Elenco Electronics 202 - Manual (Page 105)

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The voltage produced by a motor when it is spinning is called its Back
Electro-Motive-Force (Back EMF); this may be thought of as the
motor’s electrical resistance. The motor’s
Front Electro-Motive-Force
is the force it exerts in trying to spin the shaft. This circuit
demonstrates how the Back EMF increases and the current decreases
as the motor speeds up.
Place the fan on the motor (M1) and turn on the slide switch (S1). The
6V lamp (L2) will be bright, indicating that the Back EMF is low and the
current is high.
Turn off the switch, remove the fan, and turn the switch back on. The
lamp is bright when the motor (M1) starts and the lamp dims as the
motor speeds up. Now the Back EMF is high and the current is low.
BE CAREFUL NOT TO TOUCH THE MOTOR WHILE IT SPINS.
Project #247
OBJECTIVE: To demonstrate how the motor works.
Back EMF
Project #248
OBJECTIVE: To build a laser sounding circuit.
Flashing Laser LED’s
with Sound
When you press the press switch (S2) the integrated circuit (U2)
should sound like a laser gun. The red (D1) and green (D2) LED’s will
flash simulating a burst of light. You can shoot long repeating laser
burst, or short zaps by tapping the switch.
!
WARNING: Moving parts.
Do not touch the fan or
motor during operation.