This power indicator circuit is intended to be used as an extension for the water softener. It might be easier to just place a LED but it is better to make a rectifier that detects the presence of the 15 kHz signal generated by the 555 IC.
The rectifier is connected at the water softener system with C4 capacitor. D1 and D2 diodes are used to obtain a DC voltage that is filtered by C5 capacitor. This voltage turns ON the FET that lights up the LED. When there is no signal from the oscillator the LED will not glow. D1 and D2 = 1N4148.