ON (indicating a low level in the tank). When U1A and
U1B are low, the + input to U1C = (Vdd-
1.5V)*(R13//R14)/(R13//R14+R15) = 10. 5*50/150 = 3.5V,
causing U1D’s output to go low, turning OFF RLY1 and the
S2 represents the three-pin jumper selector. The fill
mode is selected by connecting the jumper from the
output of U1C to R19 and the base of Q2. This negates
the output of U1C so that when U1C is high (the tank level
is high), Q2 is OFF and Q1, RLY1, and the fill pump or fill
valve solenoid are OFF. When the tank is emptied below the
low level, U1C goes low, which turns Q2, Q1, RLY1 ON
to activate the fill pump or fill valve solenoid to refill the
tank. The pressure sensor level controller performs the
same sump pump or tank fill function, but does it in a
different way. Please refer to Figure 2 for the following
theory and operation discussion.
The pressure sensor level controller uses a Freescale
MPXM2051G sensor to measure the pressure created by
the liquid level in a sense tube that goes from the top to the
bottom of the sump or cistern vessel. It works like an
inverted drinking glass when you place it into water. The
liquid level inside the glass rises as the glass is put lower
into the water. The air trapped inside the glass is equal to
the water pressure at the bottom (lowest edge) of the
If a flexible tube was connected between the top of
the drinking glass (the air bubble inside the glass) and a
pressure sensor, the pressure measured would represent
the liquid level, or depth of the liquid in the vessel.
24 December 2016
■ FIGURE 3. ■ FIGURE 4.
■ FIGURE 2.