Dial-A-Level lives again! (Patreon)

While on my expedition to Excess Solutions to pick up my enclosure for the Apollo GSE box project, I also snatched this 1970's "Dial-A-Level" contraption. I was looking for an on/off controller to do the temperature control for my Apollo IRIG gyroscope. The gyro needs to be regulated to within 0.5 °C, and that was done in the spacecraft with a simple on/off control off a thermistor. So I need a box that can monitor a resistance, or the voltage across it, and trigger an on/off heater within a band. Which of course I could build not too difficultly in a million modern ways. 

But this box which appeared to be from the 1970s and had the best ever name: "Dial-A-Level". It seemed to have all the required controls on the front panel: a min and max setting for alarm and control, and an oversize 0-100% analog meter. 

The back looked equally promising with a Motor/Load output and what looked like an alarm relay. It said it was made by Expo Instruments Inc, in nearby Sunnyvale, CA.

Inside was straightforward looking 1970's analog electronics, pretty clean. However, none of the boards had their screws left, nor did the cover have any of its screws either. Looked like someone tried to repair it unsuccessfully before it was ditched. 

One mystery though, the input seemed to have three connections named "probe", marked with 3 cryptic circles. Hieroglyphics anyone?

I tried to find some info on the web, but my searches came up dry. However there is still an Expo Instruments Inc. with a Sunnyvale phone area code, and a minimalist, amateur-looking website. It confirmed that the company was started in 1970, and specializes in liquid measurements using special capacitive sensors. Uh oh. That might be what my three pronged probe input is for. It had a little inquiry box so I left a short message asking for info about my 50 year old box, obviously without much hope of hearing anything back. Little did I know.

Fortunately, there was a "Patent Pending" displayed prominently on the front. With a little help from my company IP consultant, we quickly dug out 3 patents from the 1970's assigned to Expo Instruments, all from a single inventor named George Rauchwerger. 

All three patents were very clearly written, exact and informative, which is very rare. Usually the lawyers that write these have only a vague idea idea of what they are talking about, and it shows. But not here - it feels like the inventor himself may have written the explanation. 

One patent explained how the probe works. It is simply two isolated conductors. The capacitance they form is proportional to the permittivity of the material between them. That of air is close to 1, that of water is 83, so when you fill the thing with a liquid or some moist earth content, the capacitance goes up dramatically.

And another one detailed the principle of the electronics, complete with schematics

This weren't the exact electronics in my box, but it gave me a good hunch on how it would work. There is an oscillator in the kHz range that feeds a signal to the probe, which is amplified using a capacitor ratio-ing op-amp, detected using a peak detector op-amp, and level compared using a Schmitt trigger op-amp.

With that, I was able to reverse engineer the schematics of the main board, and eventually find the connector contact fault that was preventing it from working. 

I also correctly concluded that at least one strap was needed on the "Control Reversing" outlets, which should have been named "Control Select" instead, or nothing would work.

Thanks to one of the figures in the patents, I was able to fabri-cobble an ersatz probe with an audio transformer and a capacitor box. Turns out the circles depict a triax, which is a coax with two shields instead of one. The smaller circle was the center conductor, the medium circle the mid shield, and the large circle the outer shield. Or the Egyptian sun God Ra, no one knows.

I connected my pseudo probe to the hieroglyphics, a lamp to the load output, and lo and behold, the whole thing worked as designed. You can see her chooching in the short clip below:

https://youtu.be/-oYiasGEqjE 

Now the best of the story  is when I read my emails just after having finally repaired the thing. Someone from Expo Instruments had actually responded to my inquiry, and rightly wondered where the hell I found this box that they had not sold in 50 years. And that someone was none other than George Rauchwerger, the inventor himself! That made my day. He went on to explain that he wound up his company in 2017 and gave everything he had to Excess Solutions. We are still conversing as we speak. I hope I can meet him over a beer when the Covid episode is over!

I wasn't expecting much of anything exciting about this quick side project. I just wanted the thing off the bench so I could finally get on to meatier space hardware that has been languishing for so long. So I did not really film anything. And as usual, it turned into an interesting mini adventure. I might try to take some footage after the fact and cobble a video equivalent of this post, we'll see. In the meantime, you get the scoop.

[Edit: Well, I did end up making a video about it after all, pretty much based on this post's story: https://youtu.be/0GpT4YlAeTo  ]

As always, thanks for the support!

Marc