But why not improve the system a bit?
I made a salt level sensor for it: it has a RGB-led which gives 4 colors with the following meaning:
green=level ok
blue=level ok but add some salt in a few days.
red=level low, add salt
white=level out of range
But this is not all ofcourse, the sensor works also with Domoticz and https://www.mysensors.org/.
So, you can monitor the salt level with your domotics system too!
Details about the sensor
Powersupply 24Volt AC, about 150mA.
You can use a 24Volt AC adapter, but don't use the 24Volt AC adapter of your softener: it has too less current. I have replaced my own adapter for an oversized adapter (1A) and placed a 800mA FUSE inside too (there was no fuse inside at all). I made a power splitter cable too, the new adapter feeds the softener and the sensor together.
The full package of all the files (details, design, STL's, photo's and more) can be downloaded here: https://www.thingiverse.com/thing:2597067.
You need the following parts:
Arduino uno (remove the powerconnector, solder the wires of the outputs of the stepdown converter to the correct + and - poles.
NRF24L01
Optional: NRF24L01 adapter: with this you can feed 5Volt instead of 3.3Volt to the NRF.
Stepdown converter (change the output voltage to 9Volts.
AC-to-DC bridge
Ultrasoon sensor
RGB-led (common ground)
Powerplug 3.5mm
Resistors: 1x 100 ohm (Red pin), 2x 470 ohm (Green and Blue pins)
Bolts 3M (10 mm) and more
By the way, the sensor can be used standalone, so without domotics system, and the sensor can be used on all kind of water softeners ofcourse.
Arthur