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.
The two schematics symbols for basic bipolar transistors are shown to the far left with the labels NPN and PNP. A small current going to the emitter and base of these transistors will cause a much larger emitter and collector current to flow. For example if 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. See the schematics below:
The emitter is the transistor pin that is connected to the ground. The Base is the pin going to the 1K resistor and the other pin is the collector. A data pin from your computer's printer port could connect to other end of the 1K resistor (marked C). You would also need a ground from the computer printer port to complete this circuit. I want to point out the relay in the picture was drawn by hand fully illustrate the pin out from the bottom of the relay. Do you see the diode that connects across the relay coil? Do you know why it's there? The diode protects the transistor and computer from voltage spike that might come back from the activation of 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 previous 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.
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. The functions of these gates are too numerous and complex to go into right now but below the most common schematics symbols for them are shown.
Relays and Speakers
Relays are electromagnetic switch. Small current flows through a coil in the relay, which creates a magnetic field that pulls the one, switch contact towards or away from another. Relays are pretty common; in fact when you signed online you probably heard the click of a relay right before your modem dialed up you internet connections! 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.
Wires and Power
Here we have a picture of what some wire connections in a schematic look like. The one with the black dot in the middle is the only one that shows connections. The first symbol shows two wire connections over top one another, but they are not connected. The middle symbol is not connected and shows the horizontal wire curving to better illustrate to schematic reads like you that no connections is intended.
These are common schematic symbols associated with power. The one with a +V is showing you where the positive voltage goes. You'll most often see the voltage value next to the V, such as 12V for 12 volts.
The symbol below the positive is the negative or ground line schematic symbol. If you are working on a project such as a robot where you might have many different circuits working together, you'll want to make sure your grounds are connected together.
The middle symbol shows the common schematic symbol for batteries, with a +indicating where the positive end needs to go. If you notice it shows a big line, with a small line, and a big line and another small line. This is indicating the proper polarity of battery placement. Say you need 6 volts for a circuit. You can take 4 AA-batteries (1.5 volts each) and connect them in a line with the negative of the first connecting to the positive of the next and so forth...the batteries then add up to create 6 volts. If you try to connect all the positive ends and all the negative ends of the batteries together you are going to have fire hazard on your hands!
The schematics symbols labeled as F1, is a fuse. This would be connected in between the positive side of your power source and the positive side of your circuit. The idea is that a fuse will blow before your batteries or circuit does. You do not always need this protection but it might be a good idea with an AC projects or projects with high voltage motors.
I hope this tutorial will give all who need it an introduction to electronics. I'll try to teach you about circuit schematics, basic component functions and uses, Multimeter use, and all other info that a beginner needs to know.
While this tutorial is no substitute for taking formal courses in electronics, here you can get an overview of the subject and familiarize yourself with the basic concepts. You'll also find links to the best electronics tutorial websites, many with advanced subject material to help you learn the more complex electronics concepts.
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