Elenco Electronics 202 - Manual (Page 50)

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Project #91
OBJECTIVE: To make a circuit that detects if the fan is on the
motor.
Press the press switch (S2). If the fan is off the motor (M1) (or flies off)
then the LED (L1) will light.
It takes a lot of current to spin the motor when the fan is on it, and the
voltage drops because the batteries (B1) cannot supply enough.
When the fan flies off, the current drops and the voltage rises. The
NPN transistor (Q2, used here as a diode) and 470
μF capacitor (C5)
are a detector circuit, which measures the voltage at the motor.
Fan Detector
Project #92
OBJECTIVE: To change siren sounds with a delay.
Turn on the slide switch (S1) and you hear a siren sound.
Now hold down the press switch (S2) until the sound becomes a fire
engine sound. This delay is due to the 10
μF capacitor (C3) charging
up and is controlled by the photoresistor (RP). If there is bright light
on the photoresistor, then the delay will be only a few seconds.
Release the press switch and after a while the sound will be a siren
again. The capacitor slowly discharges through the NPN transistor
(Q2).
Slow Siren Changer
!
WARNING: Moving parts. Do not touch the fan or
motor during operation.
! WARNING: Do not lean over the motor.