Here is a close-up of the circuit you will build above a diagram called a schematic. A schematic has information about how the parts are electrically connected, but the details of how that circuit is built on a breadboard is up to the person building the circuit. In other words, the circuit you see plugged into the breadboard is just one of many possible “correct” ways to build it.
This activity starts with a simple light circuit. It’s a lot like the lights on your micro:bit, except you can pick the color, red, green, or blue. We’ll start with green.
If this is your first time building circuits on a breadboard, follow along carefully. Along the way, you’ll also get to learn about the parts as well as what happens when electric current flows through the circuit.
The parts drawing below will help you to identify and gather the electrical components for this activity.
You will also need the completed setup from Set Power for Circuits [1].
Now, let's use the components to build the LED circuit on the breadboard.
(View full size:connect-usb-light-on.mp4 [3])
The term LED is an abbreviation for Light Emitting Diode. When current flows through an LED, it emits light. It glows more brightly with more current, and less brightly with less current. That takes care of “light emitting”, now what is a diode? A diode is a one-way valve for electric current. In other words, current can only flow through in one direction. That’s why it’s important to make sure the longer pin is in (d, 16) and the shorter pin is in (f, 16). If the pins are reversed, no current will flow, and the LED will not emit light. The longer (+) pin is called the anode and the shorter one is the cathode.
No longer pin? If an LEDs pins get cut to the same length, there’s a flat spot on the round plastic case next to the cathode pin to help identify it.
A resistor “resists” the flow of current. Resistance is measured in ohms and abbreviated with the greek letter omega (Ω). Most resistors have color bands that indicate what their resistance values are. More on that later.
The 220 Ω resistor in this activity limits the current through the LED to about 5 mA, which is 5/1000 of an amp. The maximum current for these LEDs is 20 mA. Larger values of resistance will let less current flow, making the LED dimmer. Likewise, smaller resistance values will let more current flow through, making the LED brighter.
Resistors plug in either way. Unlike diodes, you won’t have to worry about what direction to connect a resistor. Resistors resist current the same amount in either direction.
By connecting one end of the LED circuit to 3.3 V and the other end to GND (0 V), the electrical pressure propelling current (electron flow) through the LED circuit is 3.3 V.
One of the most common identification schemes for resistors is the four-band color code. Each of the four color bands on your resistors represents a value.
To read a resistor, position it so that the fourth band, which is going to be gold, silver or blank, is on the right. The left two bands are the first two digits in its resistance value. The third band is the number of zeros to append to the resistance value.
You can use the digits in the Resistor Color Codes chart to figure out what digit each color indicates. The fourth band is the tolerance. That’s a measure of how far the parts actual resistance might be from what the color bands say.
In the example of a resistor with red-red-brown-gold color bands, the first two digits are red, red. Since red is the digit 2, the red-red bands mean 22. The third band is the number of zeros to add to the value. Since the brown band is 1, it means add one zero, for 220 Ω. The gold color band means the resistor has a 5% tolerance. 5% of 220 Ω is 11 Ω, so the resistor’s actual value will be somewhere between 209 and 231 Ω.
Other Color Codes: The rules in this resistor ID table cover the four-band resistors in your kit. There are more digit and tolerance options. For more complete color band ID tables, check the Electronic color code at Wikipedia.
Look at the picture of a battery-powered LED circuit below. A battery contains chemicals with an excess of electrons connected to its (-) terminal and chemicals that are missing electrons connected to its (+) terminal.
With the LED circuit connected, the electrons escape molecules in the chemical where they are already overcrowded, flow from the battery’s (-) terminal through the circuit to its (+) terminal, and then join the chemical with molecules that are lacking electrons. The molecules that lose their extra electrons go from being negatively charged (-) to being neutral (n). Likewise, when the positively charged (+) molecules that are lacking electrons gain an electron, they also become neutral (n).
Current is represented quite differently on a schematic. It is typically indicated by an arrow pointing from the higher voltage to the lower voltage. There might be some number of amps (A) or milliamps (mA) next to the arrow. It might even contain an expression like I = 5 mA. The variable I is commonly used as the variable to store current in calculations.
Your circuit used the 220 Ω resistor. Now let's try the 1000 Ω resistor.
The LED should be noticeably dimmer.
Links
[1] https://learn.parallax.com/tutorials/language/python/breadboard-setup-and-testing-microbit/set-power-circuits
[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/breadboard/mp4/connect-usb-light-on.mp4