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I am working with TLP281-4 optoisolator, there is a board which used this IC in it. It connected like this:

connection diagram

Why can't I connect 5V to the collector and take the output directly from between emitter and GND? Output is read by microcontroller. Is there any advantage of using those transistors?

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  • \$\begingroup\$ I can't read the part numbers, but this is either to invert the logic or increase the current drive capabilities I believe. \$\endgroup\$ – Ron Beyer Oct 18 at 16:58
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You could, but the TLP2851-4 can only sink 50mA of current. The addition of a second transistor could facilitate more current and allow for faster rise times and has a potentially higher gain (acts like a darlington pair). For a high current load/source use a second transistor

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