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Requires to read 24 V input signal, and output to an Arduino as 1, at the same time it lights up the LED indicator. Is this circuit viable?

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This way the Arduino input will float so the optocoupler needs to be pulled down through a resistor:

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For other alternatives you can check: Optocouplers with non-inverting logic

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  • \$\begingroup\$ Is it absolutely alright to put the load on the emitter side instead of on the collector pin? Results found from the internet were mostly on the collector side, just like NPN transistor should have the load on Collector side. Does optocoupler follow these theory that if load on the emitter side, aka common collector (or emitter follower)...the voltage on the emitter side will be 0.7Vbe dropped? (5 - 0.7 = 4.3V?) \$\endgroup\$
    – Kenny Thum
    Mar 2, 2023 at 7:53
  • \$\begingroup\$ In this case yes it is alright (otocoupler) since the transistor is optically controlled. However, for a simple bipolar transistor (not the optocoupler case) it would be difficult to saturate it when the load is at the emitter side unless you use other components and this would be the case, for instance, of an all NPN bridge where you can use optocouplers to control the upper transistors. and for the voltage of the collector-emitter when saturated won't be a problem. \$\endgroup\$ Mar 2, 2023 at 8:02
  • \$\begingroup\$ @KennyThum The reason it matters when using an NPN transistor is that what controls the transistor is the base/emitter voltage. So the emitter has to be lower than the base to make it turn on - and the collector has no such rule. With an optocoupler it's the anode/cathode voltage that controls it, so there's no such rule for the collector or the emitter. \$\endgroup\$ Mar 2, 2023 at 17:13

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