A interesting question about use PC817 Optocoupler

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thanhmssl10

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I think this problem is so interesting, I use PC817 like Datasheet, I connect pin 1 to a resistor 200 and a pin ( transmit PWM) from MCU, pin 2 to Ground, pin 3 to Power and a resistor 3K, pin 4 to Ground
Wow, it not work!

But If i add a diod to pin 1
It Work!
I Can't understand this problem because principle of Optocoupler is very simple, diod on and Transitor on, Have Any Answer for this problem?
 

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