Issue 113 December 1999
Being Cool is Easy
A Temperature-Sensing Control Device

PROTOTYPE CONSTRUCTION

I built the X-10 temperature sensor on a microEngineering Labs PICProto3 prototyping board. The main components of the sensor are mounted on the PICProto3. This PCB provides traces and pads for the standard PIC components such as power supply circuitry, microcontroller and the oscillator circuitry.

The remaining temperature sensor components (with the exception of the LCD, digital thermometer, and TW-523 module) are mounted in the prototyping area of the PICProto3. Direct point-to-point wiring using 26-AWG solid wire was implemented.

The LCD is connected to the PICProto3 by a 10-wire ribbon cable and header connector. The digital thermometer is wired to a twisted pair cable connected via a terminal block. The TW-523 is connected by a modular cable, which plugs into a jack mounted on the PICProto3.

APPLICATIONS

The X-10 temperature sensor was designed to aid in control of a house fan in a remote location. This device can also be used in other applications where it’s desirable to determine temperature in relation to a threshold. Applications that come to mind are:

• monitoring freezer temperature to generate an alarm if the temperature is too high

• monitoring plumbing temperature to generate an alarm and/or turn on a heater if the temperature gets too low

For applications where it’s desirable to obtain a remote temperature, not just the relationship to a given threshold, the software could be easily modified to transmit the local temperature upon request. The temperature could be encoded using standard X-10 Key and Function Codes. The X-10 controller could then, for example, record hourly temperatures and daily high and low temperatures, as desired.

Although the X-10 sensor provides feedback for a closed-loop environment, care should be exercised as with any X-10 application. Because X-10 control is not 100% reliable, it shouldn’t be employed in a situation where safety might be compromised. For example, an electric heater should never be controlled by X-10 alone if it could cause a fire if left on too long.

COOLING DOWN

The temperature sensor provides remote temperature measurement without wires, using only standard X-10 Key and Function Codes. The Extended Code/Data capabilities of the X-10 standard are not required. In addition, the device uses only a single X-10 House Code.

Many other possibilities exist for this temperature sensor. The protocol could be expanded to share a single House Code among multiple sensors. The LCD could be left out to reduce the number of I/O required and permit the sensor to be used with an inexpensive microcontroller such as a PIC16C84. This technique could even be applied to motion detectors, light-level sensors, or other types of sensors.