A CMOS Algae Growth Period Monitor for Algaculture Applications

In recent years, algaculture has become increasingly important, particularly for the sustenance of aquatic organisms like shrimp. However, assessing algae growth status has traditionally been time-consuming, relying on techniques such as direct cell counts, spectrophotometry, and dry weight measurements, which often suffer from prolonged processing times and dependence on specialized equipment.

To address these challenges, this paper introduces a new approach using electricity generated through a proton exchange membrane for monitoring algae growth. The proposed Complementary Metal–Oxide–Semiconductor (CMOS) algae growth period monitor consists of an algae sensor and a CMOS converter. This system translates algae growth data into a linear duty cycle, allowing for rapid assessment of growth periods.

The converter, designed to extend the input range from -1.5 to 1.5 V, was fabricated using the CMOS Taiwan Semiconductor Manufacturing Co., Ltd. (TSMC) 2 poly/4 metal (2P4M) 035-μm process. It measured a differential rail-to-rail voltage range of -1.5 to 1.5 V, with an output duty cycle range of 1.04% to 99.3%.

Photosynthesis converts light energy into carbohydrates, generating a current as a byproduct. An algae sensor, operating based on photosynthesis principles, was developed by cultivating ‘Tetraselmis chui’ algae under controlled conditions. The sensor then uses a proton exchange membrane to generate a current, which is converted into voltage by analog signal processing circuits and subsequently translated into a duty cycle.

After the algae sensor produces current IS, it generates differential voltages across resistor RS, resulting in a broad voltage range. A wide-range differential rail-to-rail voltage to current converter and adjustment sensitivity circuits are incorporated to accommodate diverse algae growth environments.

The adjustment sensitivity circuits enable adaptability to diverse environmental conditions during algae cultivation, including the processing through analog signal processing circuits, where operational amplifiers and resistors Rm function as analog adders and subtractors, generating desired voltages.

The experimental setup included power supplies, an algae sensor, a GW GDM-8245 multimeter, a LeCroy WaveAce 2034 oscilloscope, and the proposed chip. The algae sensor and chip were interconnected, resulting in a measured algae growth period ranging from 1 to 5.5 days. The output duty cycle spanned 24.96% to 90.19% across various concentrations of algae nutrients with a recorded maximum linear error of 0.49%, confirming the circuit's correct operation.

Furthermore, experiments conducted with algae species Nannochloropsis sp. and Isochrysis galbana supported the monitor's effectiveness for algaculture applications, affirming its accuracy and suitability.

The study's findings demonstrate that the measured output duty cycle correlates directly with the algae growth period, with longer growth durations leading to higher duty cycles. The chip's adaptability to diverse algae sensors employing photosynthesis principles enhances its versatility, and the implementation of adjustment circuits enhances its sensitivity, particularly when monitoring algae with suboptimal growth conditions.

Results highlight the effectiveness of the CMOS algae growth period monitor in providing comprehensive functionalities tailored for algaculture applications. Furthermore, the potential integration of the CMOS converter into IoT devices offers promising prospects for advancing technological capabilities in algae growth status monitoring.