Elenco LIGHT - Manual (Page 72)

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Photo Current Amplifier Project 157
LEDs &
Transistors
Use the preceding circuit but
replace either LED (D6 or D8)
with the red LED (D1). Compare
all three LEDs, in both locations.
Project 156
Build the circuit, turn on the switch (S1), and vary the
amount of light on the phototransistor (Q4) using your
hand. Compare the brightness of the white LED (D6)
and color LED (D8).
Swap the locations of the white and color LEDs, and
compare the brightness now.
Project 158 PNP Amplifier
This circuit is just like the preceding one
except it uses a different type of transistor.
Build the circuit, turn on the switch (S1), and
vary the amount of light on the phototransistor
(Q4) using your hand. Compare the brightness
of the white LED (D6) and color LED (D8).
Replace either LED (D6 or D8) with the red
LED (D1). Compare all three LEDs, in both
locations.
The NPN transistor (Q2) is a current
amplifier. When a small current flows
into Q2 through the left branch
(through Q4), a larger current will
flow into Q2 through the right branch
(with R1).
Green arrows shown the
current flow. So the LED on the
right side will be brighter than the
LED on the left side. The current in
the right branch might be 100 times
larger than in the left branch.
LED brightness depends on the
materials used, construction
quality, and the current through it.
The white LED is super-bright,
the red LED is low-brightness,
and the color LED is between the
others.
The PNP transistor (Q1) is just like the
NPN transistor (Q2), except that the
currents flow in opposite directions.
Green arrows shown the current flow.