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31
Sucking Fan
OBJECTIVE: To adjust the speed of a fan.
Build the circuit, and be sure to orient the motor (M1) with the positive (+)
side down as shown. Turn it on, and set the adjustable resistor (RV) for
the fan speed you like best. If you set the speed too fast then the fan may
y off the motor. Due to the shape of the fan blades and the direction the
motor spins, air is sucked into the fan and towards the motor. Try holding
a piece of paper just above the fan to prove this. If this suction is strong
enough then it can lift the fan blades, just like in a helicopter.
The fan will not move on most settings of the resistor, because the
resistance is too high to overcome friction in the motor. If the fan does not
move at any resistor setting, then replace your batteries.
Modify the circuit by reversing the position of the motor (M1), so the
positive (+) side is towards the PNP (Q1). Turn it on, and set the
adjustable resistor (RV) for the fan speed you like best. Set it for full
speed and see if the fan ies off - it won’t! The fan is blowing air upward
now! Try holding a piece of paper just above the fan to prove this.
Project #68
Project #67
Turn on the slide switch (S1) and place your nger across points A & B.
The speaker (SP) will output a tone and the LED (D2) will ash at the
same frequency. Your nger acts as a conductor connecting points A &
B. When a person is lying, one thing the body starts to do is sweat. The
sweat makes the nger a better conductor by reducing its resistance.
As the resistance drops, the frequency of the tone increases. Lightly
wet your nger and place it across the two points again. Both the output
tone and LED ashing frequency increase, and the lamp (L2) may begin
to light. If your nger is wet enough, then the lamp will be bright and
the sound stops - indicating you are a big liar! Now change the wetness
of your nger by drying it and see how it affects the circuit. This is the
same principle used in lie detectors that are sold commercially.
The Lie Detector
OBJECTIVE: To show how sweat makes a better conductor.
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