I am using pighixxx.com for all pinout references, also for examples of basic circuits. But is it reliable source of information? I have this question after I spent some while trying to connect a relay using this example with an optocoupler. The example provided by pighiXXX does not seem to work at all. How come this pighiXXX circuit can work if microcontroller provides 1V on pin high, when 4N25 datasheet says it's forward voltage should be 1.36V?
Where do you get the idea that HIGH is only one volt? It would usually be 5V for a typical arduino...
Well, when I measure voltage at optocoupler pins 1 and 2 I see 1V on Attiny85 pin high, 0.2V on low. Since optocoupler does not switch accordingly on pins 4 and 5, that makes me think that I have not enough voltage on pins 1 and 2. Am I missing something?
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when I measure voltage at optocoupler pins 1 and 2
if you measure directly across the optocoupler, you'll be measuring the voltage drop of the LED in the optocoupler, rather than the output voltage of the AVR. You need to measure on the AVR side of the resistor to the coupler.
It's not a great circuit; they're using the NPN transistor in the optocoupler as a "high side" driver, while NPNs prefer to be in the ground leg of circuits. But I think it should work; I'm also suspecting that you forgot the "pinMode" call...