Elenco Green Projects - Manual (Page 77)

0.5mA
50mA
-76-
Project #128 Wind Direction
Current Meter
Set the slide switch (S5) to position C and
spin the fan clockwise. The rotating shaft on
the motor (M4) generates a current and the
horn (W1) sounds. Current flows from the
motor to the positive side of the horn, so the
red LED (D1) does not light. Rotate the horn
so the “+” is on top and spin the fan again. Did
the horn sound? Position the horn back so “+”
is on the bottom.
Now rotate the fan counter-clockwise,
generating a current in the opposite direction.
The LED lights but the horn does not sound.
As the fan spins faster, the horn sounds
louder or the LED lights brighter. You can use
this circuit as a wind direction or speed
indicator.
Project #127
Changing the resis-
tance of the meter
changes the scale
reading.
+
+
To make a current measurement, you need to break the circuit,
so that the meter can be connected in series. The meter should
not change the circuit, so it must have a very low resistance.
50mA Scale:
To measure the current through the red LED (D1), set the
meter to 50mA and the slide switch (S5) to position C. Look at
the meter and read the current, it should range from 20-25mA
(2-2.5 on scale).
0.5mA Scale:
Set the slide switch to position B, placing a 10K
Ω resistor in the
pivot stand in parallel with the meter. Set the meter to the
0.5mA scale and the meter pointer should show about 1.5,
which is 0.15mA. The LED will not be as bright, due to the
lower current.
5mA Scale:
You can change the scale to 5mA by pushing the press switch
(S2). This connects a 47
Ω resistor in the pivot stand in parallel
with the meter, diverting enough of the current away from the
meter to change the scale to 5mA.
See projects 1 & 3 if you need to recharge the battery (B4).