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I am trying to Optoisolate a signal from microcontroller. I am driving the signal to base of MMBT2369, and opto isolator LED is connected to Collector of the transistor.

When transistor is not in place, uC output is as per logic i.e toggles every 1 second. When Output from uC is connected to base of MMBT2369, uC stops toggling and output is always constant low.

What can be the reason for this behavior?

Circuit

Circuit is as shown. SOL_ENB is signal from Microcontroller.

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You need a current limiting resistor between the base and the microcontroller. Otherwise you are driving the micro controller pin to a low impedance input which drives it low, your micro controller pin is currently switching between 0V and 0.7V and wasting a lot of current. I recommend trying a 1k resistor between the micro and the base.

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  • \$\begingroup\$ +1 As well, R15 is probably too low, depending on the CTR of the EL817 and required speed. \$\endgroup\$ – Spehro Pefhany Jun 23 '15 at 16:40

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