#### 2. Controlling the Brightness of LEDs

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.

**Equations**

V=Vresistor + Vled

Vresistor=I*R

**Known variables**

Vled=2.5V

V=5V

**My work**

V=Vresistor + Vled

5V = Vresistor + 2.5V

Vresister = 5V-2.5V

Vresister= 2.5 V

Vresistor=I*R

2.5V= 25mA *R

Convert mA to A à 25mA/1000 = .025A

2.5V/.025A= 100 ohm

** **

**Answer**

Using a green LED and 5V of power, the resistance needed to limit current to 25mA is 100 ohm.

-0.5: V_{LED}=2.2V.

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.

Minimum resistance. Let's rearrange the above equation so that it looks like this:

2.5V/100 ohm = .025A

Here, we can see if we were to decrease the resistance (100 ohm), the current (.025A) would increase. This is not desirable since .025A is the *maximum* continuous forward voltage rating. Therefore, 100 ohm represents the *minimum* resistance necessary to ensure the LED is operating within safe limits.

**✔**

c. What is the resistance range of the potentiometer?

0 – ~10K ohm. The maximum resistance is actually slightly less than 10K ohms (9.856K ohms) due to the fact that the potentiometer wasn’t perfectly manufactured.

**✔**

#### 3. Basic LED Circuit with Switch

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

No, the arrangement of the components doesn't matter. The button simply closes the loop when depressed, and breaks the loop when idle. It doesn't matter *where *the loop is closed.

**✔**

#### 4. Battery-Powered LED with Switch on Breadboard

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

**Equations**

V=Vresistor + Vled

Vresistor=I*R

**Known variables**

Vled=2.5V

V=9V

**My work**

9V = Vresistor + 2.5V

Vresister = 9V-2.5V

Vresister= 6.5 V

6.5V= 25mA *R

Convert mA to A à 25mA/1000 = .025A

6.5V/.025A= 260 ohm

** **

**Answer**

Using a green LED and 9V of power, the minimum resistance needed to keep the current less than or equal to 25mA is 260 ohm.

**✔**

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

https://www.youtube.com/watch?v=x94ryJukUIo

## Comments (1)

## xinyi xie said

at 4:22 pm on Jul 11, 2014

Good job!!

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