{"id":14245,"date":"2018-07-19T08:29:02","date_gmt":"2018-07-19T12:29:02","guid":{"rendered":"http:\/\/embedded-lab.com\/blog\/?p=14245"},"modified":"2018-07-19T13:27:27","modified_gmt":"2018-07-19T17:27:27","slug":"acme-traffic-light-restoration","status":"publish","type":"post","link":"https:\/\/embedded-lab.com\/blog\/acme-traffic-light-restoration\/","title":{"rendered":"Acme traffic light restoration"},"content":{"rendered":"

The Southern California Transportation Museum is one of the largest private transportation museums in\u00a0the United States. We are privileged to have among our artifacts a set of Acme traffic lights. This type of\u00a0traffic signal was deployed in the Los Angeles area in the 1920s and 1930s. This was the time when every\u00a0city was experimenting with different types of traffic signals. Later the Automobile Club convinced\u00a0everyone to adopt the three-light signal they use today.\u00a0During the brief time they were in use, the ACME traffic light became the favorite of the Hollywood\u00a0cartoonist so, that’s why you see them all over the movies. It\u2019s the one with the arm and lights. The\u00a0arms were used during the day since lights used electricity and electricity cost money. Then, at\u00a0night the lights were used because, logically, you couldn\u2019t see the arm.<\/p>\n

The museum has one of the original ACME signal controllers \u2013 but it has a fatal design flaw, it thinks that\u00a0it\u2019s running a traffic signal. That means it will run it all day and night, wearing out the motors. So, the\u00a0museum 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\u00a0in the museum\u2019s signal garden. Also, because it is incredibly easy to program.<\/p>\n

\"Acme<\/a>

Acme traffic light controller<\/p><\/div>\n

The\u00a0Raspberry Pi and the relay board must work in a somewhat harsh environment. The museum moved to\u00a0Perris, CA because\u00a0the hot\u00a0dry\u00a0climate is good for the\u00a0 artifacts.\u00a0However,\u00a0the climate is not\u00a0good for the electronic parts. Most of the components sit in a cast iron box, painted black, with no ventilation, in a\u00a0semi-desert. We are not\u00a0certain\u00a0of\u00a0how hot it gets inside, but the 3d printed hinges we\u00a0implemented\u00a0in the\u00a0box\u00a0melted. Fortunately, all of the other parts are still working properly.<\/p>\n

Now, you might be wondering\u00a0why it takes so many relays to control one signal. The ACME traffic light\u00a0is not a\u00a0simple\u00a0signal.\u00a0There are arms, lights, and a bell. So, the relays are for:<\/p>\n

    \n
  1. Red Light<\/li>\n
  2. Green Light<\/li>\n
  3. Yellow Light<\/li>\n
  4. Motor power<\/li>\n
  5. Motor direction<\/li>\n
  6. Bell<\/li>\n
  7. Fold (Keeps the arms from extending when returning them to the housing.)<\/li>\n<\/ol>\n

    And everything must be doubled because we have two signal heads.<\/p>\n

    Some additional relays were needed since the arm motor works by shorting two wires and\u00a0applying power to a third. To change direction, you change which wires are shorted and\u00a0which gets power. This would have been possible with two relays on the 16-channel\u00a0board, but\u00a0in this\u00a0configuration,\u00a0it\u00a0would be possible for a software glitch to damage the motor. To ensure\u00a0that would not happen we used a DPDT relay which provided a hardware interlock.\u00a0Also,\u00a0the motor, fold magnet, and bell all use big A\/C coils. When these are turned off they\u00a0generate a huge electric pulse. This does nasty things to the Raspberry Pi. (Funny enough\u00a0the board has no problem.) To get around this problem three zero crossing solid\u00a0state relays were added. By turning off the coils when no voltage is present you avoid the\u00a0pulse problem.<\/p>\n

    Railroad equipment is designed to last a long time. The last railroad relay I used had an\u00a0inspection sticker on it of 1910 (that\u2019s when it was taken out of service, not when it was\u00a0made). I don\u2019t know if the\u00a0new parts\u00a0will last as long, but so far, the new parts have been very\u00a0reliable in a challenging environment.<\/p>\n

    Watch the demo video below:
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