As promised, a schematic for your 'Something' is attached. Here is an explanation of how the circuit works:
Two slightly unusual components are central to the design: depletion mode NMOSFET, photovoltaic (PV) coupler.
Most NMOSFETS are enhancement mode types so that when the voltage between their gate and source (VGS) is zero the resistance between their drain and source (RDS) is very high. To make an enhancement mode NMOSFET conduct the gate needs to be made more positive than the source.
But, in contrast, depletion mode MOSFETs have a low resistance between their drain and source when the voltage between their gate and source is zero. So depletion mode MOSFETs are rather like thermionic valves in this respect. To turn a depletion mode NMOSFET off the gate must be made more negative than the source.
PV couplers, unlike the more common opto couplers, produce an output voltage when current flows through their input LED. The output is only capable of sourcing a low current though. Like the opto coupler, there is complete isolation between the input and output.
The circuit works as you say in reply #18 (posted before this post was complete).
The truth table is:
SW2 | SW1 | LED2 | LED1 |
Open | Open | Off | Off |
Open | Closed | Off | On |
Closed | Open | On | On |
Closed | Closed | On | On |
Here are some points about the design:
You may be wondering, why two PV couplers. The reason is to get sufficient negative voltage to turn the depletion mode NMOSFET off (-2.1 VGS) and have an Iq of 100uA, which I thought would be a reasonably low current. But if an Iq of 200uA were acceptable, by changing R3 to 33K, you could use just one PV coupler.
R5, C1, and, C2 have no role in the general functioning of the circuit.
R5 is a gate stopper which is a precaution to discourage the NMOSFET from oscillating. R5 should have short leads and be connected directly to the NMOSFET gate terminal.
Likewise C1 should have short leads and be connected directly between the MOSFET drain and source.
C2 also helps the stability of the NMOSFET, but is is mainly for smoothing the voltage from the PV couplers.
C1 and C2 should be high frequency low ESR capacitors, ceramic X7R dielectic or metal film polypropylene. The values of C1 and C2 are not critical.
This is a paper design, so it has not been built and tested. If you build the circuit I hope it works OK.
Data Sheets:
NMOSFET DN2625
http://ww1.microchip.com/downloads/en/DeviceDoc/20005537B.pdfPV Coupler TLP3905
https://toshiba.semicon-storage.com/eu/product/opto/photocoupler/detail.TLP3905.html