Normally closed to normally open alarm circuit

[peters.carl]

I have a 12v power supply and a flow switch which is closed when water is flowing through it and open when water isnt. I want to make a simple piezo alarm + LED light that tells me (comes on) when water STOPS flowing. How do I do this? That is, how do I take a normally closed switch and make it normally open? Simple is best for me and the less power consumption the better.

Many thanks.

 

[tpetar]

You can use this :

 

[kripacharya]

use an NPN transistor (bjt) with a base resistor to invert the logic.

something like this --

https://www.falstad.com/circuit/#$+...+LED w+464+192+448+192+0 w+400+192+368+192+0

 

[peters.carl]

This looks perfect. can you point me to the correct npn transistor on rs electronics website? (excuse my ignorance) thanks heaps

 

[tpetar]

BC546 can be one choice.