BOJACK PC817 OPTOCOUPLER DIP Installation PC817C 2.54 mm Pitch 4-pin DIP-4 (Pack of 50 pcs)

While the input LED has a 50 mA max rating (!) I ran it at 2 mA. Didn't compare rise/fall times with published, but I only needed a few microseconds and this was much better than that.

In my recent immersive dive into modular synth/sequencer voltage control of circuit bent toys I have learned a cold hard truth - connecting different devices together is tricky and you need to think about isolating the connections. One thing needs to influence another thing but we don’t want the reverse. Trying to use direct connections with transistors failed due to undesirable current flows back into the source device. Enter the optocoupler or also known as optoisolator. Very simple - two pins drive a tiny LED inside, and the other two pins act as the emitter and collector of a transistor. As long as there is a potential difference you can connect those two pins like a solid-state relay by applying just enough forward current thru the LED. For a 5-volt source potential, a 330 ohm resistor works to protect the internal LED and provide enough current to light it. The LED acts as the ‘base’ of the transistor but with absolutely no connection between the LED pins and the switched (transistor) pins. Brilliant and so easy to work with. 50-pack feels like a great value and the small rectangular package that snaps tightly shut is super handy.

everything as described

100% recommend

Work as intended. No complaints.