Potentiometers or pots are a type of variable resistor. Unlike their fixed resistor cousins, the resistance in a pot can be controlled by the user.
POT Basics
The pots in your kit have two very different resistances; 10K and 500k pots. Pull them out so you can get a sense for what these feel like in your hands. Try turning the dials on each one. Feel how far they move relative to a full circle.
What are POTs? What do they want?
Pots are one of the magical creatures of the electronics world. When we place them in a circuit, we connect one outside leg to the +Ve side of our power supply (top of waterfall), and the other outside leg to ground (bottom of waterfall). The middle leg then outputs a voltage between +Ve volts (9v if battery, 5V if Arduino) ) and ground (0V).
Recall that voltage and current are related to resistance according to relationships defined by Ohms law. So, adjusting resistance alters voltage and current flow. The wiring of the pot is such that the voltage changes are passed out the wiper (middle leg).
How is that possible? We will get into more detail after the video.
POT Representations
The symbol for a pot is based on that of a fixed resistor. Its base is a zigzag line with legs on each end. Added to that is an arrow pointing at the zigzag of the fixed resistor symbol.
The arrow represents the wiper (middle leg). It may help to imagine the whole arrow moving along the fixed resistor symbol from one end to the other. As it moves, the amount of resistance on each side of the arrow changes.
Note that the pin labels would not normally be included, but the resistance would.
Let’s set up a circuit and explore.
Required Parts
- POTs
- 1k fixed resister and chart
- LED (single color)
- hook up wire ( red, black and some other color — maybe yellow )
- wire strippers
- battery pack loaded
- battery clip
Video :: POTentiometer Basics
Slides: Grab a copy
The Circuit
Here is the schematic of the circuit built in the video above.
Note the inclusion of the 470ohm resistor after the LED. This resistor limits the current through the LED and ensures its protected.
Pots are a kind of voltage divider!
As mentioned in the video and the slides, a pot is an example of a voltage divider. Voltage dividers are very important kind of circuit — they are used with many sensors.
I will have to create a voltage divider tutorial but for now, the following will hopefully give you a sense of what is happening.
How is a voltage divider connected?
A single resistor is wired between the outside legs of the pot. The size if this resistor is fixed and stamped on the body of the pot — 10K or 500K for the ones in our kit. The middle leg — called the wiper — is connected to the dial on the pot top ( couldn’t resist — it’s a palindrome!). The wiper cuts the pot’s resistor into two. nAs you turn the dial, you adjust the position of the wiper, changing the proportion of resistance to its left and right. In other words, the wiper divides the internal resistance.
Because voltage is related to resistance ( Ohms’ law) we see the impact of the resistance changes as changes in voltage.
A POT thought-experiment
Let’s think about how electricity flows in this circuit. In particular let’s think about how that 470 ohm resistor protects the LED and impacts current flow.
POT TURNED ALL THE WAY TO POSITIVE
The 470 ohm resistor is necessary when the pot is turned all the way towards the side connected to power +Ve (high side of water fall). In this state, two things happen. First, the resistance between the power supply and the wiper drops to zero (all 10k are on the other side of the wiper). Second, this allows the full voltage of the supply to flow out the wiper. If we did not include the 470ohm resistor the LED would have zero resistance in its circuit and would get full voltage –> that almost always means an immediately dead LED. IF you are lucky it will pop and the cap will shoot off (watch your eyes! — I have actually only seen this once). Most likely it will just melt, possibly making a wimpering pop and hiss sound and then smell really bad.
POT TURNED ALL THE WAY TO NEGATIVE
Now, image we turn the wiper all the way the other way — so that there is 10K ohms between +Ve and wiper. And zero resistance between the wiper and ground. At the wiper the electrical flow has two paths it can follow (ie the circuit branches are in parallel). Current can go through the zero resistance path to the battery ground or it can go through the LED and the 470 ohm resistor. Recall that electricity ALWAYS follows the path of least resistance. The current will all go the zero resistance route and the LED will no longer light up.
Going Further
There are lots of sites with info on pots and voltage dividers. Here are a couple to get you started.
voltage divider this one has a great explanation of voltage dividers.
Sparkfun tut on pots. A goog intro.
Some Khan Academy Theory. This one is WAY deeper than we need to go — but if you are into this sort of thing — go for it. I already regret sharing it a little.