For a vending machine telemetry project, we need to be able to read the Audit Data immediately after the machine is refilled. This is crucial, as it provides the baseline for all subsequent readings until the next refill.

The trouble is, the machine power is likely to be off until the filler has shut the door after they've finished refilling (safety interlocks).

Fillers are likely to be unskilled & non-technical.

Therefore we need a simple button that the filler can push while the machine's still open, and it will retain the "pushed" state until the power comes back on and the processor can read it, then the processor (8051-derivative, of course!) can reset it to the "not pushed" state.

There is a strong desire to avoid any sort of backup battery at all - principally on grounds of cost.

So, does anyone know of any suitable device?
Possibly something that latches mechanically (or magnetically?), with an electric release facility.
But it must be cheap - else we might just as well have a battery!

I had thought of something like making an SR latch with something like the Philips picoGates - which can survive on a few uA. It would need a battery - but only a small one!