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 mF capacitor connected to pin 7

provide the restart protection When oil pressure builds up

switch B opens and the 10 mF capacitor charges through

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 mA and the memory status is retained

for some time Provided pin 15 voltage does not fall below

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

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