Normally Closed Optoisolators

Instead of switching the actual load with the opto, use the opto to drive a transistor which switches the actual load.

However, note that if the opto fails short then that will open the transistor.

The risk could be potentially reduced by having two optos wired series on the output and parallel on the input so that both must close (or fail) for the transistor open.


simulate this circuit – Schematic created using CircuitLab

Yes, there are DIP opto-MOS relays with depletion-mode MOSFETs on the output.

Or you can use a photovoltaic gate-drive optocoupler with a discrete depletion-mode transistor. Just remember to connect it to drive the gate negative.