light - Restoration Project
One of the largest private transportation museums in the United States, the Southern California Transportation Museum, is extremely privileged to have a set of Acme traffic lights amongst our artefacts. Originally deployed in the Los Angeles area, this type of traffic signal was used in the 1920s and 1930s. At this time, every US city was experimenting with different types of traffic signals. The Automobile Club later convinced everyone to adopt the three-light signal still in use today.
During the relatively brief time they were in use, the Acme traffic light became the favorite of the Hollywood cartoonist, which is why you see them all over the movies. It’s the one with the arm as well as lights. The arms were used during the day since lights used electricity and electricity costs money. Then, at night the lights were used because, logically, you couldn’t see the arm.
The Southern California Transportation museum has one of the original Acme signal controllers – but it has a fatal design flaw, it thinks that it’s running a traffic signal. That means it will run it all day and night, wearing out the motors. So, the museum decided to replace the old controller with a new one consisting of a Raspberry Pi and a 16-channel relay module. The relay module was chosen because in the past it had provided reliable service in the museum’s signal garden. Also, because it is incredibly easy to program.
The Raspberry Pi and the relay board must work in a relatively harsh environment. The museum moved to Perris, CA because the hot and dry climate is good for exhibits and artefacts. However, the climate is not so good for the electronic parts. Most of the components sit in a cast iron box, which is painted black and has no ventilation, in a semi-desert. We are not sure exactly how hot it gets inside, but the 3D printed hinges we implemented in the box melted. Fortunately, all of the other parts are still working properly.
Now, you might be wondering why it takes so many relays to control one signal. The Acme traffic light is not a simple signal. There are arms, lights, and a bell. So, the relays are for:
- Red Light
- Green Light
- Yellow Light
- Motor power
- Motor direction
- Fold (Keeps the arms from extending when returning them to the housing.)
And, of course everything must be doubled because we have two signal heads.
Some additional relays were needed because the arm motor works by shorting two wires and applying power to a third. In order to change direction, you simply change which wires are shorted and which get power. This would have been possible with two relays on the 16-channel board, but in this configuration, it could also have effected a software glitch that would damage the motor. To ensure this wouldn’t happen we used a DPDT relay which provided a hardware interlock. In addition, the motor, fold magnet and bell all use big A/C coils, which, when turned off generate a huge electric pulse. Sadly, this does nasty things to the Raspberry Pi (although, funnily enough the board has no problem.) To get around the problem, three zero crossing solid state relays were added. By turning off the coils when no voltage is present you avoid the pulse problem.
Railroad equipment is designed to last for a very long time. The last railroad relay I used had an inspection sticker on it from 1910 (which is when it was taken out of service, not when it was made!) I don’t know if the new parts will last this long, but so far, they have been very reliable in a challenging environment.
CommentsAdd a comment
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.
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.
[Comment was deleted]