It is a very simple data logger project based on PIC12F683 microcontroller. The microcontroller reads temperature values from a temperature sensor on a regular interval basis and stores them into its internal EEPROM memory. The recorded temperatures can be later transferred to a PC through serial interface. I originally published this project on electronics-lab.com last summer. I thought this could be a very good learning project for beginners, and so I am posting it here for Embedded Lab’s readers too.
Theory
The sensor used in this project is DS18B20. It is a digital temperature sensor manufactured by Dallas Semiconductor (now MAXIM) that can measure temperature ranging from -55°C to +125°C with an accuracy of ±0.5°C over the range of -10°C to +85°C. The sensor provides the measured temperature (°C) output in user-configurable 9, 10, 11, or 12-bit data corresponding to the desired resolution of 0.5, 0.25, 0.125, and 0.0625 °C. The sensor communicates with a host microcontroller over a 1-wire bus. Readers are suggested to refer the datasheet on Maxim’s website for details on this sensor. Please keep in mind that there are three versions of this sensors, namely DS1820, DS18S20, and DS18B20, and they have some architectural differences. All of them have the same pin configuration and therefore the circuit diagram would be same for all three types. However, some modification in the software may be required while the microcontroller reads the temperature data from them.
PIC12F683 has 256 bytes of internal EEPROM. Each temperature reading is stored as a byte, which means only the eight most significant bits of DS18B20 output is recorded. Therefore, the temperature resolution is decreased down to 1 °C. This temperature logger can store up to 254 temperature values (254 bytes) in its internal EEPROM. The remaining two EEPROM locations are used to store the sampling time and number of samples information. Three tact switches are used to provide user inputs for controlling the operation of the data logger.
Circuit Diagram
The PIC microcontroller uses its internal clock source operated at 4.0 MHz. The DS18B20 sensor is interfaced to GP0 pin (7) of the microcontroller. An LED connected to the GP2 pin serves as the only display in the circuit to indicate various actions of the data logger. For example, it blinks every time a sample is recorded into EEPROM. The circuit is powered with +5 V derived from a 9V battery using an LM78L05 regulator IC. The LM78L05 circuit is a very common circuit and therefore, it is not shown here.
The three tact switches provide the following functions.
- Start: Starts data logging
- Stop: Stops the logging procedure
- Send/Reset: Transfers data to PC through serial port. However, if it is held pressed for 2 sec or more, the EEPROM locations are cleared and ready for new recordings.
Selection of sampling time
This data logger offers three options for sampling interval: 1 sec, 1min, and 10 min. The selection is made through the same three tact switches. Here is how it works. Suppose if 10 min sampling time is needed, then first turn OFF the power, hold the ‘Send/Reset’ button pressed, turn the power ON, and wait till the LED glows. Once the LED glows, release the button, and the sampling interval is set to 10 min. The new sampling time will be updated to EEPROM location 0 so that in case of power failure, the previous sampling time will be restored. Similarly, the use of ‘Start’ or ‘Stop’ button instead of the Send/Reset one sets the sampling time to 1 sec, or 1 min respectively. With 10 min sampling interval, this data logger can record temperature samples over 42 hours.
Serial Interface to PC
Transferring data to PC through serial port requires a voltage translation circuit to convert the TTL logic levels from PIC12F683 to appropriate RS232 voltage levels. A regular PNP transistor with few other passive components can do this job. The RS232 standard uses a -3 to -12 V for Logic 1 and +3 to +12 V for Logic 0. The required negative voltage is stolen from the TX pin of the RS232 port on PC’s side which is unused as there won’t be any data transfer from the PC to PIC12F683. Under idle condition the TX pin on PC’s side is held high (-12 V). The two figures below describe the operation of converting TTL levels for 1 and 0 to corresponding RS232 levels. The positive terminal of the 10 uF capacitor is grounded because its negative terminal has to deal with a more negative voltage.
Software
The firmware for PIC12F683 is developed in C using mikroC Pro for PIC compiler from mikroElektronika. PIC12F683 doesn’t have built-in hardware UART module for serial communication but the mikroC compiler has built-in library routines to implement software UART through any digital I/O pin of PIC microcontroller. Due to wide use of DS18B20 sensor for temperature measurements, mikroC also provides 1-wire library routines for controlling the operation of this sensor. The built-in functions for communicating with a 1-wire sensor are,
- Ow_Reset is used for reseting sensor;
- Ow_Read is used for receiving data from sensor; and
- Ow_Write is used for sending commands to sensor.
Read more: A Beginner’s data logger project using PIC12F683 microcontroller