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Naffah Tarek Lab 1

Page history last edited by zahraa@... 5 years, 6 months ago

2. Controlling the Brightness of LEDs


Question: a. What resistance do you need to limit current to 30 mA (if using red LED) or 25 mA (if using yellow or green)? Be sure to state which color LED you are using. This resistance refers to the total resistance in series with the LED.

Answer: If we have a 5V generator, we will need to use ohm's law (V=RI) to calculate the needed resistor; therefore if we want a current of 30mA, we will need: R=V/I, therefore R=5/(30*10^-3)= 166 Ohm, and for the 25mA, we will need: 5/(25*10^-3)= 200 Ohm. 

R should be R = (Vcc - Vf) / I      -0.5


Question: b. Is the resistance from question (a) a maximum or minimum resistance? That is, in which direction if you change the resistance (higher or lower) would the LED likely fail.

Answer: The resistance of question (a) is the minimal resistance; since if we use a smaller resistance the LED will be damaged much faster and therefore fail, but if we use a higher resistance, we will only to apply a higher voltage at the input in order to turn on the LED.


Question: c. What is the resistance range of the potentiometer?

Answer: The potentiometer ranges between about 0.2 Ohms to about 10 kOhm. 


3. Basic LED Circuit with Switch


Question: a. Does it matter what order the components of your circuit are arranged between power and ground? Why or why not?

Answer: The way the components are ordered does not matter in the idea that which component comes before which component does not change anything, but the polarities of the LED should still be conserved for the component to work properly, contrary to resistors or switches. Since the circuit must be closed for the current to flow, no necessary order exists. 




4. Battery-Powered LED with Switch on Breadboard


Question: a. Using this battery, what is the minimum resistance required for use with your LED?

Answer: Since we were using a 5V generator before we needed a 150 Ohm resistor to keep the current below 25mA. Since we are now using a 9V battery, and want to keep the current below 25mA, we will use Ohm's law once more, and see that we will need a resistance of R=V/I= 5/(25*10^-3). Therefore the resistance should be at least of 360 Ohms to satisfy this case. However 360 Ohms resistors are hard to come by, we could use a 390 resistor ( since we should always use a higher resistor for safety reasons) or connect the variable resistor to the multimeter and regulate it to 360 (with much dexterity) and use it afterwards. 

The correct answer is :



Vtotal = Vresister + Vled

9 volts= Vresister + 2volts

Vresister = 7 volts



7 = 25mA * R

R = 280 Ohms (minimum resistance)





5. Make a Video of Your LED Being Switched On and Off.


Please check the following link:





Comments (1)

zahraa@... said

at 4:34 pm on Jul 6, 2015


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