Application Hints

The LM1830 requires only an external capacitor to com-

plete the oscillator circuit The frequency of oscillation is

inversely proportional to the external capacitor value Using

0001mF capacitor the output frequency is approximately 6

kHz The output from the oscillator is available at pin 5 In

normal applications the output is taken from pin 13 so that

the internal 13k resistor can be used to compare with the

probe resistance Pin 13 is coupled to the probe by a block-

ing capacitor so that there is no net dc on the probe

Since the output amplitude from the oscillator is approxi-

tion) will be turned ‘‘ON’’ when the probe resistance to

ground is equal to the internal 13 kX resistor An internal

diode across the detector emitter base junction provides

symmetrical limiting of the detector input signal so that the

where the 13 kX resistor is not compatible with the probe

resistance range an external resistor may be added by cou-

pling the probe to pin 5 through the external resistor as

shown in

Figure 2 The collector of the detecting transistor

is brought out to pin 9 enabling a filter capacitor to be con-

nected so that the output will switch ‘‘ON’’ or ‘‘OFF’’ de-

pending on the probe resistance If this capacitor is omitted

the output will be switched at approximately 50% duty cycle

when the probe resistance exceeds the reference resist-

ance This can be useful when an audio output is required

and the output transistor can be used to directly drive a loud

speaker In addition LED indicators do not require dc exci-

tation Therefore the cost of a capacitor for filtering can be

saved

In the case of inductive loads or incandescent lamp loads it

is recommended that a filter capacitor be employed

In a typical application where the device is employed for

sensing low water level in a tank a simple steel probe may

be inserted in the top of the tank with the tank grounded

Then when the water level drops below the tip of the probe

the resistance will rise between the probe and the tank and

the alarm will be operated This is illustrated in

Figure 3 In

situations where a non-conductive container is used the

probe may be designed in a number of ways In some cases

a simple phono plug can be employed Other probe designs

include conductive parallel strips on printed circuit boards

It is possible to calculate the resistance of any aqueous

solution of an electrolyte for different concentrations pro-

vided the dimensions of the electrodes and their spacing is

known

The resistance of a simple parallel plate probe is given by

1000

cp



d

A

X

where

(An equivalent is the number of moles of a substance that

gives one mole of positive charge and one mole of negative

so the equivalent is 12)

Usually the probe dimensions are not measured physically

but the ratio dA is determined by measuring the resistance

of a cell of known concentration c and equivalent conduct-

ance of 1 A graph of common solutions and their equivalent

conductances is shown for reference The data was derived

from DA Maclnnes ‘‘The Principles of Electrochemistry’’

Reinhold Publishing Corp New York 1939

In automotive and other applications where the power

source is known to contain significant transient voltages the

internal regulator on the LM1830 allows protection to be

provided by the simple means of using a series resistor in

the power supply line as illustrated in

Figure 4 If the output

load is required to be returned directly to the power supply

because of the high current required it will be necessary to

provide protection for the output transistor if the voltages

are expected to exceed the data sheet limits

Although the LM1830 is designed primarily for use in sens-

ing conductive fluids it can be used with any variable resist-

ance device such as light dependent resistor or thermistor

or resistive position transducer

The following table lists some common fluids which may

and may not be detected by resistive probe techniques

Conductive Fluids

Non-Conductive Fluids

City water

Pure water

Sea water

Gasoline

Copper sulphate solution

Oil

Weak acid

Brake fluid

Weak base

Alcohol

Household ammonia

Ethylene glycol

Water and glycol mixture

Paraffin

Wet soil

Dry soil

Coffee

Whiskey

4