LM1830 National Semiconductor, LM1830 Datasheet - Page 4

• Manufacturer Part Number: LM1830
• Description: Fluid Detector
• Manufacturer: National Semiconductor

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
0 001 F 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-
mately 4 V
tion) will be turned ‘‘ON’’ when the probe resistance to
ground is equal to the internal 13 k
diode across the detector emitter base junction provides
symmetrical limiting of the detector input signal so that the
probe is excited with
where the 13 k
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
BE
the detector (which is an emitter base junc-
resistor is not compatible with the probe
g
2 V
BE
from a 13 k
City water
Sea water
Copper sulphate solution
Weak acid
Weak base
Household ammonia
Water and glycol mixture
Wet soil
Coffee
Conductive Fluids
resistor An internal
source In cases
4
Pure water
Gasoline
Oil
Brake fluid
Alcohol
Ethylene glycol
Paraffin
Dry soil
Whiskey
Non-Conductive Fluids
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
R
where
(An equivalent is the number of moles of a substance that
gives one mole of positive charge and one mole of negative
charge For example one mole of NaCl gives Na
the equivalent is 1 One mole of CaCl
so the equivalent is 1 2 )
Usually the probe dimensions are not measured physically
but the ratio d A 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 D A 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
e
1000
c p
A
d
c
p
e
e
e
e
d
A
separation of plates (cm)
concentration (gm mol equivalent litre)
equivalent conductance
(
area of plates (cm
b
1
cm
2
equiv
b
2
1
)
)
2
gives Ca
a a
a
a
a
Cl
2Cl
b
so
b