On 25/11/2019 14:13, Dave W wrote:

I was going to suggest a series of
reflectors/light-guides/fibre-optics illuminated from one 24W light
source when the gate is closed. Anyway a roadsign type of
retro-reflector would pick up the vehicle's headlights.


How about if I mount a magnetron on a big satellite dish and point it at
a sealed container of water bolted onto the gate? The water boils,
powers a little Mamod steam engine that turns a dynamo which charges the
battery. Simples.

Bill