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Novel Liquid-Level Sensor

Normally, the level of a liquid in a container is determined

by sensing changes in the capacitance or resistance

between a pair of electrodes that are immersed in the liquid.

Generally speaking, this technique requires fairly compli-

cated circuitry to protect the electrodes against electrolysis

(and associated corrosion). In addition, in many cases the

liquid must be conductive for the measurement principle

to actually be usable. The circuit presented here shows that

an alternative approach is possible.

Here we utilise the fact that a PTC resistor warms up in pro-

portion to the amount of current flowing through it, with the

result that its resistance increases. If a PTC resistor is

immersed in a liquid, the additional warmth is dissipated

in the liquid and the resistance remains nearly constant. If

the level of the liquid drops below the immersion depth of

the resistor, the change in the resistance can be easily

sensed by a subsequent comparator stage.

The PTC resistor should be isolated from the fluid into

which it is immersed, in order to prevent undesirable elec-

Ω

IC1

150

15k

LM393

Ω

014092 - 11

trolytic processes from taking place. A further improvement

in the characteristics of the circuit can be achieved by

using a logic circuit — such as a microcontroller — to

apply power to the circuit only at predefined times and then

switch off the power after sampling the comparator output.

(014092-1)