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Thanks, the series resistor saved the LED (doh!). Also, it seems the weird behavior when using a buzzer was due to EMI between piezo buzzer and trigger input for the open-collector (an iButton reader).
Sorry, I think I don't quite master the English terminology. I think what I meant is best put as "the open collector will not cut-off". Please let me know if this is better so I will update my question! Thanks.
