I have a small Wayne 12 volt battery backup pump. The wires coming from the switch had a float which I removed. When those 2 wires are touched together it activates the pump. I would like to wire the wires from the switch and the 12 volt charger to a relay or solenoid so that when power is off from the charger the circuit completes. and activates the pump. Can you help me?

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    \$\begingroup\$ Hmm, the only way I can think off would be a 12-5v regulator, to a simple micro controller, a small bit of C, when the pin connected to the charger is low, I.E off, another pin connected to the gate of a Mosfet/relay is High, connecting the circuit and enabling the pump, I personally use micro controllers for almost every thing, I'm sure a better answer is here somewhere. \$\endgroup\$
    – Alex
    Mar 25 '16 at 16:19

This is quite straightforward, and requires a 12-volt, SPDT relay with contacts adequate to handle the pump current.


simulate this circuit – Schematic created using CircuitLab

As long as the charger is providing voltage, the common (the upper in the diagram) lead contacts the NO (normally open, on the left) contact, while the NC (normally closed) is not connected. When the charger shuts off, the relay relaxes, and the common connects to the NC contact, turning on the pump.

  • \$\begingroup\$ you might want to add a diode across that coil. \$\endgroup\$ Mar 25 '16 at 16:40
  • \$\begingroup\$ This sounds like the answer I'm looking for but have yet to try it; I will forward the results. Thanks \$\endgroup\$
    – mark
    Mar 26 '16 at 17:50

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