In this instructable rather than give you a set of instructions you can follow to make something I am going to try to give you an introduction to basic electricity and how transistors work so you can understand some of the things you might try to do.
Some atoms have electrons that are tightly bound to their orbits and some atoms have rather loose electrons – we call these free electrons. Free electrons can if given enough “push” jump from one atom to another. This movement of electrons is what we call ELECTRICITY.
So a material made from atoms that have very few electrons that don't move is called an insulator and a material that is made up of atoms with lots of free electrons is called a conductor.
Atomic particles are often are charged particles, either negatively or positively charged (some are neutral and have no charge at all).
ELECTRONS have a NEGATIVE charge, remember that because it will be important in what comes next.
It is the same with atomic charges like charges repel unlike charges attract – so electrons repel each other because they are all negatively charged and they are repelled by any other negative charge such as the negative pole of a battery.
Electrons individually move very slowly, about 8.4 cm/hour, but because when an electron hops onto the next atom it repels another off that atom and so on down the line the effect of flowing electricity seem instant. A bit like having a hose pipe full of marbles and pushing another one in. The end marble falls out instantly but the new marble will take some time to get to the other end.
We can use the flow of electron to do work for us. They can heat up thin wires to make heat or light, operate electronic circuits etc.
Voltage Units Volts symbol V
Voltage is the force pushing the electrons along, generally provided by the battery, the bigger the voltage the harder they are pushed. It takes about 10,000 volts to make electrons jump about an inch, (25 mm), of dry air.
Current Units Amps Symbol I (French for Intensity the original French word for current)
Current is a measure of the number of electrons flowing past and particular point in the conductor. The larger the current the more energy we can extract from the electrons. This is why static electricity, say produced by rubbing a balloon on your jumper or getting out of your car on a dry day, is relatively safe although it has a voltage measured in tens of thousands of volts the current levels are very low.
Resistance Units Ohms Symbol R - used to be the Greek letter omega.
This is as the name suggests a measure of how hard the electrons find it is to flow through the conductor.
A law connects these measurements called OHMS law:
Where V = Volts
I= Current in Amps
R = Resistance in Ohms
V=I x R
or I=V / R
or R= V / I
This diagram is a good way to remember it.
Scientists were aware of the conducting properties of different materials and also knew that there were a group of materials that were neither good conductors or good insulators. Imaginatively they called these semiconductors.
It was discovered that mixing various materials together could produce semiconductors that had lots of free electrons or very few free electrons but lots of empty space electrons could inhabit.
The semiconductor with lots of free electrons appeared to be negatively charged because of the excess of free electrons.
The semiconductor with few free electrons had lots of space that free electrons could move into and so appeared positively charged.
These materials were called N type semiconductor and P type semiconductor. I leave it to you to work out which is which.
A diode is an electronic component that can conduct electricity in one direction only. This is useful for several reasons that are beyond the scope of this instructable but we should understand how it works so we know how to use it and it's near relation the transistor.
Putting a bit of P type and a bit of N type material together and connecting a battery resulted in some strange results. Connected as in the diagram P type to the negative pole and N type to the positive pole didn't do very much as the free electrons in the N type material were attracted to the positive pole leaving a relatively empty space at the junction of the two bits of semiconductor. This area is called a depletion zone for this reason.
As a result there aren't any free electrons available to flow over the junction and no electrical current flows.
It was discovered that a voltage of as little as 0.6 volts would make them cross the junction into the P type material, they could then freely pass through to the positive pole of the battery.
Connected in this way electricity flows and yet the other way round the electricity is blocked. These semiconductor diodes, in the form of crystal detectors, formed a vital part of early radio sets.
After a while scientists tried connecting a semiconductor sandwich, N type material, P type material and another N type material – perhaps just to see what happened.
Connecting a battery to the N type material didn't do very much because the free electrons at the positive end were attracted to the positive pole of the battery leaving a depletion zone at the junction of the P type material.
There were few free electrons in the P type material anyway and no charge to attract electrons from the negative end across the junction.
BUT when a low positive voltage was connected to the P type material in the middle a lot happened.
Electrons from the N type material were attracted across the junction into the P layer. This layer was very thin and when the electrons get in there they strongly attracted by the large positive voltage at the N type end, up to 99% of the electrons go right through and only 1% go to the smaller voltage connected to the P type layer.
So most of the large current passes through the transistor from N type material to N type material but controlled by the small voltage applied to the P type material in the middle. A bit like turning on and off a tap.
These three layers were called the Emitter because it appeared to emit electrons, the Collector because it collected most of the electrons and the Base in the middle because when they first assembled this configuration they put the middle layer on the bottom and rested the other two bits of material on top of it. (I said scientists were full of imagination!)
So what you ask? Well what this means is that if there is no voltage on the base then no electricity can flow through the transistor. If there is a small voltage there – at least 0.6 volts – then electricity can begin to flow through from emitter to collector. A small flow of current out of the base is controlling a much larger current flow between the emitter and the collector. This allows us to control a big machine with a very small electronic device.
This device was called a transistor. It required low voltages to work and produced little heat.
For much of its time in our digital world a transistor is now used as a switch and is either switched off or switched on by the current at the base. However if that base current is turned on slowly then the output current will rise slowly the output will be a copy of the input but a much larger current because it comes from a higher voltage and current source. In this form the transistor can be used as an amplifier.
To make a NPN transistor conduct the base much be more then 0.6 volts more positive than the emitter this attracts the negative electrons across the junction. To turn a NPN transistor fully on (we call this saturated because no more current can flow through it) the base should be around 1 volt or more positive than the emitter.
To know how much voltage and current your transistor can pass you need to look at the data sheet for that device. http://ww.rapidonline.com is an electronic supplier that have many data sheets freely available on their site.
ALL electronics depends on the fact that electrons are negatively charged and will be repelled by another negative charge. At some time in the distant past scientists decided for no good reason that electricity flowed from positive to negative. This has become known as conventional current flow. Now we know that actually electrons flow from negative to positive. This is known as electron flow.