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DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram

Intro Of DIY Water Quality Meter

Just after the Kerala flood, I received requests from a few victims inquiring how to build a cheap device to ensure the quality of drinking water. For the purposes of water quality monitoring, I started the design of a simple Turbidity Meter capable of analyzing a water sample but without precision optics and electronics. Frankly, the given low-profile (yet expansible) design is good only for a rough qualitative analysis (not quantitative analysis) as it’s assumed that the community at present may not have fiscal resources to build costly devices and/or water quality analyzing may not be a sufficient priority to justify that expense!

The Backdrop

Turbidity is an internationally recognized criterion for assessing drinking water quality, and a Turbidity Meter is an optoelectronic instrument that assesses turbidity by measuring the scattering of light passing through a water sample containing colloidal particles that harbor pathogens. Turbidity is most commonly quantified by the Nephelometric Turbidity Unit (NTU), or the equivalent Formazin Nephelometric Unit (FNU). Nephelometry refers to the process of aiming a beam of light at a sample of liquid and measuring the intensity of light scattered at 90° to the beam.

So a Turbidity Meter holds one light source that is directed through a water sample, a chamber to hold the water sample, and one or more photodetectors placed around the chamber. The common “single-beam” Turbidity Meter (see the design pattern shown below) only monitors light scattered by particles suspended in water to generate an output voltage proportional to turbidity or suspended solids.

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 1

However, with an optical sender and optical receiver as the front-end sensor, our Turbidity Meter operates on the principle that when light is passed through a sample of water, the amount of light transmitted through the sample is dependent on the amount of suspended particles in the water. As the amount of total suspended solids (TSS) increases, the amount of transmitted light decreases. The rest of the electronics in the system circuitry then measures the amount of transmitted light to determine the turbidity of the sampled water. In short, the Turbidity Meter simply measures the amount of light coming from the light sender to the light receiver and calculates the water turbidity.

The Frontend

As you might have guessedthe front-end sensor is an optical device comprising an LED (light sender) and a phototransistor (light receiver). Because Turbidity Sensor modules are widely used in washing machines and dishwashers, you can buy a compact front-end sensor from most online electronics retailers. The TSD-10 Turbidity Sensor from GE (www.ge-mcs.com) and from Amphenol (www.amphenol-sensors.com) are compatible. Note that you can also see a maker’s version — the Gravity Arduino Turbidity Sensor from DFRobot (www.dfrobot.com) — everywhere on the web.

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 2

Here’s the schematic of the TSD-10 Turbidity Sensor’s (GE) inside electronics. TSD-10has a three-wire interface: VCC (+5 V) — GND (0 V) — OUT/SIGNAL (Analog V).

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 3

The Sensor Driver

The internal electronics of the turbidity sensor head ensures that it gives an analog output voltage in proportion to the level of turbidity. If the primary objective is to just detect when the water is turbid, then we can use a standard comparator circuit to switch an output load (a piezo sounder, for instance) when the turbidity reaches a pre-defined threshold value. Here’s a turbidity sensor driver circuit based on LM393 for the intended task. Tweak it!

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 4

On the Other Side

While I was experimenting with my old TSD-10 turbidity sensor lifted from a defunct washing machine, the ordered Gravity Arduino Turbidity Sensor from DFRobot arrived at my doorstep from an Indian online store a little sooner. Surprisingly enough, the sensor package consists of a sensor driver module in addition to the original sensor. The LMV358 IC-based module provides a three-pin interface to connect with Arduino (or any other microcontroller), and there is also an “analog/digital” selector switch on the module to flip between analog and digital output mode. The official DFR documentation (https://www.dfrobot.com/wiki/index.php/Turbidity_sensor_SKU:_SEN0189) states that in analog mode, the output value decreases at high turbidity, while in digital mode, the output pin goes high if the turbidity reaches the threshold value set by its on-board trimpot.

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 5

From the same source, I got this graph with an equation that relates the voltage from the sensor to turbidity!

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 6

I made a few initial tests the other day, trying to have the DFR sensor module communicating with my Arduino Uno first using the serial monitor. It works! Note that with the “DFR” sensor, we get a signal output very close to VCC (in digital mode) when the sensor is not submerged or is in clear water (and a bit lower than that in analog mode). Here’s one simple Arduino code for a quick test (connect sensor module’s output to analog pin 1 and move its mode selector switch to analog position).

void setup() {
  Serial.begin(9600);

}
void loop() {
  int sensorValue = analogRead(A1);
  float voltage = sensorValue * (5.0 / 1024.0);

  Serial.println ("Sensor Output (V):");
  Serial.println (voltage);
  Serial.println();
  delay(1000);
}

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 7

Later, I decided to go directly into the design of a luxurious Digital Turbidity Meter — more soon.

DIY Water Quality Meter Using a Turbidity Sensor Schematic Circuit Diagram 8

And Finally…

Keep note while coding for your microcontroller-based project that the equation included in the relationship graph is only applicable if the sensor gives out 4.2 V roughly at zero turbidity (clear water), and it’s only true within the range of 2.5 V to 4.2 V (3,000 to 0 turbidity)!

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