A review of how to read resistors and of what the equation and schematic looked like was discussed today.

Vs = Batter Voltage

Vf = LED Voltage

I = Current of LED

In general students have been using a 9V battery for Vs. Where it gets a little more difficult is when students needed to select their LED's.

The data sheet on the package was a little difficult to read but we eventually found the maximum forward voltage for the LED's that the students selected.

I modified the original equation a little since LED1 and LED2's voltages should be the same we can just simply multiply by 2:

$\large R_1=\frac{V_s-2V_f}{I}$

So if I have a red LED's whose maximum voltage is 2.3V I can simply set up my problem like so.

First I define my variables:

$\large R_1=\frac{V_s-2V_f}{I}$ - $V_s=9$
- $V_f=2.3$
- I=20mA or since we need amps I= .02A

$\large R_1=\frac{9-2(2.3)}{.02}$

$\large R_1=\frac{9-4.6}{.02}$

*Note: We must follow the order of operations here!*

$\large R_1=\frac{4.4}{.02}$

*Note: This looks daunting but since the denominator is in hundredths lets multiply the denominator and numerator by 100!*

$\large R_1=\frac{4.4 * 100}{.02 * 100}$

*Note: Now just simply move the decimal two places to the right in both the numerator and denominator!*

$\large R_1=\frac{440}{2}$

*Note: Ahhh much better!*

$R_1=220 \Omega$

Now we have found our resistor value! We will need to see the chart below to determine the color bands of our resistor.

Since we need a resistor that is $220 \Omega$ we know that the first number relates to the first band of our resistor which 2 = red. Next we look at the second number and this is our second band of our resistor which is 2 which is red.

Recall that the third band of our resistor is reserved for our multiplier.

22 multiplied by what will give us 220?

I hope you said 10!

If you look at your chart a multiplier of 10 gives us brown for our third color.

Therefore the resistor in this example would be: Red Red Brown!

Color | First Band | Second Band | Multiplier | Tolerance |

Black | 0 | 0 | 1 | |

Brown | 1 | 1 | 10 | |

Red | 2 | 2 | 100 | |

Orange | 3 | 3 | 1000 | |

Yellow | 4 | 4 | 10000 | |

Green | 5 | 5 | 100000 | |

Blue | 6 | 6 | 1000000 | |

Violet | 7 | 7 | 10000000 | |

Gray | 8 | 8 | 100000000 | |

White | 9 | 9 | 1000000000 | |

Gold | ±5% | |||

Silver | ±10% |