Most of the current levellers use chips based on a bubble of heated air (look up MEMSIC). The ones based on a micro machined beam/pendulum (e.g.ADXL202), were more common on the first solid state levellers, which first started to appear about 10-11 years ago. The ones based on gas are more rugged. Clever stuff.
When I first got into subs, everyone referred to levellers as SALCON's, much in the way PMMA is often referred to as Perspex
I couldn't get one, as they were out of production, and information was a bit thin on the ground for making your own- the circuits in Norberts book were a bit cryptic to say the least. So I ran without one for a while, fortunately the boat I had ran quite well minus a leveller as it wasn't terribly fast, and was very stable.
The earlier designs were based around analogue devices, like the 555 timer chips. These days they're mostly based on micro controllers, usually a PIC. Those sort of little wonders didn't really start appearing until the early to mid nineties, and have since, alongside other brands like Atmel and ARM, transformed R/C electronics.
A chap in Germany still does analogue levellers, which are combined with a depth controller. They enable the operator to finely tune the PD loop directly to the boat they're fitted in, but they take a bit of setting up.