📊 INSTRUMENTATION & MEASUREMENT

Optical Signal
Conditioning System

Multi-stage analog signal processing system for optical communications. Utilizes dual-LDR differential sensing, instrumentation amplifier, cascaded bandpass filters, and output buffering to recover clean signals from noisy environments across 5Hz-200Hz bandwidth.

5-200Hz

Bandwidth

5 Stages

Signal Chain

Dual LDR

Noise Rejection

Project Type Academic Coursework

Unit ELEC7008

Institution Western Sydney University

Engineering Focus Instrumentation & Analog Signal Processing

Status ✓ Complete

01 Project Overview

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The Challenge

Optical communication systems face significant noise challenges from ambient light interference. This project demonstrates advanced signal conditioning techniques to extract a weak LED-modulated signal from a noisy environment using differential sensing and multi-stage filtering.

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The Solution

A dual-LDR configuration captures both signal+noise (A0) and noise-only (A1). A differential amplifier subtracts A1 from A0, eliminating common-mode interference. Cascaded active and passive bandpass filters then isolate the 5Hz-200Hz target frequency while rejecting out-of-band noise.

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Course Context

Part of ELEC7008 Instrumentation & Measurement at Western Sydney University. Applies fundamental concepts in sensor interfacing, noise rejection, op-amp circuits, active/passive filters, and signal chain design for real-world measurement systems.

02 Signal Processing Chain

01 - Signal Source

Labradog Signal Generator LED Modulation Optical Transmission

↓ Light Beam ↓

02 - Dual Sensor Array

LDR A0: Signal + Noise LDR A1: Noise Only Wheatstone Bridge

↓ Analog Sensing ↓

03 - Differential Amplifier

                        Common-Mode Rejection
                        Instrumentation Amp
                        LM2902N Op-Amp
                    

↓ Noise Cancelled ↓

04 - Active Bandpass Filter (A2)

5Hz - 200Hz Passband Non-Inverting Configuration Controlled Gain

↓ Filtered Signal ↓

05 - Passive BPF + Buffer (A3)

Secondary Filtering Output Buffer Clean Signal Recovery

↓ Output ↓

Arduino UNO Data Acquisition

A0: Raw Signal A2: After Active BPF A3: Final Output

03 Circuit Stages and Components

Stage 1

✓ Sensors

Wheatstone Bridge & Dual LDR Array

LDR A0 Signal + Noise

LDR A1 Noise Only (Reference)

Configuration Wheatstone Bridge

Resistors R1-R4 (10kΩ balancing)

Purpose Differential Sensing

✓ Balanced bridge configuration

✓ Common-mode noise capture

✓ Differential signal extraction

Stage 2

✓ Noise Rejection

AC Pass Instrumentation Amplifier

IC LM2902N Quad Op-Amp

Type Differential Amplifier

Function Common-Mode Rejection

Inputs PR1, PR2 from Bridge

Coupling AC Coupled (0.1µF)

→ Ambient light cancellation

→ High CMRR performance

→ Signal extraction from noise

Stage 3

✓ Active Filter

Active Non-Inverting Bandpass Filter (A2)

Topology Active BPF

Passband 5Hz - 200Hz

Gain Controlled Amplification

Components R8, R9, R10 + Caps

Op-Amp LM2902N (U1A)

→ Selective frequency passing

→ Out-of-band rejection

→ Signal amplification

Stage 4

✓ Output

Passive BPF + Output Buffer (A3)

Stage 1 Passive Bandpass Filter

Stage 2 Unity-Gain Buffer

Purpose Final Noise Removal

Buffer Type Voltage Follower

Output Clean Recovered Signal

→ Secondary filtering

→ Low output impedance

→ Load isolation

Support

✓ Biasing

Virtual Ground & Voltage Follower

Purpose Single-Supply Operation

Configuration R12/R13 Divider + Buffer

Components 1kΩ + 1µF Caps

Output Vcc/2 Reference

Function AC Signal Biasing

→ 9V single-supply design

→ AC coupling support

→ Stable bias point

05 Technical Achievements

Noise Rejection

Implemented a differential amplifier architecture to reduce common-mode ambient light interference. The dual-LDR configuration captures reference noise separately and subtracts it from the signal path before the filtering stages.

Multi-Stage Filtering

Designed and tuned cascaded active and passive bandpass filter stages to isolate the 5Hz-200Hz target frequency band. The active stage provides gain control and the passive stage cleans the buffered output.

Complete Signal Chain

Integrated all stages from sensor interfacing through signal conditioning to buffered output. Arduino UNO data acquisition at multiple test points (A0, A2, A3) shows how each stage changes the signal from raw optical sensing to filtered output.

06 System Specifications

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Signal Parameters

Frequency Range: 5Hz - 200Hz
Signal Source: LED (Labradog modulated)
Sensors: Dual LDR array
Coupling: AC coupled
Supply: ±9V (single supply with virtual ground)

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Key Components

Op-Amps: LM2902N (Quad)
Sensors: 2× LDR photoresistors
MCU: Arduino UNO
Power: 2× 9V batteries
Passive: Resistors, capacitors, LEDs

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Design Features

Topology: Multi-stage signal chain
Noise Rejection: Differential amplifier
Filtering: Active + Passive BPF
Output: Unity-gain buffer
Testing: Arduino multi-point acquisition

Optical Signal
Conditioning System