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

Abstract

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.

Chemical and biological sensing using liquid crystals

“…Past studies have demonstrated that LC-based sensors can be designed to report a range of physical stimuli, including mechanical shear, temperature, electric and magnetic fields, and light. For example, Marcos et al [9] reported the use of LCs to sense shear and temperature fields, Herzer et al [10] reported printable cholesteric LC films as temperature and humidity sensors, and Chatterjee et al [11] examined the influence of shear fields on defects in LCs. In addition, Chanishvili et al [12] reported cholesteric LC mixtures that are sensitive to different ranges of solar UV radiation.…”

Tracking capacitance of liquid crystal devices to improve polarization rotation accuracy

“…For most LCPRs, the capacitance curve as shown in Fig. 2 changes very slowly with temperature when operated far below the liquid crystal isotropic temperature [3]. The LCPRs in our study have an isotropic temperature of 363 K. In this paper, we focus on the temperature range between 283 K and 301 K which represents the expected conditions for our instruments onboard small satellites [5].…”

Liquid Crystal Temperature Sensor Based on Three Electrodes and a High-Resistivity Layer

“…For example, Marcos et. al used this feature to create a LC capacitor that was integrated in a multivibrator circuit [13]. The result is a temperature-frequency transducer with a variable frequency as an output signal; this kind of sensor has demonstrated a broad operating temperature range.…”

“…This fact can be used to introduce this electrical component in some circuit (as it was proposed in ref. [13]). This system needs extra components and special equipment to measure the output signal.…”