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Timer-Based Capacitance-to-Voltage Converter
(2026-03-01)
Kaewpoonsuk, Anucha
;
Tokampang, Sudarat
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Sisuk, Noppadon
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Rerkratn, Apinai
;
Petchmaneelumka, Wandee
This article presents the design and analysis of a capacitance-to-voltage converter based on 555-timer Integrated Circuits (ICs) as the core design element. The circuit structure consists of two primary active components: two 555-timer ICs connected in conjunction with a low-pass filter circuit. The first 555-timer IC operates in astable mode to generate a signal that controls the timing of the second IC. The target capacitor or capacitive sensor to be measured is connected to the second 555-timer IC, which operates in monostable mode. The output signal from the second 555-timer IC is then converted into a DC voltage output using a low-pass filter circuit. The circuit was tested using laboratory capacitors in the range of 52.72 pF to 807.61 pF, and the data were analyzed using Microsoft Excel. The results showed excellent agreement with the measurements obtained from the GW INSTEK LCR-819. The maximum uncalibrated error was found to be −3.32% of full scale. After applying linear curve fitting in Excel, the maximum error was reduced to 0.92% of full scale, confirming the effectiveness of the calibration approach. Furthermore, the circuit was applied to measure the moisture content of paddy rice using a cylindrical capacitive sensor within the moisture range of 12.1% to 23.2%. The experimental data were fitted to a derived equation, which was then used to predict rice moisture content, producing results that corresponded closely with those obtained from the KETT PM-450 grain moisture meter.
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Low-Cost Capacitive Sensor Front-End Circuit Based on Phase-Locked Technique
(2025-12-15)
Wongprommoon, Natapong
;
Chimnoy, Jirat
;
Prommee, Pipat
This research presents an efficient front-end circuit for grounded capacitive sensors (GCSs), based on a phase-locked technique. An all-pass filter serves as a phase shifter to obtain the desired 90° phase difference, enabling the sensor capacitance to be calculated from the locked frequency regardless of parasitic capacitances. For long-distance capacitance measurements (remote sensors), the proposed circuit integrates an improved active shield technique, allowing real-time measurements using only a low-cost microcontroller unit (MCU), display, and commercially available electronic components, without the need for expensive or bulky instruments. The design is fully automated and does not require manual adjustments. Simulation and experimental results are consistent and validate the effectiveness of the proposed method. Comparative experiments with standard instruments demonstrate accurate measurement within the 1–500 pF range through shielded cables up to 10 m, while maintaining a maximum nonlinear error (NLE) of only 0.04% full-scale span (FSS). The proposed circuit architecture effectively mitigates the effects of parasitic cable capacitance and the input capacitance of operational amplifiers. This approach provides a low-cost, compact, fast, and highly accurate solution, making it suitable for applications of liquid-level measurement in hazardous or hard-to-access environments through remote capacitive sensor measurement.
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Grounded Capacitive Sensor Readout Using Improved Guard Driving Circuit
(2025-01-01)
Chimnoy, Jirat
;
Karawanich, Khunanon
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Prommee, Pipat
A remote capacitive sensor is necessary in hazardous locations such as liquid boilers and chemical facilities. The connection of the capacitive sensor and coaxial cable is integrated with the readout circuit, encompassing intrinsic noise and parasitic capacitance limitations. This work presents a grounded capacitive sensor with an economical architecture incorporating an RC lowpass filter and an enhanced guard driving circuit. The proposed technique effectively achieves minimal parasitic capacitance and reduced noise at the output. The sensor capacitance can be readily calculated by measuring the output signal's amplitude at the cutoff frequency. Both simulation and experimental findings corroborate its efficacy and precision. The capacitance range can be quantified between 1-100 pF with less than 2% relative errors.
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Front-end interfacing circuit for capacitive sensor
(2015-01-01)
Ota, Akira
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Petchmaneelumka, Wandee
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Cheypoca, Thepjit
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Rerkratn, Apinai
;
Riewruja, Vanchai
This paper presents an interfacing circuit for capacitive sensor using charge amplifier formed a capacitance-to-voltage converter. The proposed circuit is suitable for the front-end analog-to-digital converter (ADC) and wide variable range of sensing capacitance. The achieved output voltage provides a linear transfer characteristic and fast response. The circuit configuration is implemented using only commercially available devices. The performances of proposed circuit are confirmed by experimental results.
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Synthesis of simple interface circuit based on time-period detection technique for capacitance measurement
(2011-01-01)
Kaewpoonsuk, Anucha
;
Rerkratn, Apinai
This paper presents a synthesis simple method using time-period detection technique for use in capacitance measurement. The proposed scheme consists of a square wave generator, an integrator, a sample-and-hold circuit, and a designed logic circuit. Output signals of logic circuit are used to control the integrator followed by sample-and-hold circuit in sequential operation. The proposed circuit provides measuring capacitance in 4 ranges; 0.1-1 nF, 1-10 nF, 10-100 nF, and 100-1000 nF. Each range has output voltage signal proportional to the sensing capacitor from 0.5 V to 5 V. Performances of the proposed circuit were experimentally verified. Results are in good agreement with expected values. © 2011 SICE.

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