Converting a NC switch to NO.

I have 2 hydraulic switches that I will be using with the brake light circuit of my car. But unfortunately they are normally closed. I want to know if there is a way to convert them to normally open type. They will be used at 12 V.

If there are three electrical connections on the back of the switch, then probably, but there almost certainly aren't. How hard is it to find a switch that will do what you want? And more particularly, isn't there a brake light switch already installed that you can use?

You can use the existing switch to operate a relay that produces the output sense you require.

Or you can substitute the relay with a simple electronic circuit, if you’re comfortable designing electronic circuits.

Or you can substitute the existing switch for one that does what you want. This is the recommended approach. Hydraulic brake light switches are normally of the NO type so if the ones you’re using aren’t, they’re probably not designed for what you’re doing. Brake light switches are cheap and common so swapping for the right part should be no big deal - far better than adding extra electrical complexity to bodge the existing ones.

It's probably a wild guess but you may be thinking of connecting the switch output to an Arduino. If so wouldn't it be a trivial matter for the software to invert the sense of the switches.

I would be wary of tampering with the braking arrangements of your car without getting your insurer's prior approval.