Good day, everyone!

I have a question about a lab I'm doing for Intro to EE. So... We've set up an Arduino to power 4 LED circuits. Each with a different value resistor: 100Ω, 220Ω, 330Ω, and 1KΩ. All with a 5Vdc supply voltage. That's not the problem... That's simple. HOWEVER... we're asked to make a "theoretical" calculation of the voltage across the LED... That's the problem... This is impossible, is it not...? First off, I've been building circuits (especially LED circuits) and using Arduino for years... Typically, we take our LED and either read the data sheet or test with a meter to find the forward voltage, and then we use that and our desired current to calculate which resistor we should use for the circuit.

In this case, how can we calculate the voltage dropped across the LED? Let me be clear... the only information we are given is the respective values of the resistors and the supply voltage...

Our standard formula for voltage drop cannot be applied, right?


I emailed my professor to ask and this was his reply:

"Every LED is connected in series with a resistor. Some of the voltage will be dropped across each resistor and some across LED. Remember LED is a resistor as well.

Hope this helps."

SINCE WHEN DO WE CONSIDER AN LED A RESISTOR?!

Even if we could consider it a resistor, we aren't given its resistance. We aren't given forward voltage, or current... just the value of the resistor in the circuit...

Am I missing something? Am I over or underthinking this?

Thank you all for your help with this.

Respectfully,

Hopeful EE

 

I would agree that it is improper to claim that an LED is a resistor, but within the narrow context of this discussion, it's not too unreasonable. A better way to have put it would be to say that, at any given operating point, an LED has a relationship between the voltage across it and the current through it, just like a resistor does. So when you have it in series with a fixed voltage and a fixed resistor, you can treat it as if it were a fixed, but unknown, resistance.

Here's a hint -- if I start at the edge of a lake and climb up 1200 ft to the top of a hill and then come down 400 ft to where an observation deck is, can I determine how high the observation deck is above the lack without being told anything more?

Now, you haven't said what, exactly, you are expected/allowed to do. It's not enough to know the value of the resistance. Are you allowed to measure the voltage across it or, in some other way, determine the current through it?

 

The relationship between current and voltage in an LED is exponential, not linear, so you can't calculate it from the equation which uses only resistances.
The easiest way to do it is graphically. If you have a graph of voltage v. current for the LED, you can draw a line on it which represents the voltage v. current of the resistor and where they cross you have your answer (just like designing for valves),
However, there is a large tolerance on voltage drop with LEDs so it will only give an approximate answer.

 

The relationship between current and voltage in an LED is exponential, not linear, so you can't calculate it from the equation which uses only resistances.
The easiest way to do it is graphically. If you have a graph of voltage v. current for the LED, you can draw a line on it which represents the voltage v. current of the resistor and where they cross you have your answer (just like designing for valves),
However, there is a large tolerance on voltage drop with LEDs so it will only give an approximate answer.

Good day!
This is exactly my problem... BEFORE conducting any experiments I am supposed to make a "theoretical calculation" and then conduct the "experiment" It's a sad little collection of simple circuits but this one proposition of a theoretical calculation with this given information is impossible! we don't have any information on the physical characteristics of the LED and no current... only the supply voltage of 5Vdc and and resistor value for each circuit

 

I would agree that it is improper to claim that an LED is a resistor, but within the narrow context of this discussion, it's not too unreasonable. A better way to have put it would be to say that, at any given operating point, an LED has a relationship between the voltage across it and the current through it, just like a resistor does. So when you have it in series with a fixed voltage and a fixed resistor, you can treat it as if it were a fixed, but unknown, resistance.

Here's a hint -- if I start at the edge of a lake and climb up 1200 ft to the top of a hill and then come down 400 ft to where an observation deck is, can I determine how high the observation deck is above the lack without being told anything more?

Now, you haven't said what, exactly, you are expected/allowed to do. It's not enough to know the value of the resistance. Are you allowed to measure the voltage across it or, in some other way, determine the current through it?

Good day, and thank you for your reply!
That is indeed the problem... we are not given any additional information... these are very simple LED circuits but this is a seemingly impossible question to answer. We are to make a theoretical calculation as to the voltage across the LED before we conduct and tests... How are we supposed to do that... we don't have current or any data regarding the physical characteristics of the LED.... blah! XD

 

HOWEVER... we're asked to make a "theoretical" calculation of the voltage across the LED.

Hi EE,
It is possible that the question is designed to make you realise you cannot make a "theoretical" calculation of the voltage ?

Do you have an actual copy of the question that you were given, that you could post

E

 

A theoretical calculation of the led voltage will require some data of some sort. Normally this is provided by the datasheet. Failing that, you can make justifiable assumptions of behaviour and proceed on that basis.

 

Do you know what color the LED is?

Have you been introduced to the diode equation at all?

I don't know how much current an Arduino can deliver, but assuming it can deliver at least, say, 40 mA or so, you should drag down the output too much. Your current is going to vary over a range of about 10:1, so you can expect the LED voltage to change by a couple hundred millivolts, give or take, for most LEDs in that current range.

 

The exact assignment might help us solve the mystery. Sometimes there is critical information in the wording of an assignment that the student misses. I think that might be the case here, and we have no way of knowing unless you post the exact wording of the assignment.

 

Thank you all so much for your assistance with this silly thing. Here is the assignment in its absolute entirety. I'm sorry it's so much for so little.



Assignment:
Objective: This Assignment will introduce you to the practical application of Ohm's Law using an Arduino microcontroller. You will design a simple circuit to control an LED and measure the relationship between voltage, current, and resistance.

Please follow the step-by-step instructions below (refer to screenshots for guidance).

Components:

• Arduino Mega Board
• 4x Red LEDs
• 1x 100Ω Resistor
• 1x 220Ω Resistor
• 1x 330Ω Resistor
• 1x 1KΩ (1000Ω) Resistor
• USB Cable
• Assorted Wires

Theory:
Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Formulated by Georg Simon Ohm in 1827, the law states that the current flowing through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance. Mathematically, Ohm's Law is expressed as:

V=IR

where V is the voltage in volts, I is the current in amperes, and R is the resistance in ohms.

In a series circuit, where components are connected end-to-end, the total resistance is the sum of the individual resistances, and the current remains constant throughout the circuit. According to Ohm's Law, the voltage drop across each resistor in a series circuit is proportional to its resistance. This leads to the concept of voltage division, where the total voltage in the circuit is divided among the resistors in proportion to their resistance values. The voltage across a particular resistor in a series circuit can be calculated using the below formula:

V_n=I〖 × R〗_n

where V_n is the voltage across the resistor and R_n is the resistance of the specific resistor. The sum of the voltage drops across all resistors in the circuit equals the total supply voltage.



Step-by-Step Instructions:


1) Download the Arduino IDE: Go to the Arduino Download Page [Webpage] and download the latest version for your operating system (Windows, Mac, or Linux). For Windows, select the “Windows Installer” option.
2) Save the Installer: Download and save the .exe file to your hard drive.
3) Run the Installer: Double-click the .exe file to begin the installation.
4) Accept the License Agreement: Click the button to agree to the licensing terms.
5) Proceed with Installation: In the next window, click "Next."
6) Choose Installation Folder: Select the folder where you want to install the program, then click “Install.”
7) Complete the Installation: Wait for the installation to finish, then click “Close.”
8) Launch Arduino IDE: Locate the Arduino shortcut on your Desktop and double-click it to open the IDE.
9) Confirm Installation: The IDE should open with the code editor, confirming the successful installation of the software.

10) Get the Components: Open the Arduino starter kit and take out all the components listed in the components section.
11) Assemble the Circuit: Construct the circuit as illustrated below. Pay attention to the following details:

• The shorter leg of each LED is the cathode and should be connected to the ground.
• The longer leg of each LED should be connected to 5V through resistors.
• Use resistors of the following values for each LED, starting from the leftmost LED: 100Ω, 220Ω, 330Ω, and 1000Ω.

12) Connect to the Computer: Use the USB cable to connect the Arduino Mega board to your computer.
13) Set Up the Board in the IDE: In the Arduino IDE (from step 9), click on “Tools” in the upper menu. Ensure the selected board is “Arduino Mega” and the correct COM port is chosen.
14) Enter the Example Code: Copy and paste the below example code into the Arduino IDE’s code editor.

///////////////////////////////////////////////////////////////


// This code demonstrates the application of Ohm's law by measuring


// the voltage drop across each LED to infer the current flowing through them.


// Code Written by: Dr. Satinder Gill


///////////////////////////////////////////////////////////////




int LED1_ADC = 0;  // Iniatilize LED1 ADC Value to be Zero


int LED2_ADC = 0;  // Iniatilize LED1 ADC Value to be Zero


int LED3_ADC = 0;  // Iniatilize LED1 ADC Value to be Zero


int LED4_ADC = 0;  // Iniatilize LED1 ADC Value to be Zero




float LED1_Volt = 0.0; // Initialize LED1 voltage to zero


float LED2_Volt = 0.0; // Initialize LED2 voltage to zero


float LED3_Volt = 0.0; // Initialize LED3 voltage to zero


float LED4_Volt = 0.0; // Initialize LED4 voltage to zero




void setup()


{


Serial.begin(9600); // Enable Serial Communication


pinMode(A0, INPUT); // Set A0 as an input for LED1


pinMode(A1, INPUT); // Set A1 as an input for LED1


pinMode(A2, INPUT); // Set A2 as an input for LED1


pinMode(A3, INPUT); // Set A3 as an input for LED1


}




void loop()


{


LED1_ADC = analogRead(A0); // Read the analog value from pin A0


LED2_ADC = analogRead(A1); // Read the analog value from pin A1


LED3_ADC = analogRead(A2); // Read the analog value from pin A2


LED4_ADC = analogRead(A3); // Read the analog value from pin A3




// Convert the ADC value to a voltage (5V reference voltage and 10-bit ADC resolution)


LED1_Volt = LED1_ADC*(5.0/1024);


LED2_Volt = LED2_ADC*(5.0/1024);


LED3_Volt = LED3_ADC*(5.0/1024);


LED4_Volt = LED4_ADC*(5.0/1024);




// Print the voltage values to the Serial Monitor


Serial.print("LED1 Voltage="); Serial.print(LED1_Volt); Serial.print("V; ");


Serial.print("LED2 Voltage=");Serial.print(LED2_Volt); Serial.print("V; ");


Serial.print("LED3 Voltage=");Serial.print(LED3_Volt); Serial.print("V; ");


Serial.print("LED4 Voltage=");Serial.print(LED4_Volt); Serial.print("V; ");


Serial.println();




delay(1000); // Wait for 1 second before repeating the loop


}

15) Save the Sketch: To save your sketch, go to the “File” tab and select “Save As.” Choose the desired location on your computer and name your file accordingly.
16) Upload the Code: Click “Verify” to check the code for errors, then click “Upload” to send the sketch to the Arduino Mega board.

17) Open the Serial Monitor: Go to the “Tools” menu and select “Serial Monitor.”

18) Observe the Output: Use the Serial Monitor to observe and record the voltage across each resistor.
19) Perform Theoretical Calculations: Apply Ohm’s Law to calculate the expected voltage across each LED.

Submission Instructions:
Prepare a lab report, written as a Word document, that includes the following sections:

• Cover Page: Include the title of the lab, your name, date, and course details.
• Lab Objectives: Clearly state the objectives of the lab.
• Theoretical Calculations: Show your calculations for the expected voltages across each LED using Ohm's Law.
• Circuit Setup Pictures: Include clear pictures of your circuit setup.
• Observed Results: Document the voltage readings displayed on the Serial Monitor.
• Discussion: Analyze and compare your theoretical calculations with the observed results. Discuss any discrepancies and potential reasons for them.

 

observe and record the voltage across each resistor

That gives you the necesary information to do the calculations.

 

You will design a simple circuit to control an LED and measure the relationship between voltage, current, and resistance.

Hi EE,
I read the assignment as the above. measure the relationship between voltage, current

E

 

That gives you the necesary information to do the calculations.

No... I must compare my observations to my theoretical calculations... Meaning that I should have made the theoretical calculations BEFORE any observations or measurements.
Thank you

 

Ohm's Law stated mathematically is

I = V / R

The equation V = I x R is a corollary of Ohm's Law.

If you would like to learn more about the reasons for a series resistance, see this blog:

https://forum.allaboutcircuits.com/ubs/why-you-need-a-series-resistor-when-driving-an-led.1651/

 

Hi EE,
I read the assignment as the above. measure the relationship between voltage, current

E

But I have to compare my observations to my theoretical calculations....

 

In order to solve this mathematically, you need to model the I-V curve of the LED.

 

In order to solve this mathematically, you need to model the I-V curve of the LED.

That's kind of how I feel about it... but this is an intro lab to an intro class and we have not even begun to approach such things... I wish the professor would friggin talk to me.
This lab would have been more appropriate with a second resistor instead of an LED.

Thank you

 

Don't expect your professor to teach you everything you need to know.
Look up Shockley diode equation.

 

• Arduino Mega Board
• 4x Red LEDs
• 1x 100Ω Resistor
• 1x 220Ω Resistor
• 1x 330Ω Resistor
• 1x 1KΩ (1000Ω) Resistor
• USB Cable

Hi EE,
I would suggest you select a RED LED Vfwd from this table.
E

 

Meaning that I should have made the theoretical calculations BEFORE any observations or measurements.

No. You're not following the Step-by-step instructions. Step 18 comes before step 19.