Dave Liquorice wrote:
On 22 Jul 2020 11:59:16 +0100 (BST), Theo wrote:

- 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.

Apart from that through the resistor across the LED and modern white
LEDS take naff all current. You can get LED versions of pretty well
any automotive bulb that is out there apart from some headlight
bulbs. You'd not want ones that are compatible with some cars "bulb
failure" systems as they have resistors to ensure they draw enough
current to supress the "bulb failure" warning.

In this case the circuit is carefully designed to be sensitive to voltage.
A regular automotive indicator LED bulb is a 3.2V white LED and a series
resistor which drops about 10V. The resistor limits the current so the
forward voltage of the LED is immaterial. In my setup the current is very
much dependent on the forward voltage, which is the point.

I'm not sure I'd be able to find LEDs when put in series with quite the
right voltage/current curve though. It might need a more conventional
cutoff circuit.

The correct solution is to work out what stops the boot from not
closing properly and fix that. Bit of lubrication? Small adjustment
to the pin that the lock striker engages with? Still if trying to
develop a complicated workaround, ignoring the root cause, keeps some
one happy...

In this particular case, a load of cardboard fouling the lid.
Root cause analysis doesn't help when the root cause is the idiot user.
Upgrades to the idiot may be unsuccessful.

Theo