I have an air condition condensation pump that has a float switch which interrupts the control signal from the thermostat to the outside aircon unit if the water level in it gets to high - i.e. if the pump fails for some reason. That's wired into the NC half of the float switch.

What I'm hoping to accomplish is to wire a small battery powered siren through the NO side of the switch so that if the float switch is triggered, the aircon is powered down (which is already it's current behavior) and the siren starts sounding, letting me know I have an issue. Is it a problem to have the common used for two completely different circuits? I'm thinking not since I'm just using the switch to complete circuits, but I'm not entirely sure and don't feel like burning up a control board somewhere.

Here's my diagram if it helps explain it better.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ The way it is hooked up now will work. \$\endgroup\$
    – user66377
    Jun 13, 2016 at 2:40
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    \$\begingroup\$ As wired, no. You wouldn't want to mix the 9V and 24V lines. Simpler would be to add a second float switch. \$\endgroup\$
    – Passerby
    Jun 13, 2016 at 3:28
  • \$\begingroup\$ Even though when it's NC, there's no path for the 9V to follow and NO leaves the 24V circuit open? \$\endgroup\$
    – kazoni
    Jun 13, 2016 at 14:04
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    \$\begingroup\$ Wait so you want to power the siren from the 9V battery or power the AirCon by the Thermostat which is determined by the float switch? \$\endgroup\$
    – Bradman175
    Jun 14, 2016 at 0:03
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    \$\begingroup\$ The proposed circuit does not "mix the 9V and 24V lines". I see no significant issue with the circuit as shown. Even if there IS some sort of catastrophic fault, 9V batteries have so little stored energy, and such high source impedance it would pose little risk. If you really want to be "conventional" then use a 24VAC siren. Or a rectifier to operate the DC siren from rectified 24VAC. \$\endgroup\$ Jun 14, 2016 at 0:37

1 Answer 1


The circuit shown has no significant issues. Typical rectangular 9V batteries have very limited energy capacity and rather high source impedance. So it offers little risk in case of catastrophic failure of the switch or wiring.

Using a 24VAC siren, or rectifying the 24VAC to something suitable for the DC siren would be a more conventional approach that would raise less concern from critics.

  • \$\begingroup\$ I'm trying to do this with parts that I have on hand. The other catch is the 24VAC is also switched by the AC, so if I used that for a power source for the siren as well, it'd only work while the thermostat was calling for AC. \$\endgroup\$
    – kazoni
    Jun 14, 2016 at 3:43

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