Electronics Basics

RESISTORS

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.

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 using color code. 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.

Fixed Resistor

Variable Resistor or Potentiometer

CAPACITORS

Capacitors have 3 primary functions:

1. To store a charge, much like 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.

2. 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.

3. To counteract inductive reactance in order to create a “tuned circuit”.

4. 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.

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.

SWITCHES

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 though 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.

DIODES

There are different types of diodes. The most common in small electronics is the signal diode shown in fig.2 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

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.

OPTICS

A Photo-Resistor is 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 “Opto 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 as used as a switch. Say you have a high-powered motor you want to control with your computer. You would use an opto isolator in-between your computer and motor (along with other proper control circuitry) so that you computer can control your motor with out being directly linked to the motor incase something should go wrong the motor end, nothing will happen to your computer because it is “isolated” via the opto isolator!

TRANSISTORS

Transistors are semiconductor devices with three leads. For those that don’t know “leads” simply refer to the pins or wires coming from a device or component. A very small current or voltage at one lead can control a much larger current flowing through the other two leads. This type of action turns a transistor into a mechanical switch. That’s pretty much the basic function, it’s a switch. Most integrated chips or IC’s as they are commonly called chips with several or many thousands of transistors inside. Computer processors are built up from millions of transistors. However, switching is not all a transistor can do, they can also be used as amplifiers.

The most basic transistor is probably a bipolar transistor and these transistors are made of three layers, which are the Emitter, Base and Collector.

Iif you wanted to control a relay with your computer you would need a transistor of this sort. The transistor would allow the very small current produced by your computer would go to the transistor’s base and emitter, which would allow the larger current on the emitter collector to flow to the relay.

Transistors of this sort have a few key features in common for instance the base – emitter junction and a diode will not conduct until the forward voltage exceeds 0.6 volts.

Too much current will cause a transistor to become hot and stop functioning. If a transistor is hot to the touch, disconnect the power it! Some project will force transistors to become hot and so proper heat sinks are connected to these transistors. Transistor meant for heavier loads will come with a metal tab on the back for mounting to a heat sink. A heat sink is black metal that is designed to dissipate the heat coming from these “power” transistors. As they relate to robotics heavy-duty motor controls (for motors 12 volt and up) you might see power transistors like this with heat sinks attached to them. Also power supplies often have these sort of transistors. Transistors of this sort are often MOSFET’s, which stands for Metal Oxide Semiconductor Field-Effect Transistor, or MOSFET for short. These transistor schematics symbols are picture to the right in the pervious schematics listing picture. MOSFET allow a few volts to switch or amplify many amperes at very fast speeds, this makes them perfect for control of larger motors.

The middle transistor schematics symbols are JFET’s or Junction Field-Effect Transistors. JFET’s can be used as amplifiers or switches just like all other transistors but they have a built in high resistance on their Gates (JFET’s don’t have an emitter, base and collector they have a source, gate and drain pins) so the have little effect on external components connect to their gates. If a JFET were used in the above relay circuit this would mean the computer would be even safer from voltage spikes. JFET’s are not often used for high power jobs.

INTERGRATED CIRCUITS

Integrated Circuits or IC’s for short reference are small electronics circuits contained inside a silicon chip. For instance an IC’s might have for build in transistor with 2 diodes and 2 resistors….this may never be displaced in the schematics symbols but that are build inside by tiny layers of silicon. IC’s are what make smaller electronics possible and what drive you computer; there are millions of different types of IC’s. At their most common core an IC is built up from basic transistors.

Integrated Circuits some in many different packages, the most common by far is the “dip” which stands for Dual In-line Package, in other words you have two rows of pins of a chip like this. Most IC’s will come with a little index marker, which will indicate which is pin 1, the marker looks like a little indented hole. DIP can range in pin count from 4 to 64. Most IC’s of this nature are clearly marked with the part number on them, such as 7404 or 555. Some schematic symbols for IC’s will look like the actual dip package with a box and the pins labeled and going to the other components of the circuit. This is the case with many schematics with 555 chips in them. However, most of the time the chip is cut up and parts of it are placed though out the circuit schematics. The cut up parts are the gates of the chips. The gates are like individual circuits inside the IC.

RELAYS and SPEAKERS

There are many different types of relays, but they all do the same things, which is to act like a switch. Inside a relay you’ll find a coil (as pictured) and an arrangement of contacts which provide different types of switching, such as SPST, SPDT and DPDT or Double Pole Double Through.

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