We'll finish up by introducing another part that's in your kit bag. This is the potentiometer (sometimes also called a pot because the word potentiometer is just terribly long)
Recall oh so many hours ago, when we talked about having a magic resistor that we could change from 0 ohms to infinite ohms and used that to think of how resistance changed LED brightness? Well, that isn't such an imaginary thing after all, in fact they are quite common. Potentiometers are resistor that are adjustable with a knob. We will talk about potentiometers more in detail in a future tutorial so consider this a light introduction!
Potentiometers, like resistors, have an Ohm value. For example, this potetiometer is a 2Kohm (its printed up top). Potentiometers have three pins, two 'outer' and one 'middle' pin. The middle pin is sometimes called the wiper.
You can see why its called the wiper by opening up the pot, its literally like a windshield wiper! The black-brown stuff that goes 3/4 around the circle is the resistor material, the resistance from one end to the other is say 2Kohm for this guy. As the wiper (the triple-fingered thing) moves from one end to another, the resistance between that pin and the right or left pin changes, the closer the wiper is to the side pin, the smaller the resistance. When the pot is turned all the way to the left the resistance between the left pin and middle pin is zero ohms and the resistance between the middle pin and the right pin is 2Kohms (or whatever the maximum is of that potentiometer. When the pot is turned all the way to the right, its the opposite.
The resistance between the two outer pins is always the same. The resistance between the middle pin and the left or right pin changes!
For the 2Kohm potentiometer here, if the knob is right in the middle, what is the resistance between the middle pin and the left pin?
In the middle, it will be 1/2 of the maximum, 1Kohm
What is the resistance between the middle pin and the right pin?
The schematic symbol for a potentiometer looks like this, its kinda like there is a resistor, and then the arrow pointing in the middle is the wiper. The little arrow on the left indicates which way the wiper moves when potentiometer turns clockwise (its not terribly important, if you get it wrong on a breadboard just turn the pot around.
Of course, we've just learned so much about using resistors to adjust the current going through an LED, we can use the knob of the potentiometer as a physical way to control the LED.
Use a 10K potentiometer for this excersise, find one that has 103 printed on it (this is the same as 10 (first two digits) with 3 zeros afterwards = 10,000).
Note that we are connecting to the wiper and one end, not to both ends. Also, we have a 100 ohm resistor between the potentiometer and the LED.
Please try to build this circuit, verify that the LED dims and brightens when the potentiometer is turned.
The 100 ohms gets added to the resistance of the potentiometer!
If connecting the 100 ohm resistor like in the diagram means the resistances are added, what is the total resistance when the 10K potentiometer is turned all the way 'down'?
When 'turned down' the potentiometer resistance between the middle pin and the left pin is zero ohms. 0 + 100 = 100 ohms total.
What is the total resistance when the 10K potentiometer is turned all the way 'up'?
When the potentiometer is turned all the way up, the resistance is 10Kohms = 10,000 ohms. adding 100 ohms makes it 10,100 ohms.
And if its in the middle?
1/2 or 10K is 5K, so 5,000 ohms + 100 ohms = 5100 ohms.
Why do we even have that 100 ohms? Can't we just adjust the potentiometer to get whatever resistance we want? Yes, but think about what would happen if we didn't have the 100 ohm resistor and we turned the potentiometer down…the resistance would be zero! Zero resistance is the same as no resistance, and we know that having no resistance is very bad for an LED because there is nothing to limit the current flowing through the LED. For that reason we have the additional 100 ohm resistor. This keeps the resistance from ever going under 100 ohms.
Last Quick Quiz!
How much current is flowing through the LED if when the pot is turned down? The forward voltage of the LEDs are 2.2V.
(5 - 2.2V) / 100 ohm = 28 milliamps.
What about when the potentiometer is turned all the way 'up'?
(5V - 2.2V) / 10100 ohm = 0.28 milliamps.
Whew, that was a long and very math-intensive lesson. We'll get back to doing more with software and fun blinky in future tutorials, but we hope that this tutorial helped you understand some of the details of how to wisely use LEDs!