Phototransistor
Photoresistor
ECE 456 - Mechatronics
Photodiode vs Phototransistor
•
Photodiode
–
Ideal for very fast signals but gives a much
lower output signal
•
Phototransistor
–
Provided amplified signal but with relatively
slow conversion
T02 Sensors
59
© Bharat Balagopal, North Carolina State University, All Rights Reserved
ECE 456 - Mechatronics
Subsection: Photodiode
T02 Sensors
60
© Bharat Balagopal, North Carolina State University, All Rights Reserved
ECE 456 - Mechatronics
Mechanism of Photodiode
•
Mechanism
–
A PN-junction diode that consumes light energy to produce electric current
–
Works in reverse bias mode of PN junction diode
T02 Sensors
61
© Bharat Balagopal, North Carolina State University, All Rights Reserved
V-I Characteristics of Diode
pd
I
Op
amp
-
+
-
f
R
E
+
-
+
-
out
f
pd
V
R
I
=
-
×
ECE 456 - Mechatronics
Relation between current
I
and voltage
V
of photodiode
•
V-I Characteristics equation for photodiode
T02 Sensors
62
© Bharat Balagopal, North Carolina State University, All Rights Reserved
[
]
9
:
5
10
,
;
0.026,
;
3,
;
log(
1)
1.3 ,
;
0.1 ,
;
:
0, 500
.
d
t
F
t
F
S
F
F
thermal voltage
emissi
where
IS
I
A saturation current
V
V
N
V
I
I
V
V is the forward voltage
I
A is the forward current
As
on coefficient
sume I
A
μ
-
=
=
´
=
=
»
+
=
=
Î
1
t
V
NV
I
IS
e
æ
ö
=
×
-
ç
÷
ç
÷
è
ø
ECE 456 - Mechatronics
Photodiode characteristics – Linear approximation
•
Results
T02 Sensors
63
© Bharat Balagopal, North Carolina State University, All Rights Reserved
T02_VI_curve_photodiode.m
Comments:
1.
Once the current of photodiode reaches a certain amount, for example 100
μ
A, we can assume the voltage drop on
photodiode remains the same, or changes very little (less than 0.2V).
2.
We can assume the linearity between incident light and current through photodiode when the terminal voltage of
photodiode is constant or varies in small range.
ECE 456 - Mechatronics
Example – Linear approximation
•
Based on the V-I Characteristics equation
and the related parameters given in previous slides, derive the equation for V as a function of
I and other
given parameters.
•
Plot the I-V characteristics with current ranging from 0 to 600 μA.
•
Suppose we are going to use the following piecewise linear function to represent the I-V characteristic
of photodiode. Show the non-linearity error based on the given information.
T02 Sensors
64
© Bharat Balagopal, North Carolina State University, All Rights Reserved
1
t
V
NV
I
IS
e
æ
ö
=
×
-
ç
÷
ç
÷
è
ø
3
0.7802,
1
0.2
10
0.78,
1
600
I
V
I
I
-
=
ì
=
í
´
×
+
<
£
î
ECE 456 - Mechatronics
Example – Solution
•
Equation for I-V characteristics
•
I-V characteristics of photodiode (actual curve and linear curve)
•
Non-linearity error
T02 Sensors
65
© Bharat Balagopal, North Carolina State University, All Rights Reserved
[
]
ln
1 ,
0, 600 .
t
I
V
NV
with I
IS
æ
ö
=
×
+
Î
ç
÷
è
ø
3
0.7802,
1
_
0.2
10
0.78,
1
600
_
I
V
linear
I
I
error
V
V
linear
-
=
ì
=
í
´
×
+
<
£
î
=
-
T02_VI_curve_photodiode.m
ECE 456 - Mechatronics
Ultrasonic Sensor
T02 Sensors
66
© Bharat Balagopal, North Carolina State University, All Rights Reserved
ECE 456 - Mechatronics
EV3 Ultrasonic sensor
•
Features
–
distance measure up to 255cm (~100-inch)
–
+/- 1cm (+/- 0.394-inch) accuracy
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