Manual

High Sensitivity
Photo Control
Project 103 Photo Control
Build the circuit and turn on the switch (S1). The
white LED (D6) and horn (W1) will be on if there
is light on the phototransistor (Q4); cover the
phototransistor to turn them off. If the LED and
horn turn on too easily then place the Q4
attachment on Q4 to restrict the light to it.
IFNEEDED
The phototransistor is able to
control other devices (such as
the white LED and horn) much
more easily than in projects 54-
55, because the NPN transistor
(Q2) is used as an amplifier.
The NPN transistor helps a
small electric current through
the phototransistor control a
larger electric current through
the LED and horn.
Use the preceding circuit, but
replace the white LED (D6)
with the color LED (D8).
Project 104
Photo
Control (II)
Use the preceding circuit, but
replace the horn (W1) with the
white LED (D6).
Project 105
Photo
Control (III)
Use the project 103-105
circuits but remove the 5.1kW
resistor, and see how its
sensitivity to light changes.
Project 106
Photo
Control (IV)
The 5.1kW resistor diverts some
current from the phototransistor
to keep the circuit from being
too sensitive to light.
Build the circuit and turn on the switch (S1). Vary
the amount of light shining on the phototransistor
(Q4) to see how easily the brightness of the color
LED (D8) and white LED (D6) can be adjusted.
Notice that D6 may remain bright even when D8 is
off. You may have to completely cover Q4 with your
hand and/or take the circuit into a really dark room
to get D6 off.
If desired, place the Q4 attachment on Q4 to restrict
the light to it. You can also swap the locations of the
LEDs (D6 & D8).
-62-
Project 107
A small electric current may be flowing through the color LED even though it appears to be off.
This small current, amplified by the NPN transistor (Q2) can be enough to keep the white LED on.