This circuit is used in RVs to display holding tank fluid level with 3 LEDs. 1/3, 2/3 and full. The op amps are a quad LM339 comparator. 8V power source from a 7808 voltage regulator. The 3 switches and 68K resistors are located near and on the tank, the switches represent stainless steel contact probes with a ground probe in the bottom of the tank. This circuit would be replicated for each holding tank to be measured.
