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_spectru
m.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)
• Phototransistor
http://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.p
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
The one equation that will always be used
with BJTs
I E  I B  IC
Circuit Configurations
I-V Characteristic: npn Transistor
IC = b IB when VCE > VCEsat
Measured in a Common Emitter Configuration
Modified from
https://awrcorp.com/download/faq/english/examples/images%5Cbjt_amp_oppnt_bjt_iv_curves_graph.gif
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 collector
VA – Early Voltage
b is not a constant
BVCEO – 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 biased
BC 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
iE  iC  iB
iC  b F iB
iE  (1  b F )iB
iC  a F iE
aF
bF 
1  aF
DC Equivalent Circuit for
npn in forward active
npn
BE
 qV

I E  I S  e nkT  1


pnp
EB
 qV

I E  I S  e nkT  1


Simplified DC Equivalent Circuit
IC = bF IB AND IE = (bF +1) IB
npn
VBE = 0.7V
VCE > 50mV
IB > 0mA
pnp
VEB = 0.7V
VEC > 50mV
IB > 0mA
Saturation
npn
IC ~ ISC
VBE = 0.75V VCE = 50mV
IC < bF IB
pnp
VEB = 0.75V VEC = 50mV
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.