It’s easy to see how the current flows through a circuit that has only one component, like one LED connected to the battery. If it’s a 3 Volt battery, then there’s 3 Volts across the LED also.

But what if there are two or three LEDs? How does the voltage look across each one? What if the LEDs are different sizes? Does it matter how you hook them up, meaning does one way make the LED last monger or glow brighter? Let’s take a look at the difference between series and parallel connections.

Each electrical component is connected so that there’s only one option for the current to flow. There’s no branches or alternate routes for the electricity… it’s only got one way to move through the circuit. When you add more electrical components, like motors or LEDs to this circuit, the overall resistance in the circuit decreases since there’s only one path for the current.

Each device, like a motor or LED, has its own branch lines in a parallel circuit, which means that the electricity has many different ways that it can travel along the circuit lines. When scientists and engineers draw electrical diagrams, they put one electric component on each branch, even if in reality there’s more than one when you actually build the thing.

Current and charge aren’t quite the same thing. Current is rate that charge goes through a circuit (just like acceleration is the rate of change in velocity… acceleration and velocity aren’t the same thing either, but they are related). Remember, charge doesn’t get used up by electrical components like LEDs or resistors.

One more thing to note about the difference between series and parallel circuits is that in a parallel circuit, the current in each branch can be different, but they all add up to be the same everywhere once you reduce the branches into a single branch. Just like the water analogy, when you connect the main hose into five different smaller hoses, the sum of all five is going to equal flow through the main hose.

Now let’s take a look at how Ohm’s Law and Power work together:

Physics equations are pretty useless unless you really understand how to apply them… that’s when the real magic happens. Here’s an example of how to apply Ohm’s law intelligently and easily.

We’re about to jump into making a lot of really cool experiments with different electrical components, but before we do, let’s recap what we’ve covered so you’re sure not to miss any important concepts.

It’s easy to get sucked into doing math and equations all day without understanding how it applies to the real world. Let’s take a look at how to actually hook up series and parallel circuits in everyday life, and how they are different, and when to use each one…

Although you can’t see electricity, you can certainly detect its effects – a buzzer sounding, a light flashing, a motor turning… all of these happen because of electricity. Which is why electricity experiments are among the most frustrating. You can’t always tell where the problem is in a circuit that refuses to work right.

We’re going to outline the different electronic components (resistors, capacitors, diodes, transistors, etc) so you get a better feel for how to use them in a circuit. While we’re not going to spend time on why each of these parts work (which is a topic best reserved for college courses), we are going to tackle how to use them to get your circuit to do what you want. The steps to building several different electronics projects are outlined very carefully so you can really understand this incredible micro-world.

In this video, you’ll learn how to identify each electronic component.  You’ll also learn how to use a breadboard to quickly build circuits that can be easily changed.  Plus, you’ll learn how to make sure you don’t damage your components.

Before you use a breadboard, you need to know how the “holes” in it connect to each other.  Once you get this, they’re easy to use, but until you understand their secret, they can be totally confusing.  Be sure to pay attention to this part, and it will make things a lot easier.  Once you have this down, you’ll wire up a few simple circuits on the breadboard just to try out your new knowledge.

Which part is which? Click here to access a reference sheet so you can tell which resistor is which.

Click here to go to your next lesson on Series Switches.

Switches are a lot of fun, they allow us to control when a circuit is on and when it’s off. In this experiment we’ll see how two switches can be connected in series to control when a single device is on or off.

We’ve seen how switches can be used to turn stuff on and off, now let’s take a look at how a switch in different locations can turn the same thing on or off. A lot like a doorbell with a button at the front door and a button at the back door.

Click here to go to your next lesson on Light Actuated Circuit.

In this circuit, we’re going to use a special kind of resistor, called a CdS photocell to detect light and dark.  When light is shined on the photocell, the LED will light up.  When it is dark, the LED goes out.  And with just a little light, the LED is dim.  Remember the explanation of how a transistor works?  We talked about having a small voltage (or current) control a larger one, kind of like turning the knob on a light switch dimmer in your house?  That’s what this circuit is doing.

The photocell is a kind of resistor that changes it’s resistance depending on how light or dark it is.  In this circuit, when it is light, the photocell delivers more current to the base lead of the transistor.   When this happens, the transistor allows more voltage to flow from the emitter lead to the collector lead, which in turn lights up the LED.  One resistor are simply used to reduce the amount of current that goes into the transistor (so it doesn’t get too much current)  if the photocell has a really low resistance because of how much light is on it.  The other one is called a pull-down resistor.  Think of it like a door closer spring for electricity.  A door closer closes the door when you let go of it (instead of leaving the door to sway in the wind).  A pull-down resistor makes sure that when the transistor is “off”, it will “spring” toward a connection to the negative side of the battery (a pull-up makes it spring toward the positive).

Click here to go to your next lesson on Light De-actuated Circuit.

Make sure you’ve already made the Light Actuated Circuit before starting this project!

Photoresistors (also called CdS photocells) are made of a material that reacts with light, very similar to solar cells.  When light hits the material, it knocks a few electrons loose. When you hook up the cell to a circuit, the electrons now have a place to go, and electricity flows through your wires. You’ll notice your CdS cell works when you shine a light on it from either the front side or the back side. If you want to use a phototransistor, make a note as to the frequency of light it’s been tuned to – some will only work with IR light (like your remote control or sunlight).

In this circuit, the LED is actuated only when the photoresistor is dark. If you want a faster response to your light, you can substitute a phototransistor for the photoresistor (CdS cell) and adjust the value of the 1-kOhm resistor (change it lower or higher, or use a potentiometer) to control the sensitivity.

This is basically the same as the light-actuated circuit.  The difference is that the transistor is connected to control power to the LED in the opposite way as the light-actuated circuit.  So, as there is more light on the photocell (and the base lead gets more current), the voltage to the transistor is reduced.

Click here for Unit 14 (Lesson 1) schematics.

Notice what is the same in the circuit, and what is different.

Click here to go to your next lesson on How to Read Schematics.

Do you remember the first time you tried to read a map? There were all those weird symbols and curving lines that you had to figure out before you could get anywhere. Electric circuits are kind of the same way… people use schematic diagrams to write down how their circuit is wired so others can build it, too.

This Flashing Circuit used to be a real ‘wowser!’ with students before LEDs become commonplace (around 1995). You’re going to build a circuit that has a control knob that will allow you to set the flash speed of the LED. You can try different LEDs or mini-lamps to see what kind of an effect you get. Are you ready?

NPN and PNP transistors are similar in that when current is applied to the base, electricity flows through them.  But, they way they are used is different.  NPN transistors are often used to control whether a circuit is completed by connecting it to ground or not, where PNP control the positive current going into a device (or portion of a circuit).  NPN transistors are often used where larger currents need to be controlled, because it’s easier for a transistor to control the ground side of a circuit than the plus power side of it.

So, why does the LED flash?  Remember, a capacitor is like a storage tank for electricity.  You fill it up, then empty it out.  But, it takes time to fill up and empty.  This circuit uses the time it takes to fill and empty as a delay for turning the LED on and off.  How fast it fills up depends on the value of the resistor that is connected to it.  We’re using a variable resistor, so we can adjust how fast it fills up, and thus adjust the flash rate.

Click here for Unit 14 (Lesson 1) schematics.

Click here to go to your next lesson on Audible Probes.

Resistors look like candy-striped hot dogs. Their job is to limit current to keep sensitive electronics from being overloaded. If you break open a resistor, you’ll find a pile of graphite. If you have a digital multimeter, draw a line on a sheet of paper with a graphite pencil, and place one probe near the end of the line. You can measure the change in resistance along the line with your other multimeter probe!

Make sure you’ve made the Light Flasher before starting this circuit. This circuit, the Audible Light Probe, is actually a very sensitive circuit that will emit all sorts of sounds reminiscent of junior high school boys locker rooms. The frequency from the speaker will change as the light intensity changes. One of the neat features about this circuit is that it will allow you to test different transistors (both PNP and NPN) to see (hear) the changes. You can also play with the capacitor and resistor values to change the range.

Click here for Unit 14 (Lesson 1) schematics.

TIP: If your tones won’t stop, or are too high to hear, try operating your circuit in a very dark room before adding the light. Can you get your speaker to *click*?

Click here to go to your next lesson on Lie Detectors.

Lie Detectors are electronic circuits that are able to measure your skin’s resistance. When you sweat (or if your skin is wet), the resistance is different than if it’s dry.

However since most people don’t sweat when they lie, this type of detector isn’t the most reliable type of detector around, but it’s one of the simplest to create. We’re going to build one from simple electronic components like resistors, capacitors, and transistors.

Our lie detector uses a speaker that changes pitch depending on the resistance of your skin – it’s much more entertaining than blinking an LED on or off. You can think of this circuit as more of a skin humidity indicator. Are you ready?