Direct interface circuit for capacitive humidity sensors

Polymer 0–100 180 at 55% RH 0.34 (from 33–75% RH) 163–201
Table 2 Saturated salt solutions used to subject the sensors to different values of RH
Salt
Nominal RH at 25 ◦C (%) Actual RH (%)
Low-cost hygrometers generally use capacitive RH sensors [2,3]. These sensors have a hygroscopic dielectric (such as a polymer, porous ceramic or porous silicon) that absorbs or desorbs water vapour as the ambient RH increases or decreases, which leads to a change in the dielectric constant and hence in the capacitance [4]. For commercial sensors, the sensitivity varies from 0.2 pF up to 0.5 pF per %RH, and the capacitance is between 100 pF and 500 pF at 50% RH at 25 ◦C [5].
Capacitive sensor; Sensor electronic interface; Microcontroller
1. Introduction
Humidity (i.e. water vapour content of air) has a significant effect on quality of life and products. For this reason, humidity measurements have become increasingly usual in industry (e.g. manufacturing of paper, food, tobacco, textiles and integrated circuits), services (e.g. hospitals, museums, libraries and computer rooms) and home appliances [1,2]. The relative humidity (RH), which is the ratio of the actual water vapour pressure to the saturated water vapour pressure at a given temperature, is the most frequently used parameter to specify the ambient humidity [1].
There are basically two types of interface circuit to read the signal from a capacitive RH sensor: (a) circuits based on the charge-transfer method [6–8], and (b) oscillator circuits [9–12]. The former have a voltage-modulated output signal and hence need an analogue-to-digital converter, whereas the latter have a time-modulated output signal that can be directly measured by a digital system [13]. Data sheets of commercial capacitive RH sensors also propose oscillators based on 555 timers IC that are similar to those described in Refs. [10,12].
In order to reduce the cost, complexity, area and power consumption of the interface circuit, some references have proposed direct capacitive sensor-to-microcontroller interfaces, in which the capacitive sensor is directly connected to the microcontroller (␮C) without using either a signal conditioner or an analogueto-digital converter. Such direct interface circuits basically rely on the same principles indicated in the previous paragraph. On one hand, we have direct interface circuits based on the chargetransfer method [14,15], in which the ␮C counts the number of charge-transfer cycles needed to charge a given capacitor via the unknown capacitance. On the other hand, we have direct interface circuits based on oscillator principles [16,17], in which the ␮C measures the time needed to charge/discharge the unknown capacitance through a given resistance. However, such ideas have not yet been applied to capacitive RH sensors, basically
Abstract This paper proposes and analyses a direct interface circuit for capacitive humidity sensors. Such a circuit relies on directly connecting the sensor
to a microcontroller without using either a signal conditioner or an analogue-to-digital converter, thus resulting in a simple, compact, low-cost and low-power interface circuit. In spite of its simplicity, the interface circuit performs satisfactorily when measuring commercial capacitive humidity sensors (Philips H1 and Humirel HS1101). The non-linearity error of the interface circuit itself (0.1% full scale span (FSS)) is much smaller than that of the sensor (5.6% FSS for the H1 and 2.1% FSS for the HS1101) and, therefore, the circuit does not limit the accuracy (in terms of linearity) of the measurement, as is to be expected from a well-designed interface circuit. Furthermore, the effective resolution equals 9 bits for a measuring time of 50 ms, which are fully acceptable for humidity measurements. © 2007 Elsevier B.V. All rights reserved.
Philips H1
Humirel HS1101
Dielectric Humidity range
(%RH) Typical nominal
capacitance (pF) Typical sensitivity
(pF/%RH) Typical capacitance
range (pF)
Polymer 10–90 122 at 43% RH 0.4 (from 12–75% RH) 112–144
Available online at
Sensors and Actuators A 143 (2008) 315–322
Direct interface circuit for capacitive humidity sensors
Ferran Reverter ∗, O` scar Casas
∗ Corresponding author. Tel.: +34 934137090; fax: +34 934137007. E-mail address: reverter@ (F. Reverter).