- Build a programmable LED light
- Make your light blink using Python
- Add a pushbutton switch to your circuit
Computers can interface with not just with other computers but with nearly anything in the real world, including motors, servos, lights, and sensors. Because a computer program can encode extremely complex behavior, this is how most interactive, automated objects in our world work. We're going to take the first simple steps to creating a programmable thing by connecting your Raspberry Pi to a light emitting diode.
Along the top right of the Raspberry Pi circuit board is a 2 row, 20 column matrix of "pins". These are intended for project makers like you to hook it up to the real world. Let's take a quick tour through the various pins as they all have specific functions that you can tap into.
- 3.3v and 5v DC Power - These provide low current power that can be used to drive small circuits
- Ground - These pins are the ground in an electrical circuit
- GPIO - This acronym stands for general purpose input/output. You reconfigure GPIO pins using software code to tell them how to behave (we'll dig more into this later)
Anyone can build a simple circuit, but not everyone can make it work using a Raspberry Pi!
Each student has been issued with a ziploc bag containing various components needed to complete today's project.
| Part | Notes |
|---|---|
| Solderless Breadboard | Ultra-handy pluggable playground for trying out circuits |
| Jumper Wires | These are used to make connections between circuit components on the breadboard |
| Cobbler | A special adapter that allows a Pi to be connected to a breadboard |
| LEDs | Light emitting diodes emit single-wavelength light and are available in many colors and form factors |
| Resistors | Components that reduce voltage and current flow through a circuit |
| Momentary switch | Basically, a push button |
Solderless breadboards are a cheap, reusable way to make quick circuits and test out ideas before making a permanent Printed Circuit Board. They often look like this:
You can see there are holes where things might get plugged in, but these have a special feature. Although you can't see it, inside the breadboard are many strips of metal that connect the rows and columns together. The metal strips are springy so that when you poke a wire into the hole, the clips grab onto it. In the image above you can see how there are two kinds of metal strips: short ones that connect 5 row holes at a time, and very long ones that connect 25 (or more!) column holes at a time. The long columns are called rails and the short strips are called rows.
To use a breadboard, you need 22 gauge solid-core (not stranded) wires that have been cut to size and had the insulation pulled off at their ends. You can also buy jumper wires wiht fancy plastic tips from any electronics supply store.
Cut the wire first
Pull off insulation at the tips
Voila, your completed jumper wire
⭐ A note on wire colors: Wires connected to ground are usually black, while wires connected directly to a source of current are usually red. Beyond that you are free to use whatever colors you wish!
Resistors are good at one thing: resisting the flow of electricity (otherwise known as current). This allows you, as a circuit designer, control where and how fast current flows.
A helpful way of conceptualizing electrical current and resistance is to think of it in terms of water pressure in a tube. If you have water flowing in a tube and you put a smaller pipe in line, there will be more resistance to water flow. On the other hand, attaching a larger pipe will lead to less resistance to water flow.
Reading Resistor Codes
Resistance is measured in ohms, often written as the symbol Ω. The bigger the resistance value (in ohms) the more it prevents current flow. Every resistor has printed on it a special code that lets you know what its resistance is. It's a challenge when you are starting out to remember which values are which, but you'll get the hang of it soon enough with some practice!
❓ What is the value of this resistor?
LED stands for Light Emitting Diode, and they (usually) have long lifespans and require little power. Because of these characteristics, you've probably got 100 of these in your house on various electrical devices! The light-emitting part of the name makes sense, but what does diode mean?
A diode is basically a one-way street for current. Imagine such a one-way street with a traffic enforcement officer in front. If you want to turn onto the street the wrong way, she will not let you. Likewise a diode simply does not let current go through it the wrong way. Current in a diode can only flow from the positive side to the negative side.
Most LEDs only require a small amount of current. Never connect an LED directly inline in a circuit unless you know for a fact that the current will never exceed that maximum rating for the LED. Otherwise, you will need to buy a new LED 😔
The longer lead on an LED is the positive side (anode), while the shorter lead is the negative (cathode) side.
A switch is a simple device with two positions, on and off. When on, two wires are connected inside, which allows current to flow. When off, the two wires are disconnected. You have dozens of switches in your home!
On the left, the switch is open and no current flows. On the right, the switch is closed, current flows and the light turns on.
In your kit, you have a 1-2 momentary switches, also known as push-buttons. "Momentary" switches get their name because the current only flows when the button is pressed. The keys on your keyboard are an example of momentary switches.
The momentary switches can be inserted right into the breadboard, but note the orientation. We'll cover this in detail later
The Cobbler is a utility header board and ribbon cable that makes it really easy to connect a Raspberry Pi to a standard solderless breadboard. Like the Pi's output pins, it has 2 rows of 20 pins. The layout of the pins is exactly the same as on the Pi, and as you can see they are even labeled for you. All you have to do is make sure to connect the ribbon cable in the right orientation (it's kind of hard to get wrong)
To turn the LED on and off we need to control it using one of the Raspberry Pi's GPIO pins. We'll worry about the programming later - for now, we will just wire up the circuit and hope for the best!
- If you have not already, install the Cobbler on the solderless breadboard. Make sure the first row of pins are seated in breadboard row 1.
- Connect one jumper wire from the GND pin to the - rail on the breadboard
- Connect another jumper wire from #25 to an open row on the breadboard
- Put a 220 Ohm (Red-Red-Brown-Gold) resistor on the same row, connecting to another row ❓ What is the value in Ohms of this resistor?
- Connect the Anode (+) of any Red, Green, or Yellow LED downstream of the resistor
- Connect the Cathode (-) of the LED to the common GND rail
- Finally, connect the ribbon cable from the Cobbler to the Pi.
- Your connections should resemble the ones in this diagram:
❇️ Try briefly moving the wire from #25 to 3V3 - What happens? Don't forget to plug back into #25 when you are done playing :-)
💥 Make sure the LED is in the correct orientation or it will not light when we power it up.
- The purpose of common electronics components like breadboards, LEDs, and resistors
- How to read resistor codes
- Technique for cutting and preparing jumper wires
- How to use a breadboard to prototype wired circuits
- How to follow a simple circuit diagram
- Graphical Resistor Chart
- Sparkfun
- Adafruit Industries
- Fritzing
- Materials List for "Physical Computing with Raspberry Pi"
- Lady Ada for images and inspiration pertaining to the breadboarding tutorial