Output devices come literally in all manner of shapes sizes ad functions.
Whatever produces an output is an output device really.
That's a huge definition, I mean when you think about it, an electric chair is an output device, but honestly, I'm going to try an narrow down my range of output devices to ones that you're likely to want to be using.
Light emitting devices
Light emitting devices, can really be broken down into a few different sorts,
Fluorescent light bulbs, (tube lights like in offices, CFL lights [energy saving bulbs in your home], I'll also include neon bulbs in this definition.)
Fluorescent bulbs are in general high voltage, low current devices, even the CFL bulbs that go into your house are high voltage (there is circuitry inside there that generates the high voltage). neon tubes are high voltage, and the tube lights actually work the same as CFL lights, except they are long and straight, instead of having been coiled up into a compact shape.
Florescent lights have just two states, on or off, you cannot alter the brightness of a tube.
Next in light emitting devices is the incandescent light bulb,
The incandescent light bulb, light bulb was invented by a man named Joseph Swan, a long time ago. The basic principal is that voltage is passed through a very high resistive and very thin wire, the wire heats up to be white hot, and it's so hot that it generates light. incandescent lights come in a variety of shapes and sizes, from the ones that go in your house, right down to the ones that fit inside pocket torches.
Incandescent light bulbs have a variable output, if you've ever used a light dimmer or seen a torch with mostly run down battteries, you already knew that though, incandescent light bulbs are a resistive load, that it more or less fixed (though does change as it heats and cools).
If you look at the resistors lesson, I covered ohms law, V/R = I
and the power laws V * I = P
So if you reduce the voltage, and the resistive load stays the same, then less current flows, if there is less voltage and less current, then there is less power in the bulb, less power in the bulb means that the filament (tiny coil of restive wire inside), doesn't heat up as much, and therefore glows less brightly.
and finally LEDs, Light emitting diodes are the last type of light emitting output device that we'll consider.
the fact that they are called diodes should make you realise that they have a positive and negative leg, an anode and a cathode, or a polarity (however you want to say it).
They won't conduct at all when put into a circuit one way, but will not only conduct, but conduct and produce light when connected in a forward bias.
LEDs are available in a vast array of voltages, shapes, sizes and outputs, some LEDs, don't even emit visible light, they emit infra-red light instead (like in our TV controller).
You can vary their brightness either by turning down the voltage a little or using a clever trick called pulse width modulation.
Basically, what you're doing here is turning the LED fully on, then turning it fully off, then back on again, then off again.
But you do this so fast that your brain can't actually see if turning on or off, it just see a light at half power.
By varying the ratio of on and off time you can vary the brightness.
This little trick relies on a phenomenon called persistence of vision. which is the same trick that TV's use and film cinemas (reel to reel) etc use to show a smooth moving pictures, when what we're actually seeing is a series of still shots.
Having looked at light bulbs, and included incandescent light bulbs (that get so hot they glow brightly) it seems to make sense to include heaters.
Heaters produce heat, just like incandescent light bulbs they are actually just restive loads, and the voltage and current in them generate heat, unlike light bulbs, they don't get so hot as to glow incandescently.
And so we move on to our last basic output device type, motors.
Now you might think of a motor as just something that turns, and you'd be right, there are motors that turn, but a motor is basically just something that moves something.
For that reason yes, Axial motors are included.
If you want something to Turn, you connect it to a motor, and you switch it on. I'm not going to go into motors, and how they are constructed in detail, but the basic principal is that there is a magnet, and a coil of wire, when electricity is applied to a coil of wire it becomes a magnet, and is attracted to that magnet inside. so the shaft spins to that the north field of the coil of wire magnet spins to face the south face of the permanent magnet. as it spins the electrical connection is broken, and the coil is connected backwards, so that north face appears somewhere else and spins round as it's attracted to that south face.
If you want the motor to spin the other way, you simply reverse the terminals.
I'm not going to draw pictures, if you want to see the inside of a motor, do a Google image search.
Stepper motors work in the same way, except there are more magnets and more coils of wire, which allows for many more coils to be activated to turn the motor, then the one next to it can be actuated. allowing for fine steps of motion, (most stepper motors I believe are further geared down, until there is a standard 1.8degrees of movement for each coil energisation.
stepper motors tend to have at least 4, or 6 wires for controlling the different groups of magnets inside them, and really need to be controlled with a micro controller, (you can't just hook them up to a battery.
Servos are the third type of mechanical "turny"type motor I'm going to talk about, inside a servo is a little controller board, and a motor, the motor is geared down and connected to a shaft that has a little arm mounted on top (usually), but also connected to this arm is a variable resistor (rotary potentiometer) that the device uses to know where the arm is pointed to.
Servos generally have three wires, a red and black wire to connect them to a power supply, then either a white or yellow wire to connect them to a controller. Servos are controlled using pulse width modulation.
The next group of motors that I'll be considering, most people don't even count as motors, because they make a sound, so they call them sounders.
Well to me, a motor is something that moves.
Sound is created by the movement of air molecules, so sounders, are moving air molecules, they, well, move. Fair enough it's not round and round movement, but they are still motors.
Buzzers are surely the most annoying component by far, that buzzing sound is horrible.
buzzers are basically tiny electromagnets that raise a small piece of metal inside, then let go of it, where a spring returns it to where it ones was, it slaps against the iron core of the coil that raises it, and against the bottom of the buzzer creating that horrible sound.
Piezo electric sounders are a little different from buzzers, they work by having a tiny piece of crystal at the centre of a large coin sized metal disc, when electricity flows through the crystal it vibrates (with a known frequency for a given voltage)
The metal disc is glued tightly to the small crystal, and the crystal shakes the whole disk, enabling it to move lots of air and produce lots of sound. (well relatively at least).
And finally Speakers, these are large basket shaped objects, a cone is attached to a coil of wire. As a signal is attached to that wire, the magnet that the coil of wire is entrapped in reacts with the magnetism induced in the coil of wire, causing the coil of wire to move either into or out of the magnet. as the coil is attached to the cone (made of paper/kevlar/plastic/Mylar/carbon fibre/aluminium) the coile moving causes the cone move to, the cone moving enables lots and lots of air to be moved by the speaker.
Whatever your circuits that you want to build does, you'll want to make some sort of output. those were the outputs that you could see, or feel, or hear.
There are two further type of output,
You can't see it, (at least not without measuring tools, but it's definitely an output, a digital output, might be logging data from a simple analogy circuit. you might be transmitting data over the internet, or indeed you may be creating a web page as your output. perhaps you're output is wifi, or bluetooth. USB, or a simple serial connection.
I just mentioned Wifi and bluetooth, perhaps they should have gone into this section.
Another type of output that you can't see is radio, like you listen to in your car.
Your project may output electromagnetic waves that enable you to be broadcast and heard either in your local area, or all over the world! Though you may need to check what kind of restrictions exist where you are as to what you can broadcast, how powerfully and on what frequency.