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

Characterization of the photodiode is done with three 5 mm LEDs: (i) red, 643nm; (ii) green, 570nm; and (iii) blue, 450nm to 495nm. It can be noted that the photodiode sensitivity increases with increasing wavelength of LED used.

The photodiode used in this experiment is Hamamatsu S5972 Si PIN photodiode. According to the datasheet, it has a spectral response range between 320nm to 1000nm, with a peak sensitivity at 800nm, near infrared radiation. The cutoff frequency is 0.5GHz at reverse bias of 10V. The terminal capacitance at reverse bias of 2V is close to 3pF.

From our experimental results, the photodiode has a faster response at longer wavelength.

LED	Wavelength (nm)	Rise Time (ms)	Remarks
Red	643	0.03
Green	570	N.A.	Low signal-to-noise ratio
Blue	450-495	0.15

Out of the three LEDs used, the signal-to-noise ratio of the green LED is the lowest. Powering the three LEDs reveals that with the same voltage supplied, the green LED is the dimmest. Thus, the intensity of incident green light on the photodiode is significantly lower. The signal captured by the photodiode is lower and hence, the decrease in resistance of the photodiode is smaller. As a result, the voltage across the external resistor in the circuit of a fixed resistance is lower. Because of this, the change in voltage reading (in the switching of the LED) on the oscilloscope is closer to background noise. Thus, signal-to-noise ratio is lower.

At fixed frequency and wavelength, reducing external resistance reduces time constant which replicates the signal sent into the LED (square wave). However, a lower resistance will entail lower signal-to-noise ratio.

[include pic to compare]

The time constant of the photodiode with a series resistance of 100kΩ is ___. Thus, the internal capacitance of the photodiode is calculated to be ___.