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The PLC is programmed from a laptop which runs software from the PLC manufacturer. If I had built the control system from relays it would have required about 120 relays and 9 timers to get the number of switching contacts required. It turned out that the diagram I was following (from the library book) had several mistakes and omissions in it, so I was pleased that I could solve the problems quickly by reprogramming, rather than spending ages with a soldering iron rewiring relays. As it was, there were still a couple of bugs which we found ways around during the Convention. The model didn’t quite perform flawlessly (floorlessly?), but apart from an early door jam, and the software bug, was otherwise very reliable and survived 4 days of almost continuous operation.

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