Nuts and Volts - December 2010

www.nutsvolts.com/index.php?/magazine/article/december2010_Reilly

PUT A TEMPERATURE SENSOR IN YOUR NEXT

By Rob Reilly

This project is a compost monitoring device (Figure 1)

that uses three temperature sensors (Dallas DS18B20), a photocell, an Arduino, and an Xbee digital radio to wirelessly send telemetry data to my notebook.

Temperatures are a good indicator of the need for turning the compost or when it is ready for use around my plants.

My device is housed in PVC pipe and runs on four D cell batteries.

■ FIGURE 1. Compost temperature device.

In this article, I'll cover using the Dallas DS18B20 temperature sensor with the Arduino microcontroller. I'll briefly describe the sensor, then summarize the Arduino code needed for using one or several of them. We’ll finish up with practical design implementation for putting these bad boys into your own device. The sensors are inexpensive and work well with the Arduino. You'll find a lot of uses for them.

A Cool Little Device

The Dallas DS18B20 "one-wire" temperature sensor is actually an amazing piece of technology. It can measure temperatures from - 67 degrees to 257 degrees Fahrenheit and operates on 3.0 to 5.0 volts DC. The TO- 92 version is physically similar to a medium-sized

36 December 2010

■ FIGURE 2. Pictorial schematic.

LED. Each device has a unique serial number and is hooked up to a simple bus wiring layout. You can put one, two, or 10 devices on a cable and retrieve each one's data individually. It's called a one-wire because you can chain multiple devices together and read them all using one digital input line on your microcontroller.

Code to integrate the DS18B20 and the Arduino used to be complicated and hard to understand. Today, easy-to-use function calls to pre-built libraries are the way to go. The one built by Miles Burton works very well and makes reading temperatures from the DS18B20 device almost painless (see Websites Sidebar).