## Thursday, March 14, 2013

### Ah! Electronics!

Today we lots of discussions on how the electronics were going to be put into the Alebrijes. There were many discussions and questions that were asked by students. We looked at their original concept sketches and then students quickly found interesting solutions to their design problem.

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:
• $V_s=9$
• $V_f=2.3$
• I=20mA or since we need amps I= .02A
Now we just substitute the numbers into the variables of our equation:

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

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

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