Water Turbidity Indicator Design Principles

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Water Turbidity Indicator Design Principles

We usually use turbidity to measure the quality of water, which is considered accurate and it can clearly reflect the quality of water. In fact, we consider particles in water larger than 2 microns to be turbid.

Next we will make a simple electronic project of water turbidity indicator. Before starting this project we need to prepare some cheap PCB components and understand the general concept of water turbidity indicator.

Description of Easy Water Turbidity Indicator

The above picture is the schematic diagram of this simple circuit, through which we can design water quality inspection equipment for low viscosity, low corrosive liquids, such as streams, lakes, etc. As shown in the picture above, this project is based on ASAIR's AZDM01 photoelectric turbidity sensor.

AZDM01 is a photoelectric turbidity sensor, which can be operated by applying a voltage of 5V across it, and a larger voltage requires replacing the resistor. After being powered on, AZDM01 can emit an infrared beam with a wavelength of 940nm, which can easily pass through the liquid to be tested. When the sensor inside AZDM01 receives the light signal, it will convert the light intensity into an analog output signal to estimate the turbidity of the target liquid.

The figure below shows the data obtained from the official website of the AZDM01 turbidity sensor:

The rest of the PCBA is a single-supply dual operational amplifier based on the LM358 IC (IC1A). Here the op amp is configured as a coarse inverting voltage comparator. It is worth noting that a comparator is a device that can have two inputs. Usually we use normal op amps that have their outputs not close to the supply rails, but somewhere in between, but that's not a significant number in this simple application.

In this water turbidity indicator, when the supply voltage is less than the threshold voltage, the output of the operational amplifier moves to the positive rail. Similarly, when the supply voltage is greater than the threshold voltage, it moves toward the negative rail. Resistor divider R3-R4 and supply voltage set the threshold voltage.

From the schematic diagram of the PCB, when the power supply voltage of the turbidity sensor is applied to the 2.2KΩ load resistance, the current will be greater than the rated value, and the green indicator light is on at this time, which means that the tested liquid is clear. On the contrary, when the red indicator light is on, it means that the tested liquid is cloudy. To limit the current flow, we will add a 100Ω resistor to the board to ensure proper operation of the circuit. It is worth mentioning that we have installed another resistor with a resistance value of 510Ω inside the simple version of the turbidity sensor for the same purpose as above. As shown below

Assembly and functional testing

• First of all, we need to install the core circuit board components on the PCB, and conduct a simple test on it to ensure that the circuit can operate normally according to the design principle.
• Submerge the simple version of the turbidity sensor in clean water and observe whether the green LED lights up.
• Submerge the simple version of the turbidity sensor in turbid water and watch for the red LED to light up.
• PCB assembly needs to pay attention to whether the components correspond to the positions on the schematic diagram, otherwise failures may occur, and components need to be desoldered to rebuild.

Summarize

If the prototype works fine, the water turbidity indicator circuit can be moved to a perf board or custom printed circuit board. A suitable housing can also be used to protect the entire electronics.

As a side note, a static threshold is used here, but it can be changed to a dynamic threshold by replacing the voltage divider with a 100KΩ trimpot or potentiometer. Additionally, an unused operational amplifier (IC1B) can be connected as a unity-gain buffer/voltage follower, feeding the analog output of the turbidity sensor (AN_OP) directly to a microcontroller such as an Arduino Uno or Raspberry Pi pico.