led on when switch is low

I'm building a tiny humidifier for my piano. I have a float switch in the water tank to turn off the dc fan when the water is low. What kind of circuit/component do I need to use that same float switch to both cut off the fan and turn on an led, indicating it needs to be refilled. I know I could do this with an arduino but I'm trying to do it with a circuit only. I'm guessing a transistor will be involved but I'm not sure how.

A lot depends on your float switch, don't you agree?


You haven't given us much to go on...but maybe reading this will help.

Sorry. It’s a very simple on/off switch that opens when the water level is low. I’ll manually refill the tank.

Then you need a relay to match your DC voltage. When there is voltage to operate the fan, it will also pull in the relay. When the float switch stops the fan it will also turn power off to the relay and allow the NC contacts to close, turning on your LED.

Or you can use a SPDT relay to control power to both the fan and the LED.


Cool. Thanks so much!

I'm going to guess that the fan draws a lot more current than the LED.

In which case, simply connect the LED with its series resistor, across the switch,

When the switch opens, the LED (and resistor) will see the full supply voltage but its current will not be enough for the fan to run.

So clever Paul B! I'll try it and report back.