Most all electronics components generate, store, control or switch electricity in some way. A circuit is made of various components that act together to produce a desired effect. There is a lot to learn about electronics and you may never truly understand everything about the subject. But when you break down even the most complex circuits you'll find the same basic building blocks. You'll also find the same basic principles and methods repeated in most electronic devices. The first REALLY big thing we need to learn is that there are two types of current: DC, which stands for Direct Current and AC, which stands for Alternating Current. DC is what you get from a battery and flows in only one direction. AC on the other hand is alternating current, alternating because it changes direction a number of times per second, which is specified as frequency. The AC we get from the power company has a frequency of 60 cycles per second. We will not go into details of the intricacies of AC and DC, just know that most electronics that we will work will use DC voltage. Some may use AC as power that will in turn be made in to DC current, such as a power supply. DC you can play around with more and you only need worry about ruining a component or two. For circuits that use AC power you better know what you are doing.
The next REALLY big thing we need to learn is Ohm's Law. Ohm's Law is designed to allow us to specify and measure the quantity and power of a direct current. Below you will find a drawing of Ohm's Law.
- V or sometimes E stands for Voltage which is a potential difference.
- I is Current, which is the flow of electrons. Current is the quantity of voltage passing a given point. The unit of current is the AMP or ampere.
- R stands for Resistance. Conductors like resistors and capacitors resist to some degree the flow of current. The unit of resistance is the OHM.
- P stands for Power. The work performed by an electrical current is called power. The unit of power is the WATT. The power of a direct current is its voltage multiplied by its current.
And now after a taste of Ohm's law let us move onto resistors and capacitors. Resistors come in all types of packages but they all do the something, which is to limit current. Resistors are pretty easy to keep track of because they are color-coded. They have three strips on each one that give the resistance value. Most also have a forth band to indicate the tolerance of the resistor. You can connect resistors in any which way, they have no positive or negative ends.
The way this system works is easy, say you have a resistor with the colors Orange, Blue and Red. You find your first number, which are 3. You find your next number for blue, which is 6, and then you multiply by your last number. So we take 36 X 100 and we get 3600 ohms. Now something else that is very important to know is that 1,000 ohm is equal to 1K. Likewise 1,000,000 ohm is equal to 1M. M and K are what you are more often to look up when buying a
Resistor rather than the pure ohm value. So our resistor with the colors of Orange, Blue and Red is a 3.6K resistor. Pretty simple right?
Below are some common ways resistors are depicted in schematics. The most common way is with the use of R1. In schematics you will usually find the value of the resistor next to it. R3 and R4 are variable resistors. These are resistors, which you can change the resistance of. Variable resistors are called potentiometer. They are used to adjust the volume of radios, brightness of a lamp or adjust the sensitivity of a sensor. These resistors are not color coded, but you'll most often find a stamped labeled on the bottom or inner ring giving the value of pots as they are called. Another version of the variable resistor is the trimmer. These are potentiometers with a plastic thumbwheel or slot for a screwdriver and are designed for occasional adjustments. R5 is a photo resistor, which is sensitive to light and gives a higher or lower resistance value depending on the level of light.
You can use resistors in series to make a higher value resistor. You simple add the value of each resistor in series. So if you need a 42K resistor and you have a 20K and a 22K then you connect them in series and you have your 42K resistor.
Capacitors have 3 primary functions:
- To store a charge, much likes a battery. These capacitors are normally electrolytic and are used in situations like power supplies where a fluctuating DC voltage needs to be smoothed, or, have the ripple taken out.
- A capacitor is used to block DC while allowing AC to pass through such as in an audio amplifier where we are passing the audio signal through from one stage to the next.
- To counteract inductive reactance in order to create a "tuned circuit".
- A cap can also be used as a spike filtering, which is slightly different than smoothing an AC signal. The term for this purpose is "bypass cap" in case anyone out there was wondering about that one.
When power gets to them they hold a charge right away, but will eventually discharge if left alone or you can discharge a capacitor by hitting both of it's leads together or connect a resistor between both leads. Capacitors have different levels, which are specified in farads. Below are common schematics symbols for capacitors and common farad ratings.
1-Farad = 1F, 1-Microfarad = 1mF or uF = .000001F, 1-Picofarad = 1pF - .000000000001F
C1 shows a normal fixed capacitor, these you can connect in like resistors. C2 shows one that is polarized, this means you must connect it's positive lead the most positive connection point in it's placement. With polarized capacitors you'll mind that they are marked with either a plus to show the positive side or with a negative to show the negative side. C3 shows a variable capacitor, I have yet to see one of these in a circuit.
Capacitors are not color-coded but they do have a numbering system that tells you what their value is. This can be tricky to find the value of an odd capacitor but most of the time you'll see number like this: 151K the first and second digits are the capacitors value. For the third number find it's multiplier value on the chart to the left and simple multiply. So 151K is a 150pF capacitor. You might also see a notation like this, EG 104 or 104K both of which are .1uf caps. Caps, of course here is short for capacitor.
The letter tells you the tolerance of the cap; you can look up the tolerance on this chart. A note I want to add to this capacitor section is that sometimes you may see the letter R on a cap, which would signify a decimal point. So if you see 2R2 that would equal 2.2 (pF or uF). My best advice is to keep your capacitors well organized and with their packaging if you are not confident you can distinguish one capacitor from another. An important thing to take notice of is that capacitors DO NOT add in series like resistors, just the opposite, two 1mfd capacitors in series equal 0.5 mfd.
I know, you are saying I know what a switch is. Well we are going to learn about them anyway. First let's look at S1; this is a Normally Open push button switch. NO is short for Normally Open. This would be a good simple way to add a sensor for a robot when it hits a wall. If this switch hit a wall it would close and complete the circuit and current would travel through it. S2 is a NC or Normally Closed switch. When a NC switch is hit is opens the circuit and so no current runs though it while it is depressed.
S3 is a Single Pole, Single Throw switch. You flip it and power is on...this is like a normal wall switch. S4 is a Single Pole, Double Throw. With this type of switch it is possible to switch between two different devices.
The simplified explanation of how a diode works is to tell you that it allows electricity to flow in one direct (forward) and blocks it in the opposite direction (reverse). To the left is a picture of how diodes appear in schematics. Diodes will have a band on them; this band indicated the cathode end. The other end is the anode. There are different types of diodes. The most common in small electronics is the signal diode and can be used to transform low current from AC to DC, multiply voltage, perform logic and absorb voltage spikes created by other devices. You also have your zener diodes that can function like a voltage sensitive switch. You also have your
LED's, which stand for Light Emitting Diodes, which we will discuss later. And you have your photodiode, which detects light, this also to be addressed later. Circuit schematics will always give you the name of the diode used, it will be something like 1N4003 or 1N914...this is how you will look them up to order or buy them at a local electronics store. An important note on all forms of diodes is that they are not like resistors; they have positive and negative ends. Current will flow when a diode's Anode end is more positive than it's Cathode end.
LED's and OPTICS
LED stands for Light Emitting Diode. LED's convert an electrical current directly into light. The light emitted by an LED is directly proportional to current through the LED. This means LED's are ideal for transmission of information. However, LED's need direct line of sight and they usually have a short range of light emission. Because LED's are current dependent they need to be protected from excessive current with a resistor. For most robotic applications with power sources of around 9 volts I find that a 1K resistor will always to the trick. A normal schematics symbol for a LED is pictured below along with a drawing of what an actual LED looks like. You'll notice one lead is longer than the other, in most cases a longer lead indicates that it is the positive lead. It might be easy to dismiss the LED and assume all it does is light up, but LED's are very interesting electronics components and have some surprising functions. For instance if you take a Jumbo LED (of any color) and connect your Multimeter up to it for voltage reading and point the LED towards a very bright light source you'll get a voltage reading. In direct sunlight you can get a reading of 1.5 volts from a normal LED! Some people even think it is possible to make a color sensor using a LED like this, however after much experimenting with this I have found LED's to not be suitable for use as a sensor. But it is interesting and you should not dismiss the LED as something that just lights up.
An LED can stop current flow like any normal diode. The schematic to the left shows a voltage indicator. You can apply a positive and negative voltage to the points labeled "input voltage." If you switch the order of positive and negative the opposite LED will light.
D2 looks a little bit like a LED, but is it? Nope it's a Photo- Diode. It is designed to detect light, which is why there is a little arrow going into the schematic symbol. Q1 is a Photo-Transistor, it's a bit like a Photo-Diode in the fact that it detects light waves, however photo-transistors, like transistor are designed to be like a fast switch, so for light wave communications or use as light or infrared sensors a phototransistor is what you want! The most common form of phototransistor is the NPN collector and emitter transistor with no base lead. Light or photons entering the base (which is the inside of the photo-transistor) replace the base - emitter current of normal transistors. See Transistors below for more details on this. R1 is a resistor, a Photo-Resistor! It acts like a variable resistor because it changes resistance as the light level changes. They have no positive or negative end and there resistance is very high (up to millions of ohms) when no light is present. These are great for simple robotics eye to find the darkest or brightest point in a room or detect the difference between day and night. You'll find photo resistor in many common security sensor and toys, including the Furby.
Now what is the schematics symbol that looks like a box with a LED and phototransistor inside? It is an "Opt isolator" which means simply optical isolator. It really is not much more than a box with a Light Emitting Diode and a phototransistor inside. This would not be used as a sensor. It was used as a switch. Say you have a high-powered motor you want to control with your computer. You would use an opt isolator in-between your computer and motor (along with other proper control circuitry) so that you computer can control your motor without being directly linked to the motor in case something should go wrong the motor end, nothing will happen to your computer because it is "isolated" via the opt isolator!
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