So here's a wacky part I found in a box: it's a special high voltage diode that is in a package with two IR diodes. You know an LED emits light when you apply voltage, but did you know you can change the breakdown voltage of a PN junction by shining light on it?

The trick here is that the main diode has a very high reverse breakdown voltage. You hit the bare PN junction with a slug of IR from the two on the sides and suddenly the main diode conducts. It's a high voltage, optically isolated switch

https://www.globalspec.com/FeaturedProducts/Detail/VoltageMultipliers/VMIs_OC100HG_High_Gain_Optocoupler/28771/0

This works with any diode including LEDs, which is why you could reboot those raspberry pis with a flash a few years ago: the IC package is a little light leaky and you could force the transistors within to conduct when they shouldn't

@hamster So what you’re saying is… optical logic gates incoming ;)

@bit0fun haha that would be fun

@hamster Probably would just be easier to use LEDs and photo transistors than this specific diode, but I’m assuming they all work on similar principles

@bit0fun they should! But the effect is very subtle.

Another fun thing is that PN junctions change conductance based on temperature as well. You can literally turn a common NPN transistor into a temperature sensor.

@hamster oh yeah, for sure. Pn junction temperature sensors are common on ICs, since it's stupid cheap to add temperature die sensors that way.

And yeah, the LED stuff reminds me of a project I made from a make magazine years ago, like the really early ones, that had a transimpedance amplifier with green LEDs on the inputs, which the resulting voltage fed into this spreadsheet app the guy made to calculate air quality. Doubt the spreadsheet is still available, but I'll have to dig sometime to get the article

Still have the circuit laying around at my parents place; might be able to make a schematic from it