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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.

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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.