Briefly, this project shows the current temperature on a LED display using an absolute minimum of parts and with very good accuracy. A temperature set-point can also be entered, enabling control of external devices for rudimentary heating or cooling applications via a single digital output.

Temperature measurement is realised with a DS18B20 digital temperature sensor IC, a device that will be familiar to many PICAXE experimenters. The sensor can be mounted directly on the board or via short flying leads tailored to suit the application.

For those interested in experimenting with 7-segment displays, this project can also be connected to the Schools Experimenter (see SILICON CHIP, May 2005) using an optional header.

Power for the project can come from a three or 4-cell battery pack or better still, a regulated 5V DC supply.

Three variants

First up, we must mention that the author is not releasing the BASIC code for this project. Instead, you can purchase pre-programmed PICAXE-08M chips (and PC boards) for a reasonable price. Three distinct versions of the code have been created to cater for a variety of needs. There are two thermostatic versions, identified as "heating" and "cooling", and a "temperature only" version.

Both thermostatic versions display the temperature of the DS18B20 sensor from 1°C to 124°C and allow entry of a user-selected setpoint. Above the programmed setpoint, the "cooling" version produces a logic high (+5V) on the digital output, whereas for the "heating" version, a logic high is produced while the temperature is below the setpoint.

The "temperature only" version is just that; it displays the temperature of the DS18B20 sensor, ranging from -55°C to +125°C, but has no thermostatic functions. The digital reading from the DS18B20 is accurate to ±0.5°C from -10°C to +85°C, so the displayed reading will be accurate to 1°C. The on-board LED is used to indicate negative temperature readings. Pressing S1 switches the display to read in degrees Fahrenheit, with a range of 32°F-125°F.

Note: a simplified version of the "temperature only" BASIC program (named tempdisplay.bas) is available for download from our website for those that wish to experiment with their own code.