A voltmeter is a device that measures voltage, the electrical pressure in a circuit. In this activity, you will learn how to measure the voltage across an element in a circuit using a micro:bit-based voltmeter. Knowing how to take voltage measurements is essential when working on electrical equipment. Examples include:
Let's re-use the setup from First Breadboard Circuit — Build the LED Circuit [1], shown again below. In addition to the basic setup and previous LED circuit, you will need an additional resistor.
(1) LED, green
(1) Wire, black
(1) Resistor, 220 ohm (red-red-brown)
(1) Resistor, 1000 ohm (brown-black-red)
measure_volts_with_cyberscope.hex [3]
In these tests, you will measure voltages across the LED circuit’s components and compare them to the voltage across the +/- supply rails.
If you guessed from your measurements that adding up the voltages across each part in the circuit has to add up to the supply voltage, you guessed correctly.
Think about the micro:bit module’s 3.3 V supply as a (positive) increase in voltage and the parts in the LED circuit as a (negative) decrease in voltage.
Add those positive and negative voltages, and the result should be zero, in theory. In practice, your measurements should add up to something close to zero.
This is an application of Kirchhoff's voltage law (abbreviated KVL), which states: “The directed sum of the potential differences (voltages) around any closed loop is zero.” When parts in a circuit are connected end-to-end like the resistor to the LED, they are connected in series.
Learn more: https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws [7].
You can change the resistor, and the sum of voltages measured across the LED and resistor should still add up to the supply voltage.
Did they still add up to 3.3 V?
Red LEDs use slightly less voltage.
Did you notice a difference in the voltages? Did they still add up to 3.3 V?
Links
[1] https://learn.parallax.com/tutorials/language/python/breadboard-setup-and-testing-microbit/first-breadboard-circuit/build-led
[2] https://learn.parallax.com/sites/default/files/content/Python/breadboard/mp4/led-circuit-dc-green.mp4
[3] https://learn.parallax.com/sites/default/files/content/Python/Elec/hex/measure_volts_with_cyberscope.hex
[4] https://python.microbit.org/v/2
[5] https://cyberscope.parallax.com/
[6] https://learn.parallax.com/sites/default/files/content/Python/Elec/kvl-measurements.mp4
[7] https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws