Digital Temerature Sensor

by jakeselectronics

This circuit is a Digital Temerature Sensor using a Dallas '1-wire' DS18B20 Digital Thermometer

How It Works

The DS18B20 is a direct-to-digital temperature sensor using exclusive 1-Wire bus protocol that implements bus communication using one control signal. In regards to hardware, this particular sensor is particularly easy to interface to. It only requires 1 external pull-up resistor to operate as opposed to an analogue sensor which possibly needs multiple external components such as resistors and op-amps.

In regards to software, opposed to analogue sensors, the Dallas 1-wire digital sensors are arguably as easy to interface to. While an analogue sensor will need an Analogue to Digital conversion using a voltage reference and possibly using an op-amp, the Dallas 1-wire direct to digital sensors require precise timing when it comes to communication. This program is fairly basic in principle as all it does is obtain temperature data from the DS18B20 sensor and display the temperature in Degrees Celsius on a 4 digit, 7 segment display. But when it comes to actually doing this, as you will see from the .ASM file, it is more complicated than it sounds. In words, the program first initialises the PIC16F628A Microcontroller. It assigns the Inputs and Outputs, zero's all bank 0 RAM, initialises the display column select bit and configures TIMER 2. TIMER 2 is used to interrupt the normal loop of the program to update the 7 segment LED display.

After the Microcontroller has been setup, it begins communication with the DS18B20 Temperature Sensor. Communication routines take up just under half of the program memory. After the temperature has been gathered and stored in RAM, the Microcontroller takes the 12-bit signed/fraction integer and converts it into a decimal number then stores it in four general purpose registers in RAM.

For example, take the number D'95.8'. It is stored like this:

Code:

These registers are then used within the Interrupt Service Routine to call a table to obtain display data.

The program runs continuously, updating the temperature on the display just over once per 1 second.

Features Summary:

• Temperature data gathered more than once per second.
• TIMER 2 interrupt driven display.
• Program expandable to include multiple sensors on the same 1-Wire bus.
• Temperature range of -55.0 - 127.9 Degrees Celsius.

Please see the DS18B20 Datasheet for detailed information on the device.

Video

Parts List

Downloads

1. schematic.gif
2. breadboard.jpg
3. digitaltemperaturesensorbasic.hex
4. digitaltemperaturesensorbasic.asm

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