Crash course: Embedded programming with Arduino

Try your hand at embedded programming with this project

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If you already have a well-equipped electronics workbench and don't need the two Adafruit kits, this is what you'll need for Singing Snowman:

  • Seven red and green LEDs with matching resistors
  • Arduino Uno
  • 9- or 12-volt power supply

Plus the other non-kit parts: Adafruit Wave Shield, MaxBotix EZ1, and wiring. Figure 2 shows my workbench with some of the tools and parts for this project.

fig-2-workbench.jpgSource: Carla Schroder
Figure 2: Workbench for the singing holiday snowman project

LEDs and resistors

You always need resistors with LEDs because resistors limit the current flowing to the LED to a safe level. Resistors are everywhere in electronics, and come in a huge variety of sizes and shapes. We are going to use through-hole carbon film resistors. A common beginner question is how do you know which resistors go with your LEDs? You need three pieces of information: The voltage of your power supply (Vs), and the voltage forward (Vf) and drive current (If) of the LED. Then calculate the resistor value (R), in ohms, like this: R = (Vs - Vf)/If.

For example, I have some of these 5mm red LEDs and a 9-volt power supply. So my equation is (9 - 1.85) / 0.02 = 357.5 ohms. The next highest standard resistor is 360 ohms at 1/2 watt, though most electronics gurus will go one higher and choose a 390 ohm resistor for a larger safety margin. A higher ohm value means more resistance and less current. You can damage LEDs with too much current. What does the watts value signify? That's how much power the resistor can handle. For low-power uses like our little blinky LEDS you'll see 1/8, 1/4, and 1/2 watt resistors. Try one of the many online LED calculators until you feel comfortable with your math, like this one.

fig-3-resistor.jpgSource: Carla Schroder
Figure 3: The color-coding tells us this is a 390 ohm resistor

That's for a single LED. We're using 7, so that changes our requirements. Again, there are handy online calculators to help us. The LED series/parallel array wizard calculates four different wiring layouts for you to choose from.

Resistors are color-coded. The color bars on the resistor in Figure 3 tell us this is rated at 390 ohms with 5% tolerance: Orange-White-Brown means 3-9- and one zero, or 390. The gold band = 5%. The tolerance rating is electronics-speak for plus/minus variance from the stated ohms.

Resistor color code tables are all over the place, on the Web and in books.

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