[QUOTE 3928306, member: 9609"]yes, i'm definitely wanting it to work from the wheel in the same way as a cycle computer works. I could fit a magnet and a reed switch - its the rest that mystifies me. there must be some gizmo on the market that would keep a ciruit closed when a pulse is sensed, then switch it off when no pulse has been noted for 1 minute.
Is there any lights that work this way ?[/QUOTE]
I don't know about lights that have this feature. "Watchdogs" are very widely used in CNC machine tools. The controlling software sends out a pulse regularly to show that the software is running. If the software hangs up and doesn't send out a pulse within a chosen period, the watchdog closes the machine down in a controlled fashion. They are designed to prevent robotic machine tools from running wildly out of control.
The circuit can be very simple...less than a dozen components on a piece of Veroboard.