4N26 and 4N37 are cheap optocoupler that are good for most of the power electronics applications. They are six pin ic with optical isolation. The output is inverted because the output transistor works as a switch. The frequency limit of this ic is explained in this post. The following is the working circuit of optocoupler 4N26 and 4N37.

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It is a simple circuit. Do not use more than 100 ohm resistance in the input. The circuit will not work properly. It is because it will limit the minimum current required for proper operation of the input stage of this optocoupler. Do not connect the input ground with the output ground or you will lose the isolation.