How to use a voltmeter/multimeter

(In development)

Ideally, the voltmeter will have AC and DC current and voltage, resistance, and a continuity tester that “beeps”.

Beaver/Cub: As a battery and wire tester.

  • Use new, lightly used and dead AA or AAA batteries. How do you tell the difference?
    • New: above 1.5V and as high as 1.6 volts
    • Used: 1.4V – 1.5V
    • Dead: 1.2 V
  • Show them polarity (negative, positive ends)
  • Show other batteries (9V, 12 V car battery)
  • Safety/durability:
    • Don’t short circuit
    • Keep ends isolated
    • Don’t leave in devices for long time (leaks, etc)

Continuity tester is used to check which wire is connected. You can do it with ohm meter (wire will have close to zero resistance), or with the continuity tester function that beeps when the connection works.

  • Prepare some wires with and without breaks (cut and add a heat shrink to hide the break).
  • Use a cat 5 wire (has 8 internal wires) to test connections or deliberate bad connections at an RJ 45 or other plug.

Cub/Scout: Introduce series connection, resistance

  • Combine with our spark gap radio lesson to introduce short circuit
  • Introduce the basic circuit diagram elements
    • Battery
    • Resistor as general load
    • Short circuit in a diagram

Here’s a fundamental question: which will draw down a battery faster –  hooking it up to a high resistance or a low resistance?

    • Take three light bulbs – say 40W, 60 W and 100W.
    • Which has the highest resistance? Measure for each light bulb
    • Analogy:
      • Power in the battery is like water stored
      • Resistance is like narrow straws or pipes
        • The higher the resistance, the narrower the pipe, the less water will flow
        • So higher resistance = less water flowing = water from the battery lasting longer
  • Series batteries: adding up the voltages
    • Two 1.5V batteries in series = 3V
    • Check out the battery back in our basic AM transmitter project. Are the batteries inside in series or parallel? Test with a voltmeter to see.

Scout/Venturer

Continue to answer the questions from the lesson above about which will train a battery faster – a high or low resistance.

  • Introduce Ohm’s law: V = I x R
  • Add power calculation: P = V x I
    • Light bulbs above – why is lower resistance bulb using more power?
      • At a given voltage, lower resistance means more current (V = I x R, or I = V / R)
      • More current means more power (P = V x I)

Explain voltage drop across components.

  • Show voltage across two series resistors in a circuit
    • If 3 V power power supply and two resistors of same value, each resistor will have 1.5 volts across it. That’s the voltage drop
  • In any electrical circuit, the total across the voltage drops is equal to the voltage from the supply
  • Basic introduction to AC
    • Cover how waves work in the sound and waves project
    • Do it after building a simple electric motor – by using it as a generator. Simple electric motors are always AC

Venturer/Rover

 

  • Introduce AC
    • Use analo

 

 

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