OPTICAL SENSOR

Project Objective• Minimum objective: Use the optical sensor to determine

the pulse rate in a finger.

http://www.homecaremag.com/sites/default/files/u4/Finger%20Pulse%20Oximetry%20Illustration.jpg

Reflective Optical Sensor

Contains: • an infrared light-emitting

diode (LED)• a bipolar junction transistor

(BJT) that is missing the base connection (a phototransistor)

http://www.vishay.com/docs/83752/tcrt1000.pdf

Visible Electromagnetic Spectrum

http://scienceblogs.com/startswithabang/files/2011/09/Visible-spectrum.jpeg

Infrared Electromagnetic Spectrum

http://www1.infraredtraining.com/uploadedImages/InfraredTrainingcom/About/What_is_IR/EM_spectrum.jpg

Absorption Spectrum of Silicon

• This is a plot of how well silicon (Si) absorbs light at different wavelengths.• It begins to absorb light at

~ 1.1 mm.• It strongly absorbs light

through the rest of the IR region into the visible spectrum and into the ultraviolet (UV) region.

Light and Electron-Hole Pairs

• An electron and a hole are the two particles that move in a semiconductor. • Energy can be released

when an electron and hole destroy each other (recombination).• Light Emitting Diode

• Energy can be transformed into an electron and a hole (generation)• Phototransistorhttp://www.iue.tuwien.ac.at/phd/entner/img158.png

Optical filter• There is an optical filter integrated into the optical sensor

package to prevent light at wavelengths other than the ones emitted by the LED from reaching the silicon transistor.

Biasing an LED

• You should look at your notes from Microelectronic Systems as well as your notes from Fundamentals of Analog Circuits to determine how to limit the current and voltage applied to the IR LED.

http://www.vishay.com/docs/83752/tcrt1000.pdf

BIPOLAR JUNCTION TRANSISTORS

Three Terminal Device

• Terminals• Emitter

• The dominant carriers are emitted from the region (equivalent to the Source in a MOSFET)

• Base • These now minority carriers travel through the base region

• Some recombine in the base, forcing a base current to flow

• Collector• The remaining carriers from the emitter are collected from this

region (equivalent to the Drain)

Types of BJTs• n-p-n

• Emitter is n+ type• Electrons flow from the emitter towards the collector

• Base is p type• Some of the electrons from the emitter recombine with the holes in the

base• Collector is n- type

• p-n-p• Emitter is p+ type

• Holes flow from the emitter towards the collector• Base is n type

• Some of the holes from the emitter recombine with the electrons in the base

• Collector is p- type

Cross Section of npn Transistor

Cross-Section of pnp BJT

Circuit Symbols and Current Conventions

npn pnp

CBE III

The one equation that will always be used with BJTs

Circuit Configurations

I-V Characteristic: npn Transistor

Measured in a Common Emitter ConfigurationModified from https://awrcorp.com/download/faq/english/examples/images%5Cbjt_amp_oppnt_bjt_iv_curves_graph.gif

IC = b IB when VCE > VCEsat

Nonideal I-V Characteristic

ICEO – leakage current between the collector and emitter when IB = 0, usually equal to the reverse saturation of the base-collection diode

Effects from a change in the effective distance between emitter and collectorVA – Early Voltageb is not a constantBVCEO – breakdown voltage of the transistor

Modified from: http://cnx.org/content/m29636/latest/

Current-Voltage Characteristics of a Common-Base Circuit

In Forward Active Region: IC = aF IE, where aF < 1

Modified from Microelectronic Circuit Analysis and Design by D. Neamen

Simplified I-V Characteristics

Modes of Operation• Forward-Active

• B-E junction is forward biased• B-C junction is reverse biased

• Saturation• B-E and B-C junctions are forward biased

• Cut-Off• B-E and B-C junctions are reverse biased

• Inverse-Active (or Reverse-Active)• B-E junction is reverse biased• B-C junction is forward biased

npn BJT in Forward-Active

BE junction is forward biasedBC junction is reverse biased

Currents and Carriers in npn BJT

iEn = iE – iEp

iCn = iC – iCp where iCp ~ Is of the base-collector junction

iEn > iCn because some electrons recombine with holes in the base

iB replenishes the holes in the base

Current Relationships in Forward Active Region

F

FF

EFC

BFE

BFC

BCE

a

a

iai

ii

ii

iii

1

)1(

DC Equivalent Circuit for npn in forward active

npn pnp

1nkT

qV

SE

BE

eII

1nkT

qV

SE

EB

eII

Simplified DC Equivalent Circuit

npn pnp

VBE = 0.7V VEB = 0.7V

VCE > 50mV VEC > 50mV

IB > 0mA IB > 0mA

IC = bF IB AND IE = (bF +1) IB

IC ~ ISC IC < bF IB

VBE = 0.75V VCE = 50mV VEB = 0.75V VEC = 50mV

Saturation

npn pnp

Cut-Off

IC = IB = IE = 0

VBE < 0.6V VEB < 0.6V

Phototransistor Characteristics

http://www.vishay.com/docs/83752/tcrt1000.pdf

To detect and count the pulses

• Saturation• The transistor turns on

when light is reflected out of a finger back into the sensor.• The collector current is

limited only by the external resistors in the circuit when the base current is created by the

To measure the amplitude and shape of the pulse

• Forward Active• The transistor turns on

when light is reflected out of a finger back into the sensor.• The collector current is a

function of the base current, which is determined by the amount of light that is reflected onto the sensor.

Electronic Design Project• Design a circuit using the TCRT Optical Sensor:

• To bias the LED so that it emits light.• To bias the phototransistor in forward active where the maximum

light from reflected from a finger places the phototransistor close to or in the saturation region.