LM903 NSC [National Semiconductor], LM903 Datasheet - Page 6

• Manufacturer Part Number: LM903
• Description: Fluid Level Detector
• Manufacturer: NSC [National Semiconductor]

Application Hints
INTERNAL COMBUSTION ENGINE OIL LEVEL
The basic system provides a single shot measurement
when the supply is applied and has a primary application in
automotive oil hydraulic fluid and coolant monitoring Partic-
ularly in the case of the oil level a valid measurement is
only possible before the oil is disturbed The application cir-
cuit shown is arranged such that the measurement is made
when the ignition is switched on via switch A Switch B is the
oil pressure sensor and is closed before the engine starts
keeping pin 7 low and enabling the measurement
STALLING AND RESTART PROTECTION
The 4M7 resistor and 10
provide the restart protection When oil pressure builds up
switch B opens and the 10
the bulb At switch-off the capacitor discharges slowly and
is capable of preventing a low state on pin 7 for 1 minute
Unless pin 7 is low a new measurement can not be made
and the previous measurement result stored in the memory
capacitor on pin 15 is used to control the output
MEMORY
The pin 15 memory output goes high if a correct measure-
ment is made (lamp off) If the power is removed pin 15
leakage is less than 3 A and the memory status is retained
for some time Provided pin 15 voltage does not fall below
F capacitor connected to pin 7
F capacitor charges through
6
3V the memory capacitor will be refreshed on powering up
again There is no internal pull down on detecting an incor-
rect measurement If it is required to use pin 15 as an output
indicating the measurement result an external pull down
resistor and buffer will be required
CONTINUOUS WARNING LAMP
The lamp can be arranged to light continuously by disabling
the oscillator with a resistor of 150k or less connected be-
tween pins 10 and 11
REPETITIVE MEASUREMENTS
Measurements may be repeated by strobing the supply to
pin 6 The probe current regulator transistor must have the
same supply as pin 6 but the warning lamp can be perma-
nently powered The lamp will light during each measure-
ment and will flash in between measurements when incor-
rect conditions are detected
ALTERNATIVE APPLICATIONS
Gas flow detection The cooling effect of gas flowing over
a probe could be used to provide a warning signal from the
LM903 in the event of gas failure
Automatic top up With the LM903 strobed continuously
the output may be stored buffered and used to drive sole-
noid valves to correct a fluid level as required