Optical Sensor for Noise Flickering

Group Members

Eric Cheung Jun Hao; Lee An Min, Amanda; Loh Jia Ying Whitney

Background and Introduction

The objective of this project is to characterise a given photodiode and to successfully determine the frequency of an unknown flickering light source.

With a simple potential divider, consisting a resistor and a photodiode in series, the resistance and hence sensitivity of the photodiode can be determined. By exposing the photodiode to visual noise tags, such as the flickering of LEDs, the resistance across the photodiode is expected to decrease and hence, a larger potential can be measured across the resistor. The photodiode resistance is a function of light exposure. The resistance is inversely proportional to light intensity. The output can be observed on an oscilloscope to determine the performance of the photodiode.

Set-Up

Equipment Needed

S/N	Item	Description/Model No.
1	Soldering Iron	-
2	Soldering Tin	-
3	Wires	-
4	Resistors	10k, 100k, 500k, 1M, 10M
5	LDR Photoresistor	GL5528 / GL5537
6	DC Power Supply	Matrix MPS-3005L-3
7	Signal Generator	Agilent 33220A
8	40 MHz Analog Oscilloscope	Hameg HM400

LED S/N	Colour	Wavelength (nm)	Size (mm)
1	Red	643	5
2	Orange	625	5
3	Amber	605	5
4	Green	570	5
5	Blue	-	5
6	Red	650	3
7	Orange	625	3
8	Green	570	3
9	Blue	-	3

Experimental Set-up

Circuit Design

A signal generator is used to pulse the LEDs to create a flickering light source. A square wave with a peak-to-peak voltage of 2V is used across all the measurements taken. A DC power supply is used to supply 3V across the photodiode.

Variables

The performance of the photodiode will be determined by varying the following parameters: 1. Wavelength of LED 2. Resistor in circuit 3. Frequency of blinking (1Hz - 100kHz)

The dependent variables that will be measured are: 1.

This will allow us to determine the following parameters: 1. Efficiency (Time decay) of photodiode at different wavelengths

Timeline and Milestone

Week	Dates	Tasks
Week 7	4 - 8 Mar	Collecting equipment and prelimary set-up
Week 8	11 - 15 Mar	Complete circuit set-up and prelimary measurements
Week 9	18 - 22 Mar	Data collection (vary LEDs)
Week 10	25 - 29 Mar [Well-being day (28 Mar), Good Friday (29 Mar)]	Data collection (vary resistance)
Week 11	1 - 5 Apr	Data Analysis and further measurements (if needed)
Week 12	8 - 12 Apr [Hari Raya (10 Apr)]	Data Analysis and explore other light sources (phone/computer screens)
Week 13	15 - 19 Apr	Analyse experimental results and complete documentation

Results

Characterisation of the photodiode is done with three 5 mm LEDs: (i) red, 643 nm; (ii) green, 570 nm; and (iii) blue, 450 nm to 495 nm. It can be noted that the photodiode sensitivity increases with increases wavelength of LED used. The photodiode used in this experiment is Hamamatsu S5972 Si PIN photodiode, with a peak sensitivity of 0.57 at wavelength of 800 nm. According to the datasheet, sensitivity decreases as wavelength decreases. This matches our experimental results.