[sdiy] current mode signals (Is everything digital?)

[Aaron Lanterman]

> By using currents to control systems, the current can be varied at one
> point and measured any distance away. Signals external (common mode
> signals?) to the current loop have no effect on the loop. This
> eliminates interference from external signals (mostly RF). until the
> signal is large enough to destroy a component in the loop (EMP?)

One of our senior design projects at Wash U (we didn't get to pick the 
project, the prof did) was to design a control system for an oven. The 
oven consisted of a light bulb in a trash can with a thermal sensor 
(thermistor I think, I don't recall now). You had to put the right 
histeresis in, or it the relay controlling the lightbulb would chatter and 
buzz when it got close to the right temperature.

To make this silly project just vaguely more interesting, you had to run 
the thing over this massive lenth of cable they had looped around the lab 
many many times. The trick was to use current to control it, instead of 
voltage...

The nightmare of getting that working was the ground on our breadboard. 
We'd look at our schematic and find a couple grounds, and when we went to 
measure them in the actual circuit, ground at one point would actually be 
different than ground at another. Fixing it didn't involve changes to the 
schematic, it involved physically rearranging things on the board. The 
prof we had for that half of the class was very good at suggesting 
what to rearrange. At the time I thought it was a stupid boring 
project, but in retrospect I did learn things from it.

That was one half of the semester. The other half involved designing a 
power supply regulation type thing for a solar panel that would let the 
user set the output voltage. It turned out to be pretty complex, but I 
can't recall why. The prof made a big deal about correctly chosing the 
right value of a certain capacitor. He spent an entire lecture on it. We 
breadboarded our design up, got it working, and demoed it for the prof, 
and then mentioned we found it odd that we had forgotten to put in this 
all-important capacitor but the thing still worked! He said it couldn't 
possibly work without the capacitor, but the proof it could was in the 
demo and he couldn't argue with that...

- Aaron