Temperature Data Logger
This is how my finished product looks like.
A data logger is a device that records measurements over time. The measurements could be any physical variable like temperature, pressure, voltage, humidity, etc. This project describes how to build a mini logger that records surrounding temperature values. It has following features:
- Uses just one 8?pin chip, so very compact size
The beauty of this project is that it uses just a
single chip for logging. It is PIC12F683, an 8?pin
microcontroller from Microchip. PIC12F683 has six
general purpose I/O (GP0?GP5, GP3 is input only)
pins, and 2K x 14 Flash program memory
Temperature will be measured with a DS1820 temperature sensor. DS1820 is a one wire digital temperature sensor from Dallas Semiconductor (now MAXIM). The operating temperature range of the device is -55°C to +125°C with an accuracy of ±0.5°C over the range of -10°C to +85°C. The temperature sensor output is 9?bit Celsius temperature measurement, and so the temperature resolution corresponds to the least significant bit, and which is 0.5°C. But in this project we will use only the most significant eight bits. Therefore, the temperature resolution will be 1°C. The measured temperatures will be recorded into the internal EEPROM memory of PIC12F683.
The first location of the internal EEPROM will store the sampling interval of data logger. Sampling interval defines the time gap between two successive measurements. This project will have 3 options for sampling time: 1 sec, 1 min, and 10 min. These are user selectable. The second location of EEPROM will store the number of measurements recorded so far. And the remaining 254 EEPROM locations will store 8?bit temperatures. So, using 10 min sampling interval, 254 bytes of EEPROM will provide data logging for 42 hours. The recorded measurements can be sent to PC at any time through a serial link at 9600 baud.
There will be three tact switches for user inputs, namely Start, Stop, and Send/Reset. The three switches will be able to accept the following 4 user requests.
1. Start: When 'Start' button is
pressed, data logging starts.
The sampling interval can be selected as 1 sec, 1min, or 10 min using the same three switches. Suppose if we need 1 min sampling time, first turn OFF the power, then hold the 'Stop' button, turn the power ON, and wait till the LED glows. Once the LED glows, release the button, and the sampling interval will be set to 1 min. The new set sampling time will be updated to EEPROM location 0 so that in case of power failure, the previous sampling time will be restored. Similarly, use 'Start' and 'Send' buttons for 1 sec, and 10 min sampling intervals respectively.
Every time the user presses input buttons, LED glows for a moment to indicate that the input is accepted. It also blinks thrice every time EEPROM Write operation takes place. It also blinks at the beginning when the power is turned 'ON'. It also glows when the EEPROM memory is full.
The complete circuit diagram for this project is shown below.
The six I/O pins of PIC12F683 are assigned as follows:
1. GP0 (Pin 7, I/P): This pin will be used to read the temperature value from DS1820 sensor.
2. GP1 (Pin 6, O/P): This will be used for serial data transfer to a PC.
3. GP2 (Pin 5, O/P): LED output pin.
4. GP3 (Pin 4, I/P): Send (tactile switch)
5. GP4 (Pin 3, I/P): Stop (tactile switch)
6. GP5 (Pin 2, I/P): Start (tactile switch)
A simple transistor based level shifting will be used to convert the TTL voltage from PIC12F683 to appropriate RS232 level for serial data transfer to PC. The required negative voltage is derived from PC RS232 port pin 3 (Tx), which remains idle as no data transfer occurs from PC to PIC. I soldered all the components on a general-purpose prototyping board. Besides, my finished product runs on 3-AAA batteries with a diode in series for reverse polarity protection. I also have a power switch on the board. The PIC uses internal oscillator at 4MHz, and MCLR is disabled.
- PIC12F683 microcontroller
The firmware is written in C and compiled with mikroC compiler from MikroElektronika. I used the free version of it because our HEX output is going to be less than 2K. The download link for mikroC is http://www.mikroe.com/eng/downloads/get/29/mikroc_pro_pic_2010_v380_setup.zip
The 3 user input switches work
on interrupt on change mode. That means any time the
user presses any button, an interrupt is generated
except at times when the microcontroller is reading
temperature values from DS1820 (which is
My Hyperterminal settings are bps : 9600, Data Bits: 8, Parity : None, Stop Bits : 1, Flow Control : Hardware
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