Turn circuit off when switch is on
Here's a typical low-side switch configuration:







simulate this circuit &ndash; Schematic created using CircuitLab

You need to pick a N-MOSFET that can switch completely when Vgs is equal to your power supply voltage. If your supply voltage is very low, this might be difficult to find.

You also need the FET to withstand the full power supply voltage between drain and source when the channel is not conducting.