Scott wrote:
What about a time delay switch? Could you get one that cuts off after
(say) 2 minutes?

Having taken the thing apart, it turns out the bulb holder is actually part
of the switch mechanism, and one end of the bulb moves up or down when you
flip the switch.

So mechanically it's a bit tricky, but I think I can do something with:

- intentionally blow the bulb, so I have the glass envelope to use as a
substrate

- fit some white LEDs and a resistor across it. The LEDs are chosen such
that the forward voltage is set to match say the '25% battery' voltage
(looks like 12.0V). When the battery voltage falls below that level the LED
current collapses and the load naturally falls away.

It would need some tweaking to get the current curve right, and it's
possible a more sensitve cutoff circuit would be needed. Worth a try
though.

With respect, I think the original question is the wrong question :-)
Surely the correct question is how to find the best solution to the
problem.

Well indeed, but 'I wouldn't start from the bottom of a hole' is not an
answer to a question about the best set of grappling irons.

Theo