Fluid Detector

The National LM1830 fluid detector detects the presence,

 absence, or level of a polar liquid, such as water. The LM1830 is a

linear bipolar integrated circuit that operates from a wide range

of operating voltages. The fluid detector shown in Fig. 3-6 deter

mines the presence of a fluid by comparing the resistance of the

fluid between the probes and the internal resistance of the IC. An

ac signal is used to overcome the plating problems associated

with a dc voltage source. Provisions are available for connecting

an external resistance in applications where the fluid impedance

is of a different magnitude than an internal resistor in the IC. As

the probe resistance increases above a preset value, the internal

capacitor produces a signal.

The internal oscillator is coupled to the base of an open col

lector transistor so that the output signal can drive an LED, a

speaker, or low-current relay. A O.OO1-JÅ.F timing capacitor be

tween pins I and 7 produces a 6-kHz signal on pin 5 of the fluid

detector. A filter capacitor on pin 9 enables the fluid detector to

operate at a constant output. Removing this filter allows a 50%

duty cycle. When used with a speaker, the output will cycle on

and off.

Although the LM1830 detector was primarily designed for

sensing conductive liquids, it can also be used in the direct-

coupled mode. A variable resistance sensing device, such as a light-

dependent resistor or a thermistor, could be used as the sensing

device, as shown in Fig. 3-7. The fluid detector is a complete, low-

cost level-detector system that can be used in many different

 sensing applications. The detector can drive a low-current relay as a

local alarm. The relay contacts could be wired to other sensors,

and the multiple sensors could activate a central alarm panel.