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I have connected a LM35 to ADC channel 1 of LPC1768. getting a value of 306(adc reading).

Vref =5V

For 12 bit ADC

Step size = 5V/4096=1.22mV

to calculate the output voltage.

Vout in mV = adc_reading * Step size = 306 * 1.22mV= 373.32mV

LM35 sensor gives an output of 10mV for every 1 degree centigrade. so the measured temperature is

373.32/10= 37 degree centigrade

But Bangalore's temperature reading is 27 degree centigrade.

Refering to Mazidi's "AVR Microcontroller and Embedded Systems" book, The digital output from ADC is calculated by the below formula

Vout = analog input voltage/step size

If Vout is calculated with the above formula then o/p voltage is

Vout = 306/1.22=250.819mV

Voltage to temperature conversion = 250.819mV/10mV = 25.08 degree centigrade.

Unable to figure out what is right. Kindly help. Sorry for this silly question.

Also connected an LDR to ADC channel 0 of LPC1768. Calculated the measured output voltage

Vout = adc_reading*step size.

for adc_reading = 55-> vout = 0.064713

adc_reading = 3245-> vout = 3.962149

adc_reading = 4095-> vout = 4.999995

May be you experts can throw some light on this. Confused as to why i am unable to get the right temperature as per my city.

1 Answer 1

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I was using 5V as VCC instead of 3V3. Now getting the right temperature.

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