Design and development of a quasi-digital sensor and instrument for water turbidity measurement

Abstract

In this paper, the design, realization, fabrication and assembly of a prototype water turbidity monitoring instrument based on an opto-resistive quasi-digital sensor has been proposed. The turbidity sensor has been constructed with LED (590 nm) as the light source, a light-dependent resistor, fixed capacitor and logic gate inverter. The primary output signal from the sensor is in the form of a transistor–transistor-logic pulse stream with 50% duty cycle. This form of the output signal can be directly interfaced with the microcontroller. To process pulse stream from the sensor, to convert into turbidity value in NTU and to display on an LCD, a microcontroller (PIC18f4550)-based embedded system has been developed. The turbidity monitoring instrument has been constructed, calibrated and experimentally tested by measuring the turbidity of the water in the range of 0–1000 NTU. The turbidity of 0–1000 NTU corresponded to the output frequency of 40.6–18.8 kHz with sensitivity of 22 Hz/NTU. This is a high-precision, low-power, portable instrument with low manufacturing costs compared to commercial turbidity monitoring instruments.