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Sensors 2012, 12(3), 3204-3214; doi:10.3390/s120303204

Temperature-Frequency Converter Using a Liquid Crystal Cell as a Sensing Element

Grupo de Displays y Aplicaciones Fotónicas, Departamento de Tecnología Electrónica, Universidad Carlos III de Madrid, Avda. de la Universidad s/n, E28911, Leganés, Madrid, Spain
Author to whom correspondence should be addressed.
Received: 1 February 2012 / Revised: 2 March 2012 / Accepted: 2 March 2012 / Published: 7 March 2012
(This article belongs to the Section Physical Sensors)
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A new temperature-frequency converter based on the variation of the dielectric permittivity of the Liquid Crystal (LC) material with temperature has been demonstrated. Unlike other temperature sensors based on liquid crystal processing optical signals for determining the temperature, this work presents a system that is able to sense temperature by using only electrical signals. The variation of the dielectric permittivity with temperature is used to modify the capacitance of a plain capacitor using a LC material as non-ideal dielectric. An electric oscillator with an output frequency depending on variable capacitance made of a twisted-nematic (TN) liquid crystal (LC) cell has been built. The output frequency is related to the temperature of LC cell through the equations associated to the oscillator circuit. The experimental results show excellent temperature sensitivity, with a variation of 0.40% of the initial frequency per degree Celsius in the temperature range from −6 °C to 110 °C.
Keywords: temperature sensor; multivibrator circuit; nematic liquid crystal temperature sensor; multivibrator circuit; nematic liquid crystal

This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Marcos, C.; Pena, J.M.S.; Torres, J.C.; Santos, J.I. Temperature-Frequency Converter Using a Liquid Crystal Cell as a Sensing Element. Sensors 2012, 12, 3204-3214.

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