4n25 Base pin - to ground or not?

pacman

I've recently come across some schematics that use 4n25 Opto-couplers. On many of these schematics there is a 10K resistor between the base (pin6) and ground.

Normally I don't ground the base when using opto couplers but perhaps there is a real good reason why (I presume it's to stop false triggering - but I could be wrong).

I've checked data sheets and several other schematics - mostly the base is left unconnected, but there are a few ...

Is my presumption correct (about false triggering)? What is best practice? Why?

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Chris Savage

Normally you can leave the base pin floating. If you connect it to ground you effectively disable the opto.· The base pin is brought out for pre-biasing including the ability to use it to set the saturation threshold.

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Chris Savage

Parallax Engineering
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