Elenco Electronics 202 - Manual (Page 40)

-39-
Project #64
OBJECTIVE: To compare transistor circuits.
Place the fan on the motor (M1) and turn on the slide switch (S1), then
compare this circuit to project #27. Press the press switch (S2), the
lamp (L2) doesn’t light now, but the motor still spins.
The lamp is dark because the 100 Ω resistor (R1) limits the current
through it. The NPN transistor (Q2) uses the small lamp current to
create a large current that spins the motor.
Now replace the 100 Ω resistor (R1) with the larger 1KΩ resistor (R2).
The motor spins more slowly now, because the transistor cannot
create as large of a motor current from such a small controlling
current.
The Transistor (V)
Project #65
The Transistor (VI)
Project #66
The Transistor (VII)
Compare this circuit to project #28. Press
the press switch (S2), the motor (M1) doesn’t
spin now, but the lamp (L2) still lights.
The motor doesn’t spin because the 100 Ω
resistor (R1) limits the current through it.
The NPN transistor (Q2) uses the small
motor current to create a large current that
lights the lamp.
Now replace the 100 Ω resistor (R1) with the
larger 1K Ω resistor (R2). The lamp is only
slightly less bright even though the motor
current is much lower.
Now place the 100 Ω resistor back in the
circuit and replace the press switch (S2) with
the photoresistor (RP). A bright light on the
photoresistor will turn the lamp on. But if the
light is dim then the photoresistor has high
resistance, so little current flows through the
transistor and the lamp is off.
OBJECTIVE: To compare transistor circuits. OBJECTIVE: To compare transistor circuits.
!
WARNING: Moving parts. Do not touch the fan or
motor during operation.
!
WARNING: Moving
parts. Do not touch
the fan or motor during
operation.
!
WARNING: Moving
parts. Do not touch
the fan or motor during
operation.
Compare this circuit to project #64. It uses
the photoresistor (RP) to control the current
to the NPN transistor (Q2), instead of the
press switch (S2). You can adjust the speed
of the motor (M1) by changing how much light
shines on the photoresistor.
The lamp (L2) is dark because the
photoresistor limits the current through it. The
NPN transistor uses the small lamp current to
create a large current that spins the motor.
If you tried to control the motor speed by
placing the photoresistor in series with the
motor, the motor would not spin because the
photoresistor would limit the current. But the
photoresistor can control the motor speed
with help from the transistor. You may need
to shine a light on the photoresistor if the
motor does not spin.