Foolproofing open-collector outputs

In my youth, I designed an acoustic monitoring system for NASA's Mississipi Test Facility. I used a GR electret microphone, temperature regulated with a beta-limited power transistor as the heater. We selected the base resistors.

NASA thought my telemetry was oscillating at subsonic frequencies. We actually discovered the mating call of bull alligators.

The locals enjoyed shooting the boxes up on poles.

Very sneaky. I will file that trick away!

piglet

Interesting. Beta usually goes way up with temperature, so ISTM that wouldn't regulate by itself. How did it work?

Maybe they were paid off by the young alligator dudes.

Cheers

Phil Hobbs

Oh, it had a thermistor, closed-loop. The base resistor just limited the power. We just ran the mic at around 50C to keep it stable and dry.

I used a bare-gate jfet follower (no gate resistor) and let the electret leakage keep the gate voltage near ground.

Sounds about right for Mississippi--just above the dew point in August. (Of course I live in downstate NY, which in some years isn't that different.)

Well, those phenolic PCBs and Bakelite spacers probably helped too.

We've been using Sensirion SHTC3 T/H sensors in all our temperature-controlled things to make sure we avoid condensation. Keeping at least 5C above the dew point makes life a lot better.

Cheers

Phil Hobbs