Low Tech High Safety and the NYC Subway System

The year is 1894. You are designing a train system for a large city. Your boss informs you that the mayor’s office wants assurances that trains can’t have wrecks. The system will start small, but it is going to get big and complex over time with tracks crossing and switching. Remember, it is 1894, so computing and wireless tech are barely science fiction at this point. The answer — at least for the New York City subway system — is a clever system of signals and interlocks that make great use of the technology of the day. Bernard S. Greenberg does a great job of describing the system in great detail.

The subway began operation in 1904, well over 30 years since the above-ground trains began running. A clever system of signals and the tracks themselves worked together with some mechanical devices to make the subway very safe. Even if you tried to run two trains together, the safety systems would prevent it.

On the face of it, the system is very simple. There are lights that show red, yellow, and green. If you drive, you know what these mean. But what’s really interesting is the scheme used at the time to make them light.

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Laser Cutter Exhaust Interlock is Silly, Educational, Useful

If there’s one maker space that has an excess of mad scientist type hackers, it has to be LVL1 in Louisville, KY. They sure do a lot of crazy stuff, like this simple device to defeat the laser cutter smoke monster. Nobody got the memo about the “simple” part. Instead they created a functional, educational and aesthetically pleasing element for the hackerspace.

LVL1 has a large format laser cutter. Laser cutters emit nasty smoke. Said smoke needs to be vented outside. To do so, it needs to pass through a scrubber/filter so the neighbouring Pigs don’t complain. So they installed a larger, better filter. The Pigs are happy, until the filter gets clogged and the smoke monster decides to escape. Next they install a pressure switch which disables the laser when the filter gets clogged. Laser cutters have a myriad of safety interlocks, so quite often, it isn’t apparent which one caused it to trip. Hence, the Laser Cutter Enable Module – LCEM.

The simple part was to install an indicator that lights up when the pressure switch is enabled, and off when not. But when it’s off, it isn’t clear if the pressure switch is off, or the indicator has failed. Simple, just install a bi-color LED – Red for off, Green for On. But then what about color blind folks who cannot tell the two colors apart? So, finally, two LED’s with clearly labelled text marking them as Enabled and Disabled.

A simple (this time for real) circuit was finally agreed upon. The SPDT contacts of the pressure switch drive the LED in an optoisolator. Its output drives a DPDT relay via a transistor. One set of contacts light up the two indicator LED’s and the other set of contacts goes to the laser cutter enable contacts. Of course, the optoisolator is totally redundant and over kill too – it’s input LED shares the same power supply as the output transistor! Remember the missing memo?

It was time to assemble the circuit. This is where the mad scientist dudes got really creative. On one half of a piece of acrylic, the schematic diagram was etched using the laser. This ensures n00bs get some education. And the remaining half had the circuit laid out in old-skool wire wrap fashion. Holes were drilled and connections were drawn (using the laser, of course) for the various components. Parts were inserted, and wires were soldered to make the connections. The result is what they call the PCB/Mounting Plate/Educational Schematic/Acrylic thing. Of course, exposed connections and wires are no good. So they made a sandwich consisting of a flat acrylic base, and a cut out frame in the middle to accommodate the wire connections and joints. All of this to light up an indicator. Because.

Thanks [JAC_101] from LVL1 for sending in this tip.

If you want to read more about LVL1 shenanigans, check out this post about their Rocketry group, or this post when Hackaday visited LVL1.