John Waters wrote:

> For some of the optocoupler chip, e.g. 4N27, there is a pin coming out
> from the base of the phototransistor. The phototransistor senses the
> light coming from the photodiode to turn itself on or off, so what is
> the base used for?

You can add a resistor between base and emitter for quicker turn-off.

Gerhard

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