The local community hall is infrequently used (and used mostly by women
at that). How can we minimise water usage in the men's?

I came across mention of a system which only fills the urinals' cistern
when some other water is used - possibly the wash-hand basin. Can
anyone point me to a spec and source of this sort of control.
I am aware of the sort which uses a motion detector and only initiates
filling the cistern when someone moves in the loo area. I am hoping the
mechanism which I am hoping someone can recognise from my brief
description might be cheaper and easier to set up.

TIA
Frank Stacey