Automatic Dimmer

Educator Intro

Duration 1,5 - 3h

Learning Objectives

In this chapter the students will learn…

• … the workflow with simple analog sensors (e.g. light-dependent resistors, potentiometers)
• … to read sensor values from analog pins and to store them into variables
• … to monitor sensor values using serial communication
• … to control devices like LEDs and piezo speakers based on sensor data
• … to map sensor values to a new range
• … to simulate analog signals to control LED brightness

Depending on the final exercise, the students also practice either…

• … observing light and shadow phenomena and using crafts skills to continue developing an Arduino-based instrument (Exercise 1: Improve the Musical Instrument)

or

• … building, coding and testing a burglar alarm prototype (Exercise 2: Burglar Alarm)

Slides and Lesson Plans

Feel free to use these slides to introduce the third chapter to the students!

There are some speaker notes attached to the slides. To see the speaker notes, click Options > Open Speaker Notes in the bottom panel.

Lesson plan suggestion

Total: 90-140 min

This is the so-called voltage divider circuit. It's not covered in the student material, as there is already quite a lot of new material introduced in this chapter. Understanding the voltage divider is not considered obligatory in order to proceed with Arduino exercises. But the concept may be related to your curriculum - in case you want to explain the physics behind this technique with your students, here's an explanation to get you started!

Arduino's analog pins can measure voltage changes between 0-5V. Light-dependent resistor, LDR, is a component that has a variable *resistance, *which is something that the Arduino can't measure directly. So you need to be able to measure the change in the voltage of your circuit caused by the light-dependent resistor.

If you only connect one LDR to your circuit, wouldn't it be enough? Just read the sensor values with a wire between the LDR and ground wire, or LDR and voltage wire? Feel free to test it.

If you connect a wire to A0 from between the ground wire and LDR and then check the reading with analogRead, you only get the value 0:

If you connect to A0 between the voltage wire and LDR and check the reading with analogRead, you only get the value 1023.

Why won't these circuits work?

If you try to measure a voltage change with a wire directly connected to ground (which is the case in the first image), the voltage in that point is 0V. And if the wire is connected to the other end, directly to the 5V pin, the voltage measured at that point is 5V. 

To measure the actual voltage drop the LDR causes in a circuit, you need the voltage divider:

Now, the point where you measure the voltage change makes more sense! The measuring point is between a fixed resistor (10 kilo-ohms) and a variable resistor (LDR). According to the voltage division rule, the voltage divided between two resistors in series is in direct proportion to the resistance of these resistors.

The voltage division rule is based on this equation:

• V(A0): the voltage measured at pin A0
• V(in): 5 volts, the voltage from pin 5V 
• R(fixed): 10 kilo-ohms, the fixed resistor in the circuit
• R(ldr): the resistance of the LDR

The circuit diagram:

More information:

• https://learn.sparkfun.com/tutorials/voltage-dividers/all
• https://en.wikipedia.org/wiki/Voltage_divider

Key points:

• new use for a variable: storing sensor values
• using analogRead
• checking the LDR values using the serial monitor

You need a delay of 20ms or the values will print too fast.

Code example:

//variable for storing the sensor value
int ldrValue;


void setup() {
  //start the serial communication:
  Serial.begin(9600);
}

void loop() {
  ldrValue = analogRead(A0);
  //print the LDR value to serial monitor
  Serial.println(ldrValue); 
  //delay of 20ms
  
  delay(20);
}

Key points:

• testing different LED brightnesses with analogWrite
• using the brightness variable as an argument for analogWrite

Code example:

//variable for storing the sensor value
int ldrValue;
//create an integer variable called brightness
int brightness;

void setup() {
  //start the serial communication
  Serial.begin(9600);
  //define pin 5 as output
  pinMode(5, OUTPUT);
}

void loop() {
  ldrValue = analogRead(A0);
  Serial.println(ldrValue);
  delay(20);
  //give brightness a value (0-255)
  brightness = 127;
  // turn LED on with analogWrite
  analogWrite(5, brightness);
}
In the latter part of the coding task, the loop should look like this:


void loop() {
  ldrValue = analogRead(A0);
  Serial.println(ldrValue);
  delay(20);
  //give brightness a value (0-255)
  brightness = ldrValue;
  // turn LED on with analogWrite
  analogWrite(5, brightness);
}

With this code the brightness of the LED changes when the LDR is shadowed or when light is pointed to it.

However, especially in bright light the LED will flicker and behave unpredictably.

Here, the key take away is how to use and adjust the map command.

Flipping the latter range inside the map command will make the dimmer work as intended - the LED will get dimmer when there’s more light:

brightness = map(ldrValue, 0, 1023, 255, 0);

Adjusting the range of incoming values inside the map command will make the dimming effect clearer. If the lowest LDR value in the serial monitor is around 100 and the highest around 950, the command would look like this:

brightness = map(ldrValue, 150, 950, 255, 0);

The students should investigate the actual minimum and maximum values that they get and use them.