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Measure humidity and temperature on one TTL line

-August 30, 2001

By combining the responses of an Analog Devices (www.analog.com) AD590 temperature sensor and a Humirel (www.humirel.com) HS1101 humidity sensor, you can generate a single TTL-level signal containing information from both sensors (Figure 1). This design uses a 74HC123 monostable multivibrator, IC1, to form a free-running oscillator. The AD590 current source (1 µA/K), IC2, and a fixed 1-nF capacitor, C1, control the timing of the first monostable multivibrator in the 74HC123. Another monostable multivibrator uses a fixed 1-MΩ resistor along with the capacitive output of the HS1101 (172 pF at 0% relative humidity and 222 pF at 100% relative humidity) for its timing. Combining the two monostable multivibrators creates a free-running oscillator that produces a single-line signal from both sensors. The high- and low-level pulse widths carry the information related to the sensor signals. The AD590 circuit displays pulse-width reduction with rising temperature, because of its increased output current with higher temperatures. The HS1101 circuit displays increased pulse width with rises in humidity levels. The circuit in Figure 1 represents a simple method of transmitting signals from analog sensors by digital rather than analog means. The technique eliminates noise in signal transmission over long distances. You could add an optoisolator in the output path if you need, say, 1500V isolation.

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