Prison Door System Built Using Only Timers and Relays

Prison Door System Built Using Only Timers and Relays

Reddit user enticingasthatmaybe was recently contracted to build a system to electronically control the door and gate system of a southern prison without the use of microprocessors; not even transistors.

The reasoning behind this is that should the system fail, the prison trustees wanted it to be a simple enough to fix that any electrician could do it. The idea of it being difficult to hack was also a factor. The designer drew an easy-to-use chart that explains which part controls which section of the system. All a technician has to do is find the faulty part, buy a new one, and slip it into the socket.


It may be a more cumbersome solution than using microprocessors, but in a place such as a prison, practicality takes precedence over miniaturization and elegance.

In case things go really wrong, asenticingasthatmaybe explains:

There is a keyed interlock override switch that will complete the interlock logic if there is a mechanical failure. They specifically spec’ed the lock to be center return so it would have to be manually turned every time. (More incentive to have it timely fixed).

It’s certainly a niche market he works in, but it’s interesting to see that devices are still made in this way.

38 thoughts on “Prison Door System Built Using Only Timers and Relays

  1. scottyd says:

    Looks like the control cabinet of about every machine I work on. Old school industrial plant.

  2. squishyrobot says:

    We do the same thing on movable bridges (AKA drawbridges) all the time. One other benefit to relay based systems is that, when a system is expected to last 20 years, you can be pretty sure the maintenance electrician will be able to buy a new relay 15 years from now. Whereas, we find that PLCs often go out of production in that timeframe, or sometimes the programming software is not updated by the PLC manufacturer, so you need to keep the laptop that came with the system running for 20 years or you might have no way to update or reload the program. Maintenance is a huge factor for the designers of these systems, and leads us to engineer entirely different solutions than we might devise otherwise (especially in our hobby pursuits).

    1. Michael Colombo says:

      That’s a very good point. When I was doing research on an audio tape-based project I got in touch with the folks at MRL ( ), who make reference tapes to synchronize the heads on tape machines. They’ve been in business for over 30 years and apparently still dupe the tapes on an old 286!

  3. officer 1 says:

    What I can’t believe is that a prison let inmates dictate
    How the door controls are constructed.

    1. Michael Colombo says:

      Maybe you mis-read. It was the trustees of the prison who dictated how it was built.

      1. Steve0 says:

        The term ‘trustee’ in prison can also mean inmates that have behaved well and have special privileges such as outside work duty.

      2. mark says:

        For the 3 weeks I worked for the Texas department of corrections. There a trustee was a reliable inmate that was given special treatment and access, some of the best were “turnkeys and held all of the keys to the doors.
        That was one depressing job.

      3. Marc Tompkins says:

        These are two different words, pronounced the same but spelled differently:
        – a “trustee” is one of the administrators of a trust
        – a “trusty” is a prisoner who’s been given special privileges for good behavior.

        No matter how good his behavior, I doubt that a trustee could ever become a trustee (of a prison system, anyway – in certain white-collar prisons, I suspect most of the trustys were already trustees before they arrived!)

        1. Marc Tompkins says:

          I MEANT to type “I doubt a trusty could become a trustee”. Damn homophones.

  4. ausserirdischegesund says:

    Is this requirement so that doors will open in case of fire, or is it to deter hacking?

  5. davehillier says:

    Wow. Control logic in relays; that is special.

    I’m sure they can get “any electrician” to diagnose and fix a race condition as the switching of relays speed slows with age.

    1. squishyrobot says:

      *Hopefully* the engineer designed it so that there are no race conditions… Or hopefully the whole thing was simple enough that none would come up.

    2. mark says:

      Sounds like you’ve run down a few floating grounds and half-tripped overloads in your day.

  6. mark says:

    When I started working on automatic car washes in the ’80s most were relay and micro switch triggered electric motor powered programmer based. Going to PLCs and proximity switches increased the reliability and simplicity by orders of magnitude. And get an electrician to troubleshoot relay logic? Not most that I’ve dealt with, it’s just not what they do. Save the money paying a skilled service tech to replace the relays and components on a monthly basis and replace the system in 20 years. Believe me, there is nothing simple or reliable about relay logic controls.

  7. FM says:

    Forty relays, two TDRs, probably multiple self- hold paths, Unless they have a very clear logic diagram, this will be difficult for anyone to fix “logical errors”. Plus, it is very, very difficult to add any new features or capabilities to this system. It doesn’t look like they left any room in the control cabinet to add any new relays.

    Repairs in the future will be similar to fixing a string of Christmas lights. Take a spare relay and swap it into each socket until the problem goes away.

  8. T.E.Stuart says:

    Race conditions? Logic programming? Timing? Gentlemen, this isn’t complicated, we are just opening doors here. The logic is hard wired, with actual wires. The troubleshooting here will be’ “This door doesn’t open, replace this relay. This group of doors wont respond, replace that relay.” I have seen “Ice Cubes” operate reliably for 20 years under harsher conditions than this..You guys sound like a bunch of EE’s.

    1. None Thistime says:

      Most likely the trick will be finding the loose wire on one of the relay blacks. Twenty nine years building and troubleshooting relay logic and most problems in old systems begin and end with a lose wire or two in the interlock circuits. nd yes, I build detention systems…

    2. None Thistime says:

      Most likely the trick will be finding the loose wire on one of the relay blocks. Twenty nine years building and troubleshooting relay logic and most problems in old systems begin and end with a lose wire or two in the interlock circuits. And yes, I build detention systems…

      Spelling Corrected.. Geez these late nights…

  9. Dave Bechtel says:

    That black box on the right side sure looks like a PLC…

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In addition to being an online editor for MAKE Magazine, Michael Colombo works in fabrication, electronics, sound design, music production and performance (Yes. All that.) In the past he has also been a childrens' educator and entertainer, and holds a Masters degree from NYU's Interactive Telecommunications Program.

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