Elenco Upgrade Kit SC300 to SC750 - Manual (Page 15)

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Project #323
OBJECTIVE: To build a 3mA meter circuit.
3mA Meter
Project #324
OBJECTIVE: To build a voltmeter.
0-3V Voltmeter
Build this 0-3V voltmeter circuit. Set the meter (M2) to the LOW (or
10mA) setting. Using new batteries, place the battery holder between
points A & B. Adjust the adjustable resistor (RV) so the meter deflects
full scale.
Now you can check your other “AA” batteries by inserting them into the
battery holder.
Set the meter (M2) to the LOW (or 10mA) scale. Inside the meter,
there is a fixed magnet and a moveable coil around it. As current flows
through the coil, it creates a magnetic field. The interaction of the two
magnetic fields cause the coil (connected to the pointer) to move
(deflect). By itself, the meter can measure 300
μ A. To increase its
range, resistors are connected in parallel or in series to the meter.
Build the circuit shown. Placing the 100 Ω resistor (R1) in parallel with
the meter increases the range by 10 times to 3mA. More current flows
through the resistor than the meter. The lower the resistor value, the
wider the range of the meter.