This video shows how to code an interrupt on an ATtiny85 with just a few coding lines. It's really useful when it comes to adjust sensor values for example.
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It is a simple but very useful idea.
A weak light, for example for orientation at dark moonless night.
The circurit is as simple as possible. It includes all the calulations.
The way of creating it makes it a very useful part.
And its done very fast.
Just a small part of a perforated circuit board, 2 parts and some soldering.