Insight - Learn the Working of a Turbidity Sensor

Figure 1. A typical turbidity sensor.

Turbidity is a quantitative measure of suspended particles in a fluid. These particles can range from soil in water to chocolate flakes in a milkshake. While chocolate is a welcome addition to a drink, soil particles are undesirable.

Beyond potable water applications, numerous industrial and household systems rely on water in some capacity. For example, a car uses water to clean the windshield, a power plant requires it to cool reactors, and washing machines and dishwashers depend on water much like fish rely on it for survival.

This raises an important question: how do these machines detect turbidity? Humans can rely on their senses to identify soil in water, but what about a washing machine? It has no eyes to see, no tongue to taste, and no skin to feel — just a plastic body, buttons, and a motor. How does it function intelligently based on the level of suspended soil?

(If you’re wondering whether it even does this, the answer is a resounding yes!)

The key lies in a 4.7 cm device: the turbidity sensor (Figure 1). Working in conjunction with a microcontroller unit, this sensor is responsible for turbidity measurements. Made of plastic and metal-alloy traces, the turbidity sensor uses light to determine water clarity and detect suspended particles.

Image of Outer Structure of Turbidity Sensor

Figure 2. The outer structure of a turbidity sensor.

Figure 2 showcases the outer structure of a turbidity sensor. When viewed from the front, the sensor bears a resemblance to an Android bot, with two horn-like structures and a single-material body extending from top to bottom. A black cap is positioned at the bottom of the sensor, while thick alloyed contact legs allow for secure connections with various connectors. A white plastic slab protects these legs from damage and serves as a fixture to ensure a firm grip on the sensor.

The plastic used for the outer structure is designed to withstand significant temperature variations and resist mechanical abrasions, ensuring durability in various operating conditions.

The thermistor and the pinouts

Shape and parts of Sensor from Lateral View

Figure 3. The lateral view and parts of the sensor.

Figure 3 is the lateral view of the sensor. It features external scales designed for a secure grip. Between the “horns,” a noticeable bulge houses the thermistor, enabling the sensor to measure temperature with precision.

Layout of Connecting Pins at Bottom of Sensor

Figure 4. The connecting pins at the base of the sensor.

Connecting pins are at the base or bottom of the sensor (Figure 4). Extending from the base of the white plastic fixture, these pins are robust enough to withstand sudden jolts and unexpected mechanical forces.

Figure 5. The description of the pin functions.

The pinouts are arranged from right to left (Figure 5).

Here’s what each pin represents:

Pin 1: Connects to the sensor’s thermistor and transmits its signal to the microcontroller of the device it is integrated with.
Pin 2: Links to the photodiode, which serves as the output component for turbidity measurements.
Pin 3: Acts as an extension of the phototransistor, functioning as the sensor’s input unit.
Pin 4: Serves as the common voltage connection for the sensor.

Notably, there is no dedicated ground pin. Instead, Pin 2 and Pin 3 are externally connected to ground when the sensor is mounted on a device.

Placement of the PCB

Figure 6. The placement of the PCB within the sensor.

Image showing Tracks Aiding in Placement of PCB

Figure 7. The tracking aid in the PCB.

The cap is securely attached to the transparent casing. Removing it reveals the internal circuitry in a single step. Inside the plastic chamber, all components are mounted on a PCB (Figure 6).

The PCB is positioned on a dedicated track that ensures its stability within the sensor (Figure 7).

Figure 8. The components of the PCB.

The brown-colored PCB is largely covered by a black frame, with a thermistor positioned in a bent configuration (Figure 8). This bending allows the thermistor to align perfectly within the bulge of the transparent plastic casing, ensuring accurate temperature sensing.

Phototransistor and Photodiode Soldered on PCB

Figure 9. The phototransistor and a photodiode, which are soldered onto the PCB.

Upon close inspection, a phototransistor and a photodiode (Figure 9) can be seen soldered onto the PCB. These two components are positioned directly beneath the horn-like projections of the plastic casing, ensuring optimal light detection for turbidity measurement.

Arrangement of Components of Photodiode Region of PCB

Figure 10. The arrangement of the photodiode region’s component on the PCB.

The black plastic frame is lifted upward to provide a clear view of the phototransistor and photodiode regions of the sensor (Figure 10). The component soldered in red is the transistor, while the one in black is the diode.

The PCB’s components and connections

Figure 11. The phototransistor.

Figure 12. The photodiode.

Phototransistors and photodiodes have similar structures, requiring a reliable method for differentiation. One way to distinguish them is by measuring resistance in darkness — while a phototransistor (Figure 11) will produce an output, a photodiode (Figure 12) will not respond in terms of resistance due to the absence of light. This is why the sensor’s transparent casing is essential, as it facilitates light transmission between the phototransistor and photodiode.

Another distinguishing method is color coding, as implemented in this sensor.

Additionally, analyzing the PCB tracks on the reverse side can help identify which component is the photodiode and which is the phototransistor.

Figure 13. The layout of the PCB and its connections.

Since the sensor itself does not contain a microcontroller, understanding the function of its pinouts is essential. A closer look at the sensor’s PCB reveals the tracks connecting embedded components to their respective pins (Figure 13). Additionally, a faint sulfuric acid scent can be detected, indicating the presence of a conformal coating applied for protection.

The working principle of the turbidity sensor

Water flows between the two projections of the transparent plastic casing, where the phototransistor and photodiode are housed. The phototransistor emits light rays intended to reach the photodiode. However, when suspended particles are present in the water, they scatter the light, reducing the amount of light that reaches the photodiode.

This reduction in light intensity is then transmitted to an external microcontroller, which interprets the data and triggers appropriate actions. For example, in a dishwasher, if the water is highly turbid, the system may extend the washing cycle and reduce water recycling to ensure proper cleaning.