Category: child-page

Last updated on October 30th, 2022

Introduction to Ohm’s Law and the basic characteristics of electricity.

Video :: Terminology

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Last updated on September 5th, 2022

RGB LEDs are fun components to explore expressive and technical aspects of tangible media. You can control them in circuits with fixed resistors, or with code as digital or analog devices. The possibilities and combinations are endless.

RGB LEDs are also examples of parallel components — the three color channels are organized internally in parallel. This arrangement makes setup a little more difficult than simple LEDs but the potential color variations make it worth the effort.

Required Parts

• breadboard
• battery pack
• battery clip (wire)
• resistors assorted (start with 1kΩ)
• resistor 470Ω
• RGB LEDs (big white gumdrop)

Power connections

In this video I use a voltage regulator that we no longer have in our kits (hmm) . For a raw circuit (no code), you have two solutions possible with the current kit:

First, use the 9V battery pack we have been using in class and connect to the breadboard power rails. The resistors suggested will protect the LED at 9V

Second — and we may not have covered this in class — but you can use your Arduino as a power source. Connect the Arduino pin labelled +5V to the red breadboard rail and GND connected to the blue breadboard rail. This will provide a stable 5V supply.

Use dupont wires or cut your own to make these connections — double check for short circuits before you power up.

Arduino Connections for power

Video

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The Circuit

The RGB LED has three single color LEDs inside – red, green and blue. Each of these behaves in essentially the same way as a single color LED on its own. There are slight difference in the amount of voltage it takes to turn on each color in the RGB LED.

Finding Ground

Take a moment to find the ground leg of your RGB LEDs. The ground (GND) leg is the longest. There is no flat side on the case of these LEDs.

Hold your RGB led such that there is one shorter leg to the left of ground, and two shorter legs to the right of ground. If held like this, the legs go in order from left-to-right : RED – GROUND (GND)-GREEN-BLUE. In the symbol above, the leg below the LEDs is a common ground connection for all three internal LEDs. Keep track of the ground leg as you build.

Single resistor circuit.

Use this single resistor circuit to test each channel one at a time.

Try moving the top resistor ( 1k in schematic — connected between Arduino 5V [+ve] and the BLUE channel) to the RED and GREEN LED legs (excluding ground). You should be able to confirm the leg order described above and the channel colors.

Mixing Colors

Using the fixed resistors in your kit, try creating your favourite color. Try different values for the top 3 resistors in the schematic below. You can use any resistor from your kit AS LONG AS YOU ALSO LEAVE the 470 ohm resistor shown in place.

What is your favourite color?

Going Further

marshmallow clouds uses LOTS of RGB leds

so does forest

forest was made in collaboration with New Media!

Last updated on September 5th, 2023

After building a few circuits you should start to see that LED brightness can be controlled by altering the size (resistance) of the current limiting resistor used in the circuit.

LED Brightness and Current Flow

We know from Ohm’s law (V=IR) that for a fixed voltage (5v in case of an Arduino digital pin) current is inversely proportional to resistance.

What does that mean?

If we hold voltage constant:
– increases in resistance will reduce current.
– decreases in resistance will increase current.

LED brightness is proportional to current — so small resistors give bright LEDs and big resistors give dim LEDs.

Note if resistance gets too small, you will have dead LEDs not bright ones — see why here.

Let’s build some circuits to explore this relationship.

Required Parts

• breadboard
• battery pack
• wire battery clip
• leds (single color all the same)
• resistors — fixed several
• Ohm’s law chart

Video

UPDATE FOR 2023:

In the following video I use a battery pack to provide power (we will look at batteries soon). Remember circuits have two complimentary halves: the source side and the sink side. You can work through this video with an Arduino for power instead of battery power. The build will follow a parallel path after you have your power source setup.

One big difference is that a battery pack provides 9V — instead of 5V. You can imagine this as a tall vs short waterfall (charge difference). So in the video i talk about 9V — where, if powered from Arduino, you will only have 5V. The principles discussed remain the same.

GETTING SET UP: To use your Arduino for power, set up the power side of your circuit like the image below — the action on the breadboard does not change. At this stage dupont wires are your friends — use red and black if you have them — or others colors that are available. (Remember you need to connect your Arduino to a plugged in USB cable — not shown — so that the Arduino has power!).

In the image above the red wire is connected to 5V from the Arduino and the top red rail on the breadboard. The black wire is connected to Arduino Ground and the blue rail on the bottom of the breadboard.

With that completed you are ready to watch the video!

Going Further

How to safely select current limiting resistors

Last updated on October 12th, 2021

Potentiometers or pots are a type of variable resistor. Unlike their fixed resistor cousins, the resistance in a pot can be controlled by the user.

POT Basics

The pots in your kit have two very different resistances; 10K and 500k pots. Pull them out so you can get a sense for what these feel like in your hands. Try turning the dials on each one. Feel how far they move relative to a full circle.

What are POTs? What do they want?

Pots are one of the magical creatures of the electronics world. When we place them in a circuit, we connect one outside leg to the +Ve side of our power supply (top of waterfall), and the other outside leg to ground (bottom of waterfall). The middle leg then outputs a voltage between +Ve volts (9v if battery, 5V if Arduino) ) and ground (0V).

Recall that voltage and current are related to resistance according to relationships defined by Ohms law. So, adjusting resistance alters voltage and current flow. The wiring of the pot is such that the voltage changes are passed out the wiper (middle leg).

How is that possible? We will get into more detail after the video.

POT Representations

The symbol for a pot is based on that of a fixed resistor. Its base is a zigzag line with legs on each end. Added to that is an arrow pointing at the zigzag of the fixed resistor symbol.

The arrow represents the wiper (middle leg). It may help to imagine the whole arrow moving along the fixed resistor symbol from one end to the other. As it moves, the amount of resistance on each side of the arrow changes.

Note that the pin labels would not normally be included, but the resistance would.

Let’s set up a circuit and explore.

Required Parts

• POTs
• 1k fixed resister and chart
• LED (single color)
• hook up wire ( red, black and some other color — maybe yellow )
• wire strippers
• battery pack loaded
• battery clip

Video :: POTentiometer Basics

Slides: Grab a copy

The Circuit

Here is the schematic of the circuit built in the video above.

Note the inclusion of the 470ohm resistor after the LED. This resistor limits the current through the LED and ensures its protected.

Pots are a kind of voltage divider!

As mentioned in the video and the slides, a pot is an example of a voltage divider. Voltage dividers are very important kind of circuit — they are used with many sensors.

I will have to create a voltage divider tutorial but for now, the following will hopefully give you a sense of what is happening.

How is a voltage divider connected?

A single resistor is wired between the outside legs of the pot. The size if this resistor is fixed and stamped on the body of the pot — 10K or 500K for the ones in our kit. The middle leg — called the wiper — is connected to the dial on the pot top ( couldn’t resist — it’s a palindrome!). The wiper cuts the pot’s resistor into two. nAs you turn the dial, you adjust the position of the wiper, changing the proportion of resistance to its left and right. In other words, the wiper divides the internal resistance.

Because voltage is related to resistance ( Ohms’ law) we see the impact of the resistance changes as changes in voltage.

A POT thought-experiment

Let’s think about how electricity flows in this circuit. In particular let’s think about how that 470 ohm resistor protects the LED and impacts current flow.

POT TURNED ALL THE WAY TO POSITIVE

The 470 ohm resistor is necessary when the pot is turned all the way towards the side connected to power +Ve (high side of water fall). In this state, two things happen. First, the resistance between the power supply and the wiper drops to zero (all 10k are on the other side of the wiper). Second, this allows the full voltage of the supply to flow out the wiper. If we did not include the 470ohm resistor the LED would have zero resistance in its circuit and would get full voltage –> that almost always means an immediately dead LED. IF you are lucky it will pop and the cap will shoot off (watch your eyes! — I have actually only seen this once). Most likely it will just melt, possibly making a wimpering pop and hiss sound and then smell really bad.

POT TURNED ALL THE WAY TO NEGATIVE

Now, image we turn the wiper all the way the other way — so that there is 10K ohms between +Ve and wiper. And zero resistance between the wiper and ground. At the wiper the electrical flow has two paths it can follow (ie the circuit branches are in parallel). Current can go through the zero resistance path to the battery ground or it can go through the LED and the 470 ohm resistor. Recall that electricity ALWAYS follows the path of least resistance. The current will all go the zero resistance route and the LED will no longer light up.

Going Further

There are lots of sites with info on pots and voltage dividers. Here are a couple to get you started.

voltage divider this one has a great explanation of voltage dividers.

Sparkfun tut on pots. A goog intro.

Some Khan Academy Theory. This one is WAY deeper than we need to go — but if you are into this sort of thing — go for it. I already regret sharing it a little.

Last updated on October 17th, 2021

Analog Inputs (AI) are one of the four contexts or types of circuits that are core to tangible.

If you have not yet looked at the tangible matrix Building Block, you should check it out now. It will introduce you to all four contexts.

Context :: Analog Input

Analog refers to signals, circuits, or logical systems that are variable or graded. They can be the top or bottom of the waterfall and every shade in between.

Input and Output are defined from the perspective of our Arduino.

Electrical signals generated in circuits and sent into the Arduino are INPUTS.

So, Analog Inputs are variable signals (V) that move INTO the Arduino.

Card

Your kit contains a reference card for analog inputs.

Each card indicates the context, command, circuit and offers a code sample.

Video :: Analog Inputs

Get the Slides

Classic Example

At the introductory level, two circuits can be used as classic examples of an analog input: the POTentiometer, or POT and a photoCell.

Circuit

POTs and photoCells are both resistive circuits that make use of a concept called a voltage divider. The electrical similarity of these examples means they can be swapped out and the core behaviours will remain the same. The Analog Input card in your kit depicts a photocell circuit. We will look at both in this Building Block.

Potentiometer Circuit

The circuit for reading a potentiometer is very similar to the basic POT setup. In this case we will use the Arduino 5V for power (not your battery!) and connect the POT wiper to an Analog IN pin.

Photocell Circuit

As above, the basic photocell circuit is modified to connect to an Analog In pin.

Command :: analogRead()

The code that creates an Analog Input is:

    state = analogRead(pin);

where:
pin = A0 – A5 (inclusive)
state = 0 – 1023

_{Note 1: these pins are prefixed with a capital A}

The line of code above means:

Read the analog (variabel, staircase) state of pinAx and store that reading in a variable called state.

How Does it Work?

Much like we did for Analog Outputs we can imagine the input signal as a staircase of changing voltages.

The steps on the staircase of an Analog Input are much shorter than their output sibling, so even though we still cover a range of 5V, there are 4x as many steps! The state range for Analog Input therefore is 0 – 1023. (Note: analog inputs store 10-bit numbers while analog outputs use 8-bit numbers).

As mentioned above, both examples belong to a class of circuits known as voltage dividers.

When two resistors are placed in series with one another and we measure the voltage at the point between them we see a voltage that falls between max power (top of waterfall, 5V in Arduino Uno) and ground (bottom of waterfall, 0V). The exact voltage depends on the ratio of the resistors.

POTentiometer
The wiper in the POT, acts to continuously divide the total resistance of the device. We there for have two resistors that change in opposition to each other. When one gets big, the other gets small. Technically we say that they are in an inverse relationship.

When you turn the dial of the POT all the way towards the leg connected to ground you make the resistance between 5V and the wiper relatively LARGE. The resistance between wiper and ground (0V) gets close to zero. In this case the signal to the Arduino is small and the value (state) of an analogRead is close to zero.

When you turn the dial of the POT all the way towards the leg connected to 5V the resistance between 5V and the wiper gets close to zero. The resistance between wiper and ground gets relatively LARGE. In this case the signal to the Arduino is close to 5V and the value (state) of an analog read is close to the upper limit of 1023.

PhotoCell
In the case of the photocell we have one fixed resistor and one variable resistor that depends on light.

When the environment is dim, the resistance of the photoCell is HUGE ( ~1 Million Ohms). In this case the signal to the Arduino is small and the value (state) of an analog read is close to zero.

When the environment is bright, the resistance of the photoCell drops rapidly (~1k or less in bright light). In this case the signal to the Arduino is close to 5V and the value (state) of an analog read is close to the upper limit of 1023.

Important

In Arduino, we read inputs.

Check the Arduino Docs for this topic

Code Sample

Connect your circuit to pin A0. Upload the code below. Open the Serial Plotter.

With a photocell circuit; cover the sensor with your hand watch the plotter trace. With the POT circuit; turn the dial, watch the trace. Notice that the POT holds it value.

int sensorPin = A0;
int state = 0;

void setup() {
  pinMode(sensorPin, INPUT); 
  Serial.begin(9600);
}
void loop() {
  state = analogRead( sensorPin ); 
  Serial.println(state); 
  delay(50); 
}

What to expect

The trace below is a screen cap from my serial plotter while running the above code with a photocell connected. The relatively flat parts of the curve on the left and right are from moments when full room (ambient) light fell on the sensor.

The wobbly bit in the middle is from me quickly covering and uncovering the sensor with my hand.

Give it a try.

Going Further

Some follow up Arduino references:

*pinMode

*Serial

*see analogInputs in action with a photocell or a pot

*learn about thresholding analog sensor values.