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12F675 Tutorial 4 : Making an LM35 temperature recorder.
Measuring temperature is easy
using an LM35...
It generates serial output so that you can view the results on a PC and it also calculates the temperature reading in Fahrenheit sending both to the serial port at half second intervals.
The project uses the code from the previous tutorials to report the temperature to the PC using the serial port so the serial RS232 data format is generated in software.
Jump to circuit.
Jump to Solderless breadboard.
Jump to Circuit Diagram.
Jump to Software.
LM35DZThe LM35 is a precision temperature sensor. It is guaranteed accurate to ±¼°C at 25°C (At different temperatures it is less accurate! but it is never more than 2°C inaccurate and it probably is not this inaccurate anyway it's just the manufacturers maximum limits that may apply).
Typically is stays accurate to within ±¾°C over its temperature range so this is a good general purpose sensor and it's easy to use.
It generates a linear output voltage using a centigrade scale - generating 10mV of output voltage for every degree centigrade change and there are several versions for operation over different temperature ranges:
Note: The project code calculates the temperature in Fahrenheit and generates both Centigrade and Fahrenheit outputs to the serial port.
Temperature recorder : LM35 pinoutTemperature recorder : pinout for the LM35DZ (from the top).
ICSP so you need a way of connecting the sensor and the programming input at the same time with the programming input overriding the sensor output (and not damaging the sensor!).
This is done here by using 1k resistor that reduces the current flowing back into the sensor and at the same time is not too large (so that the ADC can easily convert the sensor output value - the impedance must be equal to or smaller than 10k Ohm from the sensor).
The voltage reference for the circuit is taken from pin 6 using a resistor divider giving a 2.5V reference. This is simply done to increase the resolution of the ADC as for the LM35 only 0-1V is generated so you loose ADC range when using a 5V reference. You could use a lower reference value but this value gives reasonable results.
Alternatively you could use an amplifier to scale the LM35 output up which would make the ADC less sensitive to noise but for this project it is simpler not to do so.
Note: The large decoupling capacitor on the supply input of the 12F675. This reduces noise overall and gives a more consistent reading. However using a plug block and ADC is not a very good idea as there is no ground plane and no control over current paths which you would be able control in a pcb.
In a commercial system the internal ADC is often not used at all as it is essential to separate the noise introduced to the ADC using separate grounds and shielding - some designs encase the ADC in a custom metal shield and along with a ground plane connecting to the shield gives the best possible result.
To overcome noise problems on the ADC the software averages the input readings so you get a better result.
Temperature recorder measurement accuracy
The analogue reference for the ADC is taken from the power supply via a resistive divider to the 12F675 input pin 6 and for the 7805 its accuracy is specified as ±5% so the accuracy of the ADC is only 5% due to the reference -the divider also introduces a 1% error giving a 6% error overall.
Note: Since the 7805 is only accurate to ±5% the accuracy of the temperature reading will be accurate to ±5% (plus errors in the ADC and temperature sensor itself and any noise introduced the the analogue input and the reference). However the reference source gives you the biggest error - the overriding accuracy - if you used a more accurate voltage supply then the ADC accuracy would become more important as well as the temperature sensor accuracy etc.
The software uses the Soft USART (transmit only) described in the previous tutorial and uses the built in MikroC routines to get the data from analogue input pin AN0.
Source code files :
To get the file software project files and c source code click here.
Software operationThe most interesting parts of the software are shown above. The variable val is an unsigned int so the maximum value it can store is 65535
The reference in use is 2.5V so for the 10bit ADC each ADC bit is worth
2.5/1023 = 2.44mV
If you work out values generated for a maximum temperature of 100°C using the scale factor 2.44mV (or 244/100)
100 * 10mV = 1.0V
1.0V/2.44mV = 410
410 * 244 = 100,040 which will not fit into an unsigned int.
So this scale factor does not work for all input values
By using a little maths it can be made to fit -you need to reduce the top number to fit. e.g.
410 * 122 = 50,020 which does fit.
Dividing by 50 gets back to the correct scale factor of 244.
So the scale 122/50 works for all input values.
This is an example of avoiding the use of floating point variables which take up too much resources. You can still make the system work but you have to be careful when using fixed types and you have to check all input values and outputs to make sure they fit.
AveragingAveraging would be better done in the PC as it has more resources - the same goes for calculating and displaying the temperature in Fahrenheit but this gives a demonstration of what you can do.
Note: The RAM is used up since a bug in MikroC 188.8.131.52 puts strings int RAM - in future versions this will be corrected.
Typical output from the temperature recorder
The left most value is the RAW ADC value, the next is the temperature sensor output in degrees centigrade and the next is the temperature sensor output in degrees Fahrenheit.
Note: You have to put in the decimal point so the above readings are:
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