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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.

schematic

simulate this circuit – Schematic created using CircuitLab

The neutral wires are inaccessible from the control circuit.

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    \$\begingroup\$ There are latch-relays. Would that solve your problem or did I misunderstand you? \$\endgroup\$
    – Wesley Lee
    May 3 '17 at 23:32
  • \$\begingroup\$ I've got a latching relay with an exposed manual control, you can push the orange bit up or down with your finger or by using a pulse to the relay coil. \$\endgroup\$
    – Jasen
    May 4 '17 at 2:44
  • \$\begingroup\$ you can use a capacitor-backed SetReset flip-flop in front of the relay to resume the 1-bit state when relay power returns, which is usually co-incidental with the switched device's power, so that's what's important. then you don't need batteries to maintain. \$\endgroup\$
    – dandavis
    May 4 '17 at 8:23
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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.

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    \$\begingroup\$ What do you mean by "Fused" in this context? \$\endgroup\$ May 4 '17 at 2:42
  • \$\begingroup\$ @ThreePhaseEel Oops! Meant to say Diode-protected! Nice catch! +1 \$\endgroup\$ May 4 '17 at 3:48
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    \$\begingroup\$ (BTW -- you can flyback protect a single coil latching relay using a bidirectional TVSS.) \$\endgroup\$ May 4 '17 at 11:36
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An alternative to using a latching relay, (which is probably the best solution, and mentioned earlier) would be to add a mains operated relay to switch between the two switch lines, and power the relay from one of the switch lines (i.e. the NO contact), so the relay would stay on once energised, then add a NO pushbutton to turn the relay on , and a NC button to turn the relay off, this approach doesn't need a battery.

If you want to keep the battery, then use the contacts of the battery operated relay to do the function of the switches.

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