Issue 154 May 2003
Automatic Temp Controller
Data Logger for Slow Cooker

PROJECT GOALS

The original goal for this project was to implement a simple closed-loop temperature controller using an 8-bit microprocessor. Modeled after a kitchen oven, I wanted the controller to have a knob for setting the desired temperature. A temperature probe would monitor the ceramic cooker’s actual temperature, and the controller would adjust the airflow to keep the desired and actual temperatures as close as possible.

Being microprocessor controlled, I thought it would be easy to add other functionality, such as additional probes to monitor the food temperature, and another control to indicate the desired final food temperature. An alarm could ring when the food reached a desired temperature or if an error condition occurred. An RS-232 port would allow cooker and food temperatures to be logged, either to a local computer or perhaps a web site for remote access. Photo 2 shows the front panel of my final controller implementation.

As for the control system’s accuracy, my intention was make it at least as good as a kitchen oven. Temperature tolerances only needed to be within ±5°, because barbequing is more of an art than a science. Temperature is just one factor to be consider when you’re trying to determine if the food is ready to be taken off the fire.

TEMPERATURE ACQUISITION

Temperatures are measured by taking an analog measurement from a temperature probe. Converting it to a digital value using the micro’s A/D converter, and then converting the raw measurement to an actual temperature achieves this.

The temperature probe I used is a thermocouple, which is constructed simply by connecting two dissimilar types of metal wire. When this junction is heated, a voltage proportional to the temperature can be measured at the other end of the wires. Different types of metals have been standardized, and their voltage-versus-temperature properties characterized. This project uses a type-K thermocouple.

Generally, type-K can be used for temperatures up to 2300°F, although the insulating materials used in the low-cost probes limit the upper temperature to approximately 500°F for Teflon and 900° for glass braid. Photo 3 shows typical thermocouple configurations. Although thermocouples are simple, rugged, and inexpensive, they have some properties that must be accommodated.