SIPMOS as current source
I think that in principle you can use the BSS126 (depletion type NMOS) as a crude current source, just like the BFR30 (N-channel FET, looks like a JFET to me)

Both are depletion (normally on) types so both will can conduct a current when Gate and source are shorted (Vgs = 0).

However, the datasheet of the BFR30 tells is that the drain current at Vgs = 0 is between 4 mA and 10 mA. For the BSS126 the range is less tight, there is only a minimum current guaranteed of 7 mA. So the current could be much larger as well, it is not guaranteed to be less than some value.

But don't think that since in the other question the issue (5 to 24 V input range) was solved with a JFET, it now has to be solved like that was well. There are many other ways as well. The JFET was there to keep the current through the optocoupler's LED somewhat constant, is that really needed? Many LEDs will work (light up) with a current between 1 mA to 20 mA. So is there a need to keep that current constant?

If you just need to detect that the 5 to 24 V is present or not, there might be no need to keep the current constant.

Maybe a couple of resistors and a zener diode is all that's needed. If the current does need to be somewhat constant there are other solutions, for example these circuits:



I know it says 6 - 15 V, with some small changes that range can easily be made 5 V to 24 V.