# A Portable Optical Sensor for Microplastic Detection: Development and Calibration
## 2 Materials and Methods
### Detector BP34W  
In the selection of the photodiode, the primary consideration was the detection range, given the implementation of various LEDs ranging from ultraviolet (UV) to near infrared. We decided to use the BP34W, which has a detection range spanning from 300 nm to 1100 nm. When the sample is exposed to the light source, the BP34W detects the transmitted light from the microplastic. Consequently, the photodiode generates an electrical signal proportional to the intensity of the detected light, thereby providing the attenuation value [37]. Figure 6 shows the relative spectral sensitivity of the BP34W, as provided in the manufacturer's datasheet by [PERSON] [38]. Subsequently, an operational amplifier converts the photodiode current into a voltage signal that can be readily interpreted. Finally, the system maps this amplified voltage to the attenuation scale, allowing microplastics to be differentiated based on their optical properties. The relationship between the optical power (\(P_{opt}\)) of the detected light and the output current (\(I_{photo}\)) of the photodiode is expressed as:  
\[I_{photo}=R_{\lambda}\cdot P_{opt} \tag{3}\]  
Equation (3) is the photodetector output equation. where:  
* \(I_{photo}\) is the output current generated by the photodiode;
* \(R_{\lambda}\) is the responsivity of the photodiode at the wavelength of the incident light (units: amperes per watt);
* \(P_{opt}\) is the optical power of the transmitted light detected by the photodiode (units: watts). To enhance the performance of the BPW34 photodiode mentioned, the TLC27L2 BCD operational amplifier was utilized. The following section will elucidate the alterations made to the operational amplifier. The modification entailed the implementation of a positive gain configuration. Specifically, the configuration of a non-inverting amplifier was  
Figure 5: The figure shows the double-sided PCB design. Panel (**A**) illustrates the physical connections between components, such as LEDs and resistors, while panel (**B**) presents a detailed electrical schematic. implemented, resulting in a gain of 1,000,000. This methodology aligns with the approach outlined by authors [PERSON] and [PERSON] in their article [39]. We used the circuit illustrated in Figure 7. In the article, they used the TLC271. One reason for this is the supply voltage range, as it supports a supply voltage range of 3 V to 16 V, providing flexibility for various applications. The gain of the non-inverting amplifier is calculated using the following formula:  
\[A_{v}1+\frac{R_{f}}{R_{1}} \tag{4}\]  
Equation (4) is the low-pass filter equation. * \(A_{v}\): amplifier gain, representing the ratio between the output signal and the input signal;  
Figure 6: The figure shows the spectral sensitivity of the BP34W photodiode, highlighting its detection range from 430 to 1100 nm. The photodiode’s peak sensitivity is at 900 nm, ensuring effective performance across the spectrum [38]. Figure 7: The figure shows the schematic of the amplifier circuit designed to boost the BPW34 photodiode’s signal. The circuit uses a TLC27L2 BCD operational amplifier to achieve high gain and maintain signal integrity [39]. * \(R_{f}\): feedback resistance connected between the output and the negative input terminal of the operational amplifier;
* \(R_{1}\): resistance connected between the negative input terminal and the ground. TLC271 is characterized by its high input impedance, which mitigates the loading effect on the preceding stage and ensures precise signal amplification. This characteristic allowed us to maintain signal integrity while enhancing the performance of our prototype. For our application, we used the TLC27L2 BCD operational amplifier (TME Group, Lodz, Poland) instead of the TCL271, which is ideal for applications requiring multiple Op-Amps (operational amplifiers) in a single package, and due to its superior slew rate. The higher slew rate facilitates expeditious signal processing, a prerequisite for attaining the precision and expediency demanded in our work.