Quartz crystal-based remote thermometer features direct Celsius readout

Microchip PIC16F73

Jim Williams & Mark Thoren

EDN

Although quartz crystals have served as temperature sensors, designers haven't taken advantage of the technology because few manufacturers offer the sensors as standard products (references 1 and 2). In contrast to conventional resistance- or semiconductor-based sensors, a quartz-based sensor provides inherently digital-signal conditioning, good stability, and a direct digital output that's immune to noise and thus ideally suited to remote-sensor placement (Figure 1).

Quartz crystal-based remote thermometer features direct Celsius readout
Figure 1. A quartz-crystal temperature sensor provides Celsius-temperature readouts
accurate to 2% over a –40 to +85 °C range.

An economical and commercially available quartz temperature sensor, Y1 and IC1, an LTC485 RS-485 transceiver in transmitter mode, form a Pierce crystal oscillator. The sensor, an Epson HTS-206, presents a nominal frequency of 40 kHz at 25 °C and a temperature coefficient of –29.6 ppm/°C. The transceiver's differential-line-driver outputs deliver a frequency-coded temperature signal over a twisted-pair cable at distances as far as 1000 ft.

A second LTC485, IC2, in receiving mode, accepts the differential data and presents a single-ended output to IC3, a PIC16F73 processor that converts the frequency-coded temperature data and presents the temperature in Celsius format on LCD1.

References

  1. Benjamin, Albert, "The Linear Quartz Thermometer—A New Tool for Measuring Absolute and Differential Temperature," Hewlett-Packard Journal, March 1965.
  2. Williams, Jim, "Practical Circuitry for Measurement and Control Problems," Application Note 61, August 1994, Linear Technology Corp.

Materials on the topic

  1. Datasheet Linear Technology LTC485
  2. Datasheet Microchip PIC16F73
  3. Datasheet Seiko Epson HTS-206

Downloads

  1. The conversion program's source code

EDN

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