Mystery Box Gives Up Its Patented Secrets

[CuriousMarc] likes to go to surplus stores even though there are fewer of them around. On a recent trip, he found a box that had some parts he thought would work for a temperature controller project. It was marked Dial-A-Level and proudly proclaimed that it had a patent pending. The box was from the 1970s and [Marc] was wondering what the device was meant to do.

The device was a bit of a puzzle since it had three oddly-marked probe inputs. A search through the patent database revealed the device was a “capacitance probe for detecting moisture with very long cables.” The idea was to create a capacitor at the end of the cable and use the liquid as a dielectric. The sensor creates a 10 kHz sine wave it uses to excite the probe and an op amp measures the relative capacitive reactance of the probe versus a reference capacitor. The rest of the circuit is a comparator that reacts when the level is at a threshold.

We love seeing the old hand-drawn boards from that era. Component designations are in copper and there’s no solder mask visible. There was a clever application of a silicon controlled rectifiers and a relay to create a type of flip flop, that [Marc] explains.

Interestingly, the company that made the device, Expo Instruments, is still around and [Marc] contacted them. The actual patent holder replied and was amazed that [Marc] had possession of this antique. You can only wonder if anything you build today will wind up on whatever passes for YouTube three or four decades from now.

Capacitive sensing is quite versatile. Of course, there are many other ways to sense liquid level, too.

21 thoughts on “Mystery Box Gives Up Its Patented Secrets

  1. I had a piece of equipment I was trying to reverse engineer a while ago due to it having a bug that rendered it useless after trying to use the “factory reset” option (a serial console server, if you’re curious). There were a great many comments in the various shell scripts and other bits of readable code inside it that made it work, and I decided to try to track down the author to ask if they could help with the problem. Ultimately, it took me down a long and winding path that revealed the author had died prematurely due to complications with diabetes. I was sad, because after reading all that code and all the associated comments (of which there were many filled with bits of both wisdom and snark) I felt like I had gotten to know them and looked forward to a chance to properly meet them.

    I think that, with all the technology the hackers and makers of today try to analyze and repurpose, it’s important to remember a fellow human created it. It’s a rare occasion when we can contact the original creators of said technology, but it shouldn’t be so rare. It’s nice to hear someone, for once, took it upon themselves to go that extra step to research and contact the original creator. As nifty and curious as these items are, there’s more to them than just the BOM needed to create them, and honoring the motivations and efforts of the designer seems so rare. There’s a lot to be learned from “the thing”, but even more to be learned from “the creator”.

    1. “You can only wonder if anything you build today will wind up on whatever passes for YouTube three or four decades from now.”

      The only things that will be available on YouTube in 3 decades will have the approval of The Ministry of Truth.

  2. That’s actually some pretty useful info. I was toying with some automation ideas for keeping aquariums and plants watered without using a probe that would end up being sacrificial due to corrosion, but hadn’t seen a good, insulated, capacitance example yet. Mind you, I hadn’t actively searched for one, either. Hack-a-day keeps me lazy, like that.
    I could easily adopt this to allow the logic to be handled by one of the many programmable household thermostats I have laying around, which would also handle temperature and filtration flow without having to build a controller circuit or enclosure, and give me WiFi monitoring and control as well!

  3. I’m confused by this one … isn’t it just a capacitive level probe? There’s nothing odd or unusual about this. This is a very common level probe in the industrial world. Ametek/Drexelbrook has made them for years.

  4. Just an FYI, PCB layouts like this are taped, not drawn. Probably at 4x size and photo-reduced later. Sheets of adhesive hole patterns and sometimes rounded corners are combined with adhesive tape of various widths. You can get pretty good at tear-up and reroute — using tape is where “tear-up” comes from. Placing all the little fiddly bits was a PITA, like part names (from sheets of letters and numbers of several sizes) and fine lines for outlines and locations.

    1. Yeah we used to do that on mylar, but we still called it drawing because it was on a drafting table. We had a lady who was a whiz at it and we would sit with her while she did the layouts. She was busy one time and I needed a small power supply board so I did it myself. My boss asked who laid out the board. “I did!” I said. He said, “Don’t do that anymore.”

  5. Here is the manufacturer’s web site:

    Expo Instruments, “liquid level specialists”. No address on the Web site, but from the 408 area code one may assume Expo Instruments is located south and east of San Fransisco. Indeed, the labeling on the back of the instrument says that at build-time Expo Instruments was located in Sunnyvale, CA, which is in the 408 area code.

    Side rant: Man, YouTube has become unwatchable these days. I have to dismiss three or four pop-ups just to get to the video, then I get smacked in the face by two ads even before the video starts, then there are three more adds breaking up a 20 minute video. Sheesh, when the third ad started I just closed YouTube out of disgust. A shame, [CuriousMarc’s] video was quite good. One thing is certain, I will NEVER use Grammerly, EVER. There are waaay too many Grammerly ads on YouTube. So now I hate Grammerly. A lot of people have been moving over to a YouTube-ish clone called Rumble. I followed some fed-up content creators over to Rumble and the difference is like night and day. No in your face ads, no pop-ups, pages load quickly, no Grammerly. But who knows how long that will last. The down-side to Rumble is that it seems you need to register to see comments. Nah, that’s bad. Sigh…

    1. I finally sprang for the premium to watch movies and longer videos without the ads. It comes out to 2 or 3 bucks a week and since I quit cable and TV, it is a bargain. But many of the more popular providers are now doing very long in-video ads for sponsors and seeing those more than once is painful. Rumble? I will check it out.

    2. uBlocker works very well to block all the annoying YouTube ads in Chrome. I can’t stand watching YouTube without it anymore. I get that ds are a good way to pay for otherwise free content, and, in most cases, I just ignore them, but YouTube has really gotten unusable without an ad blocker.

      1. mythoughts62 says: “uBlocker works very well to block all the annoying YouTube ads in Chrome.”

        Huh, I didn’t know that. Thanks for the tip. I agree with you, YouTube has become abusive. I’m using uMatrix which is related to uBlock Origin but was recently abandoned by the original aurthor. Normally I don’t block ads because I feel the content creators I visit deserve some compensation. If I’m visiting a new (untrusted) site I crank up uMatrix to fully protect me, then inspect all the nonsense on the site in uMatrix and decide what to white-list. I much prefer uMatrix to uBlock, but I’m probably going to have to move over to uBlock soon. I’m running mainstream Firefox not Chrome, there’s nothing Google related on my machine if I can help it. I wonder if uBlock will block YouTube ads on Firefox like it does in Chrome. Hmmm…

  6. Back in the mid ’80s, I worked at a small factory where I, among other things, did 4X circuit board layouts on Mylar. But that’s not the interesting part. We had a large air compressor that supplied air to most of the factory, pretty much everything except the paint area. It ran on a huge (to me at least) 3 phase electric motor. I think it was around 100 HP. Every 10 years, it was taken down over a long weekend to be rebuilt, new rings, seals, etc. I’m not sure exactly what, as that was very out of my bailiwick. When the time came to do this, the maintenance people discovered that the supply of spare parts they’d been using for decades needed to be replenished. They checked the name of the manufacturer on a nameplate and discovered that they had moved, but were still in business. The interesting part is that they discovered that the compressor was well over 100 years old, and was originally powered by a steam engine, someone, possibly long before anyone currently working at the company was born, converted it to an electric motor. The company representative was amazed to find it still in use, and, by a miracle, they had all the original specifications and blueprints. They agreed to make us a few decades more spare parts in exchange for using the story of our compressor in trade magazine ads.

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