| Digital
Thermometer
built with the AT89S52 micro-controller
By Ibrahim
Kamal
Last update:
4/4/08
 Overview
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This article shows you how to build a digital thermometer
from the beginning to the end, using a thermistor and a
8051 microcontroller.
Being based on our tutorial
about Analog to Digital conversion, it is very easy to understand
the functioning of the device, and you can build it with
any microcontroller even if it doesn't have a builtin ADC.
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The following flow chart shows the general principle of operation
of a digital thermometer, and each one of those four sections
shall be studied in this article.
1-The
temperature sensor
The
temperature sensor used in this project is thermistor
which is very suitable for measuring ambient atmospheric
temperatures, but you could replace it with any other
type of temperature sensor, that would function in a range
that is more adequate to your application. The
response time of thermistors is relatively big, but again,
the performance of a thermistor is accepted for our application.
A thermistor (figure 1.A), is a resistor whose resistance
varies with temperature. They come in two types: NTC and
PTC. NTC stands for negative temperature coefficient,
meaning that the resistance of an NTC thermistor will
decrease if temperature increases, while PTC stands for
Positive temperature. coefficient, and PTC thermistors
behave in the opposite way: An increase in temperature
causes an increase in resistance. |
Figure 1.A |
The thermistor used in this project is an NTC type (the small
one in figure 1.A). In order to be able to read the resistance
variation (corresponding to temperature variation), we need to
convert this resistance variation into a voltage variation, which
will be proportional to the temperature.
To do this we simply insert the thermistor into a voltage divider
configuration shown in figure 1.B (The 10K resistor is not part
of the voltage divider).
Figure 1.B |
The
following equation describes the relation between the
voltage of the point Vout:
 |
where
resistances are expressed in K ohms
and Rth is the resistance of the thermistor |
It is clear That Vout increases
when R decreases, then Vout
increases proportionally
with temperature. Sometimes the response of thermistors
are not quite linear, so adding a 10K parallel resistor
increase the linearity of the response of the thermistor
over a specific range which corresponds to ambient temperatures.
The mathematical proof for this application of increasing
the linearity of a signal would require a dedicated article,
which is not the target of this one.
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| 2-The
Analog to digital converter
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As you noticed in
the last paragraph, the Temperature was translated to a directly
proportional voltage, and the relation was considered to be
linear over the concerned operating range. Now we need to convert
those signals to digital signals, so that the microcontroller
can read it, store it and display it. Many of the recent microcontrollers incorporate integrated digital to analog converters,
but for educational purposes we are going to assemble our own
counting type ADC which is shown in the blue shading in figure
2. The operation of such a converter is explained in detail
in this tutorial about counting
type ADCs. One important advantage of manually building
your own DAC is that you can easily increase the resolution
of your converter while this is not always possible when you
are working with integrated ADCs. In this application,
we used a 10 bit ADC.
The output of the the last stage (the analog voltage)
is fed to the ADC via the non-inverting pin of the LM358 Op-Amp
(U2). The rest of the components are the 7-segments display
system (in the red shading), the voltage regulator to
provide clean 5V for the microcontroller, the Jack J1 for the
ISP (In System Programming) and finally the reset circuitry
coposed of R12 and C3 and the 24 Mhz crystal.
If the voltage being converted changes from 0 to 5 volts, then
the resolution of our converter would be:
Which is sufficient for our application.
Figure 2
Q1, Q2,Q3: 2n2907 ,R1 to R11: 220 Ohm, R14 to R23: 1 MOhm
R24 to R32: 500 KOhm, J1: connection for ISP
programmer
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So, with the help of the ADC, the microcontroller is able to
determine a 10 bits digital value (from 0 to 1024) that corresponds
to a certain temperature. Those values still don't provide any
direct temperature indication, but they are considered to be directly
proportional to the temperature.
| 3-Callibration
and signal processing
|
After the analog signals
are converted to 10 bit digital values, some operations have to
be done to convert that number into a temperature in Celsius.
This is done using the following formula:
Actually the formula above is the canonical form of the equation
of a line, where T is the temperature in °C and D the raw
reading obtained from the ADC. C is a constant number.
The calibration process is all about determining the value
of the conversion factor Fc
and the constant C. IF you have the datasheet of the thermistor,
you could calculate those variables, but still it would
be a very complicated operation. The method I am proposing
is called line fitting, which consist of drawing the line
that passe as near as possible from a maximum of readings
so that it can be used later on to calculate new points.
It is the reverse process of readings points from a graph,
now you have the points, but you want to build the graph
that is at the origin of those points. This graph can
never be precisely found because it never existed, but
a very similar one can be 'fitted' and used later to simulate
a linear relation ship over a wide range of readings. |
Figure 3.A |
Lot of free software on the internet will calculate for you the
two variables Fc and C, Personally
I used my scientific calculator.
In other words, in order to precisely calibrate the digital
thermometer, we need to run a multitude of test measurements,
noting at the end of each test the actual temperature measured
from a reference thermometer (equivalent to T in the graph) and
the reading of our thermometer (equivalent to D). At the end of
those test you obtain a series of temperatures and corresponding
ADC readings. Feeding those values into a scientific calculator
or a line fitting software will provide you with the values of
Fc and C
It is clear that the Calibration process can only be started
when the device is properly functioning and already incorporates
a display system to read the results.
Then all we have to do is to display the temperature that was
just calculated from the formula above, but before doing this,we
can apply some digital filtering to the signals, to reduce the
effect of any eventual noise. Note that temperature is a measurand
that is relatively stable so any fast fluctuation in the reading
would be unlogical, would cause a lot of undesired flickering
in the display and would probably be caused by the noise anyway.
That digital filter can be implemented simply
by displaying the average of the last five readings rather
that directly displaying the values calculated by the microcontroller.
This can be implemented in C code as in the following example:
temperature
= (adc_data * conversion_factor) + constant;
reading_5 = reading_4;
reading_4
= reading_3;
reading_3
= reading_2;
reading_2 =
reading_1;
reading_1 =
temperature;
actual_temperature = (reading_1+reading_2+reading_3+reading_4+reading_5)/5;
Where the variable 'actual_reading' is the reading to be displayed.
| 4-The
display system
|
As
you can see in figure 4A, 3 Seven-Segments cells are used
to display the temperature. The main idea for this display
system, is to connect all the 7-segment cells together
in parallel (all the cells show the same digit), but only
power the first cell, then switch the first cell off,
and power the second one, then do the same thing with
the last one and repeat this cycle at a very fast rate.
If you provide the appropriate DATA to the cells at the
appropriate time, the number will be displayed without
any noticeable flickering. See the display system of this
frequecny meter for more
information.
The decimal point is fixed
(from the software)
at the middle cell, so there is always one number after
the point. All the cells are |
Figure 4.A |
common
anode type, the cathodes of each cell are connected
with the cathodes of the other cells, then directly connected
to the Port 0 of the microcontroller. We could have saved 4 pins
of port 0 by using a BCD to 7-Segment decoder, but since we don't
need those pins, we choose
to
connect them directly and decrease the number
of components. Figure 4.B is a small slice of the full
schematic, showing the electrical connection for the display
system. DD1
to DD3 are the 7-segment displays. Port
0 of the 89S52 outputs the the bits configuration corresponding
to the numbers to be displayed, while pin 2,3, and 4 of
Port 1 switch on or off the cells via a 2N2907 PNP Transistor.
(NPN
would have been used for common cathode displays).
Note that PNP transistors are 'turned ON' when their base
is connected to ground.
The software routine to operate this display can |
Figure 4.B |
seem
complicated, but it only executes
the sequence described above, which consists of turning ON one
cell of the display, displaying the corresponding number and keep
cycling between the three cells abundantly.
The following function is the one used by the thermometer to display
the temperature, it is periodically called by the timer 0 interrupt:
display_sequence() interrupt 1{
display_period++;
if (display_period > 15){
display_period = 0;
segment_counter++;
if (segment_counter > 2) segment_counter = 0;
switch(segment_counter){
case 0:
P1 = 255; //Write 1's on the pins of Port 1
P1_2 = 0; //Then switch on the transistor Q1
P0 = bcd[dig[segment_counter]];//Display the corresponding digit
break;
case 1:
P1 = 255;
P1_3 = 0;
P0 = bcd[dig[segment_counter]]-128;
break;
case 2:
P1 = 255;
P1_4 = 0;
P0 = bcd[dig[segment_counter]];
break;
}
}
} |
You can notice that the whole function's execution rate can be
adjusted using the variable 'display_rate' when the maximum timing
of the timer 0 is still too fast. In order to understand the code,
it's important to note that the temperature is stored in the array
'dig', where:
dig[0] = first digit to be displayed
dig[1] = second digit to be displayed
dig[2] = third digit to be displayed (the decimal)
So each time the function is executed, the variable 'segment_counter'
is incremented in its cycle from 0 to 1, to 2, and then back to
0, and each time the corresponding digit is written on port 0
using the following statement:
P0 = bcd[dig[segment_counter];
Where bcd[] is an array where are stored the bits configurations
to show the digits 0 to 9 on the seven segment display, according
to the connection diagram in figure 4.B:
bcd[0] = 192;
bcd[1] = 249;
bcd[2] = 164;
bcd[3] = 176;
bcd[4] = 153;
bcd[5] = 146;
bcd[6] = 130;
bcd[7] = 248;
bcd[8] = 128;
bcd[9] = 144;
You will also notice that in case of the middle cell, the data
written to P0 is different:
P0 = bcd[dig[segment_counter]]-128;
The fact of subtracting 128 to the number being displayed makes
the last bit (P0_7) always Low, this way the led of the decimal
dot is always ON.
One last note about the display system, is that there is another
function that have to be called before displaying the digits of
the temperature:
int_to_digits(number);
Which will take as an argument the the temperature in a single
variable, then store the seperate digits into the array dig[0,1,2].
The detailed function, which was modified from a code provided
by KEIL™, is as the following:
void int_to_digits(unsigned long number){
float itd_a,itd_b;
number = number * 10;
itd_a = number / 10.0;
dig[0] = floor((modf(itd_a,&itd_b)* 10)+0.5);
itd_a = itd_b / 10.0;
dig[1] = floor((modf(itd_a,&itd_b)* 10)+0.5);
itd_a = itd_b / 10.0;
dig[2] = floor((modf(itd_a,&itd_b)* 10)+0.5);
itd_a = itd_b / 10.0;
dig[3] = floor((modf(itd_a,&itd_b)* 10)+0.5);
}
As you can see in the function above, the number
is multiplied by 10, this way we don't have to worry about
the decimal point which is fixed on the middle cell. This way,
a number like '24.3' will become '243', each cell will display
one of those 3 digits, and the decimal point being fixed at the
middle cell, the number displayed will be '24.3'. An integer like
'28' for example will be displayed as '28.0'.
I hope this article covered all the main aspects of the construction
of a simple digital thermometer.
Download
the zip file for this project
Containing the HEX file for the microcontroller, the
PCB layout and C Source CODE.
[note: I use ExpressPCB(FREEWARE)
to design the schematics and the PCB]
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Preview of the last 15
messages discussing this page. Messages are sorted from the newest to
the oldest. |
Posted
by:
ikalogic
on:
08 Sep 2011 |
Re: Digital Thermometer Project |
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| Quoting ikalogic: main machine controller |
what is that exactly..? mcu? PLC?
in anyway, you need about 11 digital lines to implement this ADC..
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Posted
by:
pankajg
on:
08 Sep 2011 |
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Re: Digital Thermometer Project |
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I mean to say I discuss this with my friend who is Electronic engg. "he said it wont be possible bcz your requirement is deifferent than this circuitry"
Here are some details of my set up,
I've position sensor (A simple potentiometer, 5VDC 100mA rating), I dont want to use digital sensor bcz of the limitations of the pins availabe on main machine controller.
On the main machine controller, 4 analog and 3 digital inputs are avaliable out of which i've already used 4 analog and 3 digital inputs are still there.
So any of these 3 digital inputs I'd like to connect analog signal of 2-potentiometers to this main machine controller.
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Posted
by:
pankajg
on:
08 Sep 2011 |
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Digital Thermometer Project |
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Hey I'm trying to put a tiny A to D convertor for my machine sensor which works with 0-5 VDC, 100 mA. Can you please suggest me the way to do this conversion of A-signal to D-signal. I need 2-Channel conversion that means I'd like to do two A-signal conversion at a time. I tried above (only) ADC ckt but it didn't work out. 
Please help..!!!
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Posted
by:
ikalogic
on:
18 May 2011 |
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Re: Digital Thermometer Project |
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| Quoting meerub: then what changing will i have to do..??can u please tell me...??? |
i don't know exactly.. you may have to deport the sensor via a wire to put it in contact with the skin.. then see the result.. try to take the time to understand the project, and be able to modify it according to your needs.
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Posted
by:
meerub
on:
15 May 2011 |
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Posted
by:
meerub
on:
14 May 2011 |
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Posted
by:
ahmad yasser
on:
04 Mar 2011 |
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Digital Thermometer Project |
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Al Salam Alykom
Thanks Ibrahim for this tutorial, but I have a question.
What is the sensitivity of this Thermometer?
Because I want to design something similar but with a sensitivity of 0.1 Degree Centigrade.
it will be part of a control system to keep the temperature an incubator constant at 37 Degree Centigrades.
and if you had some resources about linearizing the thermistor, Please attach it.
Thank you!
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Posted
by:
krupp
on:
12 Feb 2011 |
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Posted
by:
ikalogic
on:
09 Jan 2011 |
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Re: Digital Thermometer Project |
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Quoting casamiro: thanks for reply sir i have made it using r-2r(150k-300k) but i have problem when i give 5v to lm358 then at heating the thermistor, it's shows maximum temperature as 20C..but as i applying 10v or 12v then it's range increase to 74C.and also it's always jumps 36c to 56 and skip 40s line.
secondly decimal point digit is always shows zero only two digit change when temperature goes up.
Plz help me out i m very thankful to u guy
regards
casamiro |
The program need to be adapted. the conversion to °C need to be re calibrated according to the new sensor.. you have to find your way into the source code.. i don't have KEIL anymore, and i have stopped workin on 8051 years ago..
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Posted
by:
casamiro
on:
09 Jan 2011 |
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Digital Thermometer Project |
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thanks for reply sir i have made it using r-2r(150k-300k) but i have problem when i give 5v to lm358 then at heating the thermistor, it's shows maximum temperature as 20C..but as i applying 10v or 12v then it's range increase to 74C.and also it's always jumps 36c to 56 and skip 40s line.
secondly decimal point digit is always shows zero only two digit change when temperature goes up.
Plz help me out i m very thankful to u guy
regards
casamiro
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Posted
by:
ikalogic
on:
08 Jan 2011 |
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Re: Digital Thermometer Project |
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| Quoting casamiro: can we use lm35 instead of thermister??? and can we use r-2r resistance as 1k and 2k??instead of 1m and 500k??? or any other combination plz reply me.. |
Hello,
Yes you can use LM35 instead of the thermistor and the series resistor.
Yes you can use other resistors combination.
In both cases, the code will have to be adapted.
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