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I have a momentary switch that is usually on and I want my circuit to be off when the switch is letting the signal through, how would I do this?

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  • \$\begingroup\$ When you say "usually on" do you mean "usually open" or "usually closed"? \$\endgroup\$ – The Photon Feb 16 '15 at 2:33
  • \$\begingroup\$ Also, what voltage do you need to supply to your load and how much current does it draw when it is powered? \$\endgroup\$ – The Photon Feb 16 '15 at 2:34
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Here's a typical low-side switch configuration:

schematic

simulate this circuit – 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.

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what you can opt for is a relay. a relay can be configured to

  1. switch on the circuit with on state
  2. switch off the circuit with on state(the case you mentioned)
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