A simple, inexpensive method for building a temperature strip-chart recorder involves a parallel port that communicates with four TMP121 IC temperature sensors (Fig. 1). The sensors communicate via the parallel port using a basic serial peripheral interface. Because the sensor draws extremely low quiescent current (typically 50 µA), the parallel port can also be used to supply power for the device.
All pins for each TMP121 are connected in parallel, except the chip-select lines. During operation, the devices are accessed independently using the chip-select lines. In other words, the SO data output of the selected device is active, and the data outputs of the other devices are in a high-impedance state. The circuit can be expanded to include 10 temperature sensors by simply linking all common pins of the additional device in parallel and deriving the chip-select signals from the parallel port outputs on pins 6, 7, 8, 9, 16, and 17 of the 25-pin connector.
A Visual Basic program controls the circuit's operation. The program lets the user select the data-acquisition rate. Data is displayed in real time as a line graph and stored in a CSV file for later use. Figure 2 illustrates the program interface. The CSV file can be imported into Excel to generate a graph.