If you’ve read Getting Started with Arduino (click on link to read my review of the book) you must have noticed that the circuit on page 43 uses a 10K Ohm resistor in series with the pushbutton. (If you haven’t read it yet, the example in the schematic on the right is similar to the setup featured on the book).

Consider a microcontroller with a digital input pin connected to a switch. When the switch is closed, the pin is connected to GND. When the switch is open, the signal is not connected to anything and is left at an unknown state. The signal is in this case said to be “floating”. This is pictured in circuit (1).

To remedy this problem we pull the signal up to VCC, as shown in circuit (2). Now when the switch is open the signal is connected directly to VCC and is at a known state. But notice that when the switch is closed, VCC and GND are connected, creating a short.

To prevent the short we place a resistor between the input pin and VCC, as shown in circuit (3). This resistor is called a “pull up” resistor.

Pull down resistors work almost in the same way, except the circuit is connected so that when the switch is closed the input pin is connected to VCC, and when switch is open the pin is connected to GND using a “pull down” resistor, as shown in circuit (4).

A common value for pull up resistors is 10K Ohm. Higher resistance may be necessary for circuits that sink greater currents. We can use Ohm’s Law to determine the appropriate value for the resistor.

You might also enjoy:

1. Basic Hobby Electronics Vocabulary Part 3 of 4 (I-R)
2. Arduino LED Control Using DIP Switch: Schematic | Part 2

When looking at the parts list for the Arduino RGB LED spinning night light you must have noticed that current limiting resistors of different values were used for the Red and the Green/Blue pins of the RGB LED. That is due to them having different forward voltage ratings. You can find complete specs for the LED in the datasheet (when buying an electronic component you will have the option to download its datasheet, or the relevant information will be provided by the vendor).

We use Ohm’s Law to calculate current limiting resistor values:

Forward voltage ratings:

RED: 2.1V
GREEN: 3.3V
BLUE: 3.3V

Current:

I = 20mA

Supply voltage:

V = 5V

Ohm’s Law:

I = V/R => R = V/I

So for Red:

(5 – 2.1)/0.02 => R = 145 Ohm

For Green/Blue:

(5 – 3.3)/0.02 => R = 85 Ohm

As for the Arduino sketch, I chose to have the lamp fade between two colors, aqua (#00FFFF) and magenta (#FF00FF). For that I kept the Blue value at 255 and varied the Green and Red values between 0-255 to achieve the desired colors, as shown in the diagram:
(You can pick your favorite colors, cycle through the entire spectrum, or go psychedelic and show random colors with random delays)

// fade from aqua to magenta
  for (int i = 0; i < 256; i++) {
    analogWrite(RED, 255-i);
    analogWrite(GREEN, i);
    analogWrite(BLUE, 0);
    delay(50);
  }

  // fade from magenta to aqua
  for (int i = 0; i < 256; i++) {
    analogWrite(RED, i);
    analogWrite(GREEN, 255-i);
    analogWrite(BLUE, 0);
    delay(50);
  }

Here's the full sketch for the night light.

You might also enjoy:

1. Arduino RGB LED Spinning Night Light | Part 1
2. Arduino Motor Control for the Spinning Night Light | Part 3
3. Arduino RGB LED Spinning Night Light: Assembly | Part 2
4. Arduino LED Control Using DIP Switch | Part 1

TGIMBOEJ, The Great Internet Migratory Box Of Electronics Junk is a progressive lending repository of electronic components. An open collaborative project, TGIMBOEJ can be seen as a cross between P2P file sharing and a flea market, an internet meme in physical form. It arrives full of wonderful (and possibly useless) components, but you will surely find some treasures to keep. You will be inspired to look through your own stuff and find more “electronic junk” that can be donated to the box before it is passed on again. And on and on and on. There are several incarnations of TGIMBOEJ being passed around the US and some parts of the world.

What I took from the box:

• Straight thru-hole connector headers
• Alligator clips
• Assorted capacitors
• Assorted resistors
• A hobby motor
• A LED flashlight

What I added to the box:

• A surface mount accelerometer
• Right-angle thru-hole connector headers
• Assorted capacitors
• Assorted adapters
• A multi-card reader
• Assorted wires
• Several unidentified components from previous orders containing wrong items that were not worth exchanging

No related posts.

Insulator: A material that has high electrical resistance and is therefore a poor conductor of electricity.

Integrated Circuit (IC): A type of digital circuit in which transistors, diodes, resistors and capacitors are constructed on a semiconductor base.

Light Emitting Diode (LED): A type of diode that generates light when current flows through it.

Ohm’s Law: The relationship between voltage, current and resistance.

Ohm (?): The unit of measure for resistance.

Printed Circuit Board (PCB): A board in which components are connected using a thin coat of conductive material “printed” on the board instead of wires. It is used for mounting electrical components.

Resistance: The electrical friction between an electric current and the material it is flowing through that causes electricity to be dissipated as heat.

Resistor: An electrical component used to introduce resistance into a circuit.

You might also enjoy:

1. Basic Hobby Electronics Vocabulary Part 2 of 4 (D-G)
2. Basic Hobby Electronics Vocabulary Part 4 of 4 (S-V)
3. Basic Hobby Electronics Vocabulary Part 1 of 4 (A-C)
4. Starting Out on an Electronics Hobby – Tools