[sdiy] KN-75 optocoupler

[Paul Perry]

If it is a LED/LDR opto, and you have 'tested' it by connecting a 9v battery
directly across the diode, then it is now just a LDR, the LED will have been
destroyed (no current limiting resistor).

In Finnish, the KN-75 is called an "opto-putki" which I think is "light
pipe" or "light tube" FWIW.

As for neons in answering machines, I've seen them as static electricity
discharge gaps.

Paul Perry Melbourne Australia