Nuts and Volts - May 2014

own, I created a bottom copper mask (available at the article link).

Since this is for a single-sided PCB, the design isn’t as compact as my professionally-made boards.

The Thermometer Program

There’s a sample of the thermometer’s program also online at the article link. Below is the fifty cent tour of the program.

The program begins by setting up I2C communications with the 24LC128 EEPROM. The data will be stored here, and the PICAXE needs to communicate at 400 kHz and in one word addresses. Next, the code zeros out the LED displays to show 00.

Prior to measuring the temperature with the LM335, the PICAXE checks to see if the thermometer’s switch is set to Run or Download. If set to Download, the PICAXE starts at the beginning of the memory’s address and gets each value stored.

The data — which is stored in words (since the PICAXE uses the 10- bit A-to-D (analog-to-digital)

command for greater temperature resolution) — is dumped onto a PC using the SERTXD command. This means the user must open the

Terminal program under the PICAXE Editor.

In Run mode, the PICAXE uses the READADC10 command to digitize the voltage produced by the LM335 temperature sensor. At 10-bit resolution, the thermometer program can measure changes as small as a single degree Fahrenheit. This also means the results must be stored in one word records (rather than smaller one byte records).

The math routine that converts digitized voltage readings into a two-digit display first determines if the temperature is above or below zero. This occurs at a digitized reading of 522 for the sensor I am using. You, however, might find you’ll need to adjust this value up or down one number.

Every change in one digitized voltage reading (say, from 522 to 523) is a change by 1.16 degrees Fahrenheit. Therefore, for every change in seven digitized voltage readings, the temperature is decremented by one.

Whether the temperature is above or below a reading of 522 determines whether I/O pin B.0 is set high (to illuminate the red LED) or low (to illuminate the blue LED). The LED digits represent the tens and ones of the temperature reading. Each is set separately, so the program must divide the temperature value by 10 to separate out each digit. There’s a subroutine to show the digits zero through nine on each display, and the program jumps execution to the proper subroutine.

After displaying the temperature, the thermometer program records the digitized temperature reading into the next address in the 24LC128 memory. After a pause (you program the length of the pause), the program repeats itself.

That’s about it for the Totable TVC (t-TVC) Thermometer. It’s a fun project that’s useful for applications off the work bench. We had some pretty cold days in Idaho this winter (although not as bad as the upper Midwest and East experienced), and I would have liked to use to record and display temps overnight. Perhaps you’ll find this project equally interesting and useful.