Design Idea

Quartz crystal-based remote thermometer features direct Celsius readout

Jim Williams and Mark Thoren, Linear Technology Corp; Edited by Brad Thompson -- EDN, 3/17/2005

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).

An economical and commercially available quartz temperature sensor, Y₁ and IC₁, an LTC-485 RS485 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 (Reference 3). 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 LTC-485, IC₂, in receiving mode, accepts the differential data and presents a single-ended output to IC₃, a PIC-16F73 processor that converts the frequency-coded temperature data and presents the temperature in Celsius format on LCD₁. Click here to download the conversion program's source code.