Chapter 5
Phototransistors
and
Optointerrupters
1
Phototransistor
• Phototransistor is photodiode with base
– Greater gain
– Greater sensitivity
• Can use as regular transistor
– Apply current to base
• Phototransistor is slower than photodiode
– Ways to increase switching speed
2
• Add resistor to base to reduce turn-off time.
3
4
5
6
7
Phototransistor
• Phototransistor can be used as
– Phototransistor
– Photodiode
– Avalanche photodiode
– Photodiode with external amplifier
• Photocurrent, speed of response, and
available light determines which type is
used.
8
Photon Effect
• E=hf
h = Planck’s constant
f = frequency
E = energy
• Frequency, not intensity determines if
electrons are energetic enough to pass the
band gap.
9
10
11
12
13
14
15
16
Optointerrupter detects
presence or absence of
light beam. Has space
between emitter and
detector for disk to pass
through. LED is always
on.
Optocoupler is used to
turn on/off a device that
may have much higher
voltage, so need
electrical isolation only.
No space between
emitter and detector for
disk to pass through.
17
18
19
20
Unijunction transistor
21