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LM903 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM903 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page 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 6 |
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