I fall into the jack of all trades, master of none category. You can pick a subject and I may know something about it. HVAC (EPA certified small appliance and auto AC). Ductwork design. Help put an AC in a house in my teens. Automotive: put an AC in a car from a box, replaced a clutch, steering rack harmonic balancer etc. No formal training. Workig on cars since pre-teens. Small engine: Dad said, you break it, you fix it. I through a rod in my early teens. Rebuilt engines from crankcase up.
Making better is always my mindset. So at 15 before I had a driver's license i had a car. A severely broken one (wrecked).
A 1968 model car got 18 miles per gallon. Electronic ignition retrofit, carb rebuild, head rebuild.
A home drainage system design. I have a general 4 year Engineering degree. 2 years in EE and a 2 year AAS degree with a 4.0 GPA. So, thermo, strength of materials, relativity, solid state physics are all in the cards.
So, I spent most of my employment, making measurement systems, doing measurements in a semiconductor electronics lab and maintaining electronics in the said lab. Anywhere from high power high voltage to very small currents to RF 1000 W power supplies. Played with system that generated 100 kV @ 0.1 A and 13 kV at about 2 Amps DC. And system that used 1e-12A and 100 V. Light bulbs that take 40 kV to start and 22V 40 A to run.
Then there is mechanical design and construction having full machine shop access. I was basically trained by the shop personnel. I can't set up systems for welding, but I do nearly perfect welds once set up (MIG/TIG and stick)
Glassblowing and sealing stuff under vacuum is also in the cards. e.g. I basically have the skills to seal a CRT neck or vacuum tube although have never done it.
when our cheap SEM became unserviceable via contract. I did it. I set up a fairly decent EDAX unit using a plug in card and a multi-channel analyzer.
Then don;t forget the maintaining of PDP-11 micro-computers (the PDP-1 mini-computer was another life) and various programming languages. In the 80's a Direct Digital Control system written in Fortran using a real-time OS was a big deal. 7 PID loops and recipes.
And a fair amount of stuff with vacuum systems and pumps. e.g. 1e-9 Torr. Fingerprints matter at that range.
I put together a very comprehensive semiconductor gas safety system which actually worked when the lab blew up.
A1500 lb pressure burst of hydrogen was released into an plexiglass hood by accident, someone thought the Hydrogen cylinder was empty and released 1500 lbs of hydrogen in a line rated for vacuum. No evidence of fire. A safety device failed.
Really had the mindset to look at whatever I'm doing with safety first. You don't always have the ability to evaluate what went wrong in a disaster. It's much better to pre-plan.
Redesigning was hard. My part was somewhat easy. Expand to real hood velocity meters instead of toys. Install the hydrogen alarm that was sitting in a box (per management) and make the valves controlling the cylinder pneumatic rather than direct electric.
The hydrogen explosion destroyed the velocity sensors and shut the gases down at the cylinder. This happened during a cylinder change operation,
It doesn't even touch the surface. Built custom instrumentation. Custom mechanical stuff.