[Tungsten filaments, CdSe photocells, MOV resistor with exponential
curves]
Those are all interesting implementations, but they aren't a new
mathematical identity like Max was asking for. They're just varying
implementations of the exponent/logarithmic circuit.
One interesting thing about the CdSe photocells is that they're very
linear in electrical response while being nonlinear in optical
response. That has very real advantages in many applications.
Tim.
CdSe? I've heard of CdS, although I can recall, as a kid, dismantling
Selenium rectifiers and using a single plate as a photocell.
...Jim Thompson
CdSe cells are similar to CdS but have faster response. They were once
popular as choppers, among other things. HP made a cool DC voltmeter
that went down to 1 mV full-scale (or 1 uV? I forget) that used them
to chop and demodulate, all clocked by a synchronous motor and a
spinning slotted disc in front of a lightbulb.
When *I* was a kid, I generally used photomultiplier tubes, or
germanium transistors with the tops of the cans cut off.
I made an IR detector once that used a flashlight reflector and a
black-painted germanium transistor, measuring just the Ge leakage
current. Now that I think about it, it probably accidentally used
thermal runaway as a positive-feedback gain enhancement mechanism. It
could easily detect the heat of your hand from six feet away.
John
