Is it possible to build a battery powered circuit that switches an electrical load (e.g. lamp), controlled both mechanically with a physical switch and electrically with a relay and preserve the mechanical control when the battery
dies.
The only way I can think of is a mechanical relay that is normally closed, but it will be impossible to power it from a battery for a reasonable amount of time.
simulate this circuit – Schematic created using CircuitLab
The neutral wires are inaccessible from the control circuit.
Best Answer
Your circuit checks out ok, but to solve your mechanical switch preservation issue, you would need a keep relay (also known as a latch or stay relay). These relays utilize a permanent magnet that keep their position depending on which direction the coil was last energized at. There are also double-coil variants that utilize two coils that can be energized with the same polarity. The switch is flipped when the coil opposite to the switch's current position is energized, thus creating a magnetic attraction and flipping the switch. The single-coil variants use only one coil, where the position of the switch is changed by inverting the coil's polarity to the magnet's polarity. Flyback diode-protected versions of single coil variants do not exist due to the reason stated above, so keep a watch out for them, as they are definitely scams.