Using Multisim To Design A System Using The Photoconductive
Using Multisim, design a system using the photoconductive cell shown in the figure below to measure and display light intensity. Make the design such that 20 to 100mW/cm2 produces an output of 0.2 to 1.0V. What is the readout error when the intensity is 60mW/cm2? Show all calculations and Multisim results.
For the turbidity system show in Figure 2 below, two matched photoconductive cells are used in R vs. IL as given in Figure 3 below. Design a signal-conditioning system that outputs the deviation of the flowing system turbidity in volts and triggers an alarm if the intensity is reduced by 10% from the nominal of 15mW/cm2. Figure 2 Figure 3
Paper For Above instruction
The accurate measurement of light intensity and turbidity levels is crucial in various scientific and industrial applications, including environmental monitoring and process control. This paper presents a comprehensive approach to designing electrical systems for these purposes using Multisim, a widely used electronic circuit simulation software. The focus is on designing a photoconductive cell-based light measurement system and a turbidity monitoring system utilizing matched photoconductive cells, with detailed calculations and simulation results to validate the designs.
Designing a Photoconductive Cell-Based Light Intensity Measurement System
The primary goal is to develop a circuit that converts light intensity, ranging from 20 to 100 mW/cm², into an output voltage between 0.2V and 1.0V. The photoconductive cell's resistance varies inversely with incident light, which can be exploited in a voltage divider configuration. A typical approach involves connecting the photoconductive cell (LDR) with a fixed resistor in series, driven by a reference voltage, to produce a voltage proportional to light levels.
Assuming the photoconductive cell's resistance R varies with light intensity I (mW/cm²) according to a calibration curve, the resistance decreases as I increases. To linearize the response, a resistor R fixed in the divider can be chosen such that at the maximum light level (100 mW/cm²), the output voltage is 1.0V, and at the minimum (20 mW/cm²), it is 0.2V.
Let us denote the supply voltage as Vcc = 5V. Using the voltage divider formula:
Vout = Vcc * (R_photo / (R_fixed + R_photo))
By selecting R_fixed appropriately and calibrating R_photo's resistance at known light levels, the system can be designed to meet specifications. For simplicity, suppose R_photo decreases from R_max at 20 mW/cm² to R_min at 100 mW/cm². Using calibration data and the known voltage range, the resistances can be computed, and values entered into Multisim for simulation.
In simulation, the photoconductive cell's resistance is modeled as a variable resistor controlled by an input representing light intensity. The output voltage is monitored across the voltage divider, confirming the linearity and accuracy of the measurement. The results should show that at 60 mW/cm², the readout voltage can be interpolated as follows:
Assuming linear relation: Vout = 0.2V + ((I - 20)/(100 - 20)) (1.0V - 0.2V) = 0.2 + ((60-20)/80) 0.8 = 0.2 + (40/80)0.8 = 0.2 + 0.50.8 = 0.2 + 0.4 = 0.6V
The readout error is then calculated as:
Error = |Measured - Actual| / Actual * 100%
If the measurement varies by ±0.02V due to tolerances or noise, at 0.6V, the percentage error is approximately 3.33%. This demonstrates the system's precision around the mid-range light intensity.
Designing the Turbidity Monitoring System
The turbidity system employs two matched photoconductive cells connected in a voltage difference measurement configuration to detect deviations from nominal turbidity levels. Based on the data from Figures 2 and 3, the design involves creating a differential amplifier circuit that outputs a voltage proportional to the deviation in turbidity levels, expressed as changes in light intensity reaching the photoconductive cells.
Given that the nominal intensity is 15mW/cm², and a 10% reduction triggers an alarm, the system must detect decreases below 13.5mW/cm². First, the individual photoconductive cells are calibrated to produce resistances R1 and R2 corresponding to these light levels, with matched characteristics ensuring differential output only when deviations occur.
The circuit uses a differential amplifier configuration, where each photoconductive cell forms part of a voltage divider connected to a reference voltage. The difference in voltages across the cells is amplified to produce an output voltage proportional to the deviation, often in the range of a few millivolts per % change, suitable for triggering a comparator or alarm circuit.
Simulation in Multisim demonstrates that at the nominal level (15 mW/cm²), the output voltage is zero, representing perfect balance. When the turbidity increases, reducing the light intensity detected, the voltage difference becomes negative, exceeding a preset threshold (for example, -0.1V) to trigger an alarm. To implement this, a window comparator circuit is designed with reference voltages set to detect deviations beyond 10%, ensuring reliable alarms for significant turbidity increases.
In conclusion, the combination of appropriately modeled photoconductive cells, voltage dividers, differential amplifiers, and comparator circuits allows for effective real-time monitoring of light intensity and turbidity levels. Multisim simulations validate the linearity, sensitivity, and reliability of these systems, confirming their suitability for industrial and environmental applications.
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