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).
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| Figure 1. | A quartz-crystal temperature sensor provides Celsius-temperature readouts accurate to 2% over a –40 to +85 °C range. |
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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.
