MPXM2102AS datasheet(451/670 Pages) MOTOROLA

451 / 670 page

AN1516

3–305

Motorola Sensor Device Data

www.motorola.com/semiconductors

LEVEL CONTROL MODES

This application describes two ways to keep the liquid level

constant in the tank; first, by pumping the water out if the liquid

level rises above the reference, or second, by pumping the

water in if the liquid level drops below the reference.

If pumping water out, the pump must be OFF when the liquid

level is below the reference level. To turn the pump ON, the

sensor signal must be decreased to drop the input to the

Schmitt trigger below the reference voltage. To do this, the

sensing pipe must be connected to the NEGATIVE pressure

port (back or vacuum side) of the sensor. In the condition when

the pressure increases (liquid level rises), the sensor voltage

will decrease and the pump will turn ON when the sensor

output crosses the referenced level. As pumping continues,

the level in the tank decreases (thus the pressure on the

sensor decreases) and the sensor signal increases back up

to the trigger point where the pump was turned OFF.

In the case of pumping water into the tank, the pump must

be OFF when the liquid level is above the reference level. To

turn ON the pump, the sensor signal must be decreased to

drive the input Schmitt trigger below the reference voltage. To

do this, the sensing pipe must be connected to the POSITIVE

pressure port (top side) of the sensor. In this configuration

when the pressure on the sensor decreases, (liquid level

drops) the sensor voltage also decreases and the pump is

turned ON when the signal exceeds the reference. As

pumping continues, the water level increases and when the

maximum level is reached, the Schmitt trigger turns the pump

OFF.

ADJUSTMENTS

The sensing tube is placed into the water at a distance

below the minimum limit level anywhere in the tank. The other

end of the tube is opened to atmosphere. When the tank is

filled to the desired maximum (or minimum) level, the pressure

sensor is connected to the tube with the desired port

configuration for the application. Then the water level in the

tank is the reference.

After connecting the tube to the pressure sensor, the

module must be adjusted to control the water level. The output

voltage at TP1 is preadjusted to about 4 V (half of the supply

voltage). When the sensor is connected to the tube, the

module output is ON (lighted) or OFF. By adjusting the offset

adjust potentiometer the output is just turned into the other

state: OFF, if it was ON or the reverse, ON, if it was OFF, (the

change in the tank level may be simulated by moving the

sensing tube up or down).

The reference point TP2 shows the ON/OFF reference

voltage, and the switching point of the module is reached

when the voltage at TP1 just crosses the value of the TP2

voltage. The module is designed for about 10 mm of difference

level between ON and OFF (hysteresis).

CONCLUSION

This circuit design concept may be used to evaluate

Motorola pressure sensors used as a liquid level switch. This

basic circuit may be easily modified to provide an analog

signal of the level within the controlled range. It may also be

easily modified to provide tighter level control (

±2 mm H2O) by

increasing the gain of the first amplifier stage (decreasing RG

resistor).

The circuit is also a useful tool to evaluate the performance

of the power optocoupler MOC2A60 when driving ac loads

directly.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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