phototransistor based image scanner1

“ Phototransistor Based Image Scanner”Submitted by:Gaurav KumarMeenakshi Panwar

Project Objective

Our goal for this project is to explore the concept of using an array of phototransistors to replace the function of the photoactive silicon layer.

We want to build a camera that would be able to capture still-images using phototransistors or photodiodes.

Requirement of the project

Normally, image scan is done with a CCD(charge couple device) overlaid with a photoactive layer of silicon.

To overcome the disadvantages of CCD overlaid with a photoactive layer of silicon we will use photo transistor array.

Normally, image scan is done with a CCD(charge couple device) overlaid with a photoactive layer of silicon.

A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value.

This is achieved by "shifting" the signals between stages within the device one at a time.

 CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins.

Charge Couple device(CCD)

More complex control electronics are needed

Disadvantages of CCD

CCD is the higher cost

The cell area is basically doubled

A faster shifting requires a faster readout, and a faster readout can introduce errors in the cell charge measurement, leading to a higher noise level

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Phototransistor   A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction.

The electrons that are generated by photons in the base-collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfe).

Background maths and physics

 When light reaches a phototransistor, the electrons that are generated by photons in the base-collector junction are injected into the base.This current is amplified by the transistor's current gain.We used phototransistors as emitter followers, as shown in Figure.Vout is determined by the current flowing through resistor Re so we have Vout = I*Re.

Vout is also proportional to the intensity of the light that strikes on the phototransistor.For our scanner, the light reaches the phototransistor is reflected from the image it is scanning.

It introduce low noise.

Advantages of Phototransistor Array over CCD

More suited for applications where the light levelis relatively high

Low cost

High quantum efficiency, typically 80%

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Block diagram of project

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Image processing

Mechanical structure

Stepper motor

Photo transistor array

Wood base

Electronic Components used

Microcontroller Atmega16RS-232 (& MAX 232)Phototransistors (QRB114)Stepper MotorStepper Motor driver(ULN2003)MATLAB ( for image processing)

ATmega16High-performance, Low-power AVR® 8-bit Microcontroller

Peripheral Features– Two 8-bit Timer/Counters – One 16-bit Timer/Counter e– Real Time Counter with Separate Oscillator– Four PWM Channels– 8-channel, 10-bit ADC8 Single-ended Channels7 Differential Channels in TQFP Package Only2 Differential Channels with Programmable Gain at 1x, 10x, or 200x– Byte-oriented Two-wire Serial Interface– Programmable Serial USART– Master/Slave SPI Serial Interface– Programmable Watchdog Timer with Separate On-chip Oscillator– On-chip Analog Comparator

RS-232Communication as defined in the RS232 standard is an asynchronous serial communication method.Asynchronous tells us that the information is not sent in predefined time slots. Data transfer can start at any given time and it is the task of the receiver to detect when a message starts and ends.The RS232 standard describes a communication method where information is sent bit by bit on a physical channel.The information must be broken up in data words.The length of a data word is variable. On PC's a length between 5 and 8bits can be selected. 

Stepper Motor

A stepper motor is an electromechanicaldevice which converts electrical pulses intodiscrete mechanical movements.

The rotation angle of the motor isproportional to the input pulse.

The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.

MATLAB

MATLAB, which stands for MATrix LABoratory, is a state-of-the-art mathematical software package, which is used extensively in both academia and industry.

It is an interactive programfor numerical computation and data visualization,

MATLAB deals mainly with matrices.

Applications of MATLAB

Technical ComputingControl SystemsDigital Signal ProcessingCommunications SystemsImage and Video ProcessingTest and MeasurementComputational BiologyComputational Finance

THANK YOU