Tank level control with two float switches

Parts in this circuit

Wiring, pin by pin

Red = 5 V, orange = 3.3 V, black = GND, other colours = signals. “Pin” is the position on the 40-pin header; the GPIO number is what the code uses. Power the Pi from a 5.1 V supply and wire with the power off.

Python code (gpiozero)

from gpiozero import Button, OutputDevice
from time import sleep

low = Button(17)       # pressed = water has reached the low float
high = Button(27)      # pressed = water has reached the high float
pump = OutputDevice(22, active_high=False)

# Ladder rung:  PUMP = (NOT low  OR  PUMP)  AND  NOT high
#   - Water below the low float: the pump starts.
#   - The PUMP contact holds it on while the water rises.
#   - Water at the high float: the pump stops.
while True:
    run = ((not low.is_pressed) or pump.value) and not high.is_pressed
    if run != bool(pump.value):
        pump.value = run
        print("Pump ON  (tank filling)" if run else "Pump OFF (tank full)")
    sleep(0.1)

How the code works

• The low float is wet when the water level is above it. When the level falls below it, "not low.is_pressed" becomes true and the pump starts.
• Once the pump runs, the PUMP contact in parallel keeps the rung powered even though the low float is wet again. That gap between the floats is the hysteresis: the pump does not switch on and off at every ripple.
• When the water reaches the high float, the NC HIGH contact opens, the rung loses power and the pump stops.
• The tank in the simulator fills at 9 % per second with the pump on and empties according to the Water use slider.

Parts to buy

The links open a search on each store, so you see what is in stock today. Check the model, voltage and price before you order.