I can't understand why the other unit isn't going. Just coincidence? I used to carry a pillowcase full of (unboxed) valves for tuning accurately. All to do with chance messing up the equation. Thems were the days.
I had vast steel doors on my Frinton home. None of those tinsel ones they supply these days. The real half-horse motor had its power conveyed by a huge motorcycle-type chain to doors that would chop a Lotus in half. An no, they did not have a detector to stop them.
They were actioned by a loop in the drive and a chatterbox in the car sending pulses to a coil. When my mate (who built the house) was given the loop, he said, no, it looks like a ring of wood with wire round it. The builders said, gosh ( or something like that ) we've buried that thing in the drive. They drilled up an 8" thick drive to get it and set in the correct one.
This loop sent pulses to a cold-cathode valve in a big steel box. It all worked - if you got the car in the right place. Also, lightning would set it off - and one stormy day, there was a Lotus half way in. That's how I know.
Not quite in half, but may as well have been.
In about 1975 I brought back a Geni Door controller system from Texas. The two 'beepers' were still going 25 years later.
One last memory, then I must do some work. Wife arrives in E-Type with a police car following her. They were all smiles, and just wanted to know what the round wooden thing was under the tail of the car. 'Thought it might be a mine.' said the smiley Bobbie.