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BASIC ELECTRONICS TUTORIAL FOR ROBOTS

Power Resistors Potentiometers Capacitors

Transistors MOSFETs Diodes Relays

Electronics can at first seem extremely complicated to understand and learn. One look at a circuit board with all those little blinky LED's and black chips and unidentifiable circle pointy things can make anyone quit before starting.

But actually electronics can be much simpler than you think. Learning electronics is more like learning a foreign language alphabet. At first glance it is all a bunch of squiggles. But actually each letter has its own pronounciation and its own rules of use. And certain combinations of letters in a certain order form a word of some meaning. And a combination of words forms a sentence. This is the same for a circuit board. Each tiny component, such as a resistor or capacitor or transistor, has special rules and abilities. Combining a few into a circuit can create interesting effects. Combine a bunch of unrelated circuits together and suddenly you have a robot. So your first step would just to be to learn and understand the smallest of the components. Once there you can learn about combining them. Just like learning a foreign alphabet, no?

Ok first a quick crash course in electron physics.
All electronics is designed to manipulate a flow of electrons. Electrons have mass and volume so you can almost think of electrons in circuits as water flowing through plumbing. The analogy is amazingly helpful if you think about it. Also note, the more electrons you have in one place, the higher the voltage. The more electrons moving together, the higher the current. The same as with water.


POWER
Power is simply the energy required to do something. If you are moving a large amount of electrons, and moving them through something that is resistant of that movement, power is used. Power is voltage times current. Power is also voltage squared divided by resistance.
P = I * V
P = (V^2)/2

Ground and Source
Source is the positive part of your circuit. The plus end of your battery would go here. Ground is the negative node of your circuit. When you design your circuit, imagine a flow of electrons coming from the source, and heading to the ground. A quick note, in reality electrons move from gound to source. The confusion has historical reasons I dont want to get in to. But just know this fact, and pretend electrons move from source to ground.
Now think of this as water. Water flows down the easiest quickest path between these two points. More resistance to flow, less will flow.



RESISTORS
These do exactly what they say. They resist the flow of electrons. These are necessary for several reasons:
- they can control how much current goes down each wire
- they can control power usage
- they can control voltages (since current, resistance, and voltage are interrelated)

The last point is important as it is the basis of Ohm's law, V=IR. Voltage = Current x Resistance. For example, suppose you take a resistor and connect the two ends of a battery with it. You know that your battery is 9V (or whatever) and you know the resistor is 3Kohm (determined by the color stripes on the resistor), so 9V divided by 3Kohm is .003amps (3 milliamps). So why is this information useful? Well now that you know the current, you can determine other useful things such as power. P=IV. You will notice that if you increase resistance, you decrease current. If you decrease current, you decrease power use. Put a 1ohm resistor between the battery and it will get so hot it could burn because of the power use. Use a 100Kohm resistor and almost no power at all will be used.
So about determining the value of a resistor, all resistors have the value labled on them. You will notice colored stripes on the resistor. Each stripe means a certain number. This has been explained a billion times online already so I won't, just google search 'resistor color tutorial.'
Click for a quick resistor color code reference chart.



POTENTIOMETERS (VARIABLE RESISTORS)
There are also neat things such as variable resistors (called potentiometers) that can change resistance at your will. Quite often you can change the resistance by simply turning a dial. A great way for experimenters to tweak, no? Most robot sensors are actually resistors that change resistance based on some environmental effect. Photoresistors are perfect examples. A change in resistance changes the current crossing the sensor. A change in current causes a change in voltage - voltage is what computers measure to detect the environment.

So how can you use resistors for you robot? Well LED's require resistors in series or they burn out. Voltage Amplifiers will need them. Almost every circuit you build will require them for some particular reason. Even if you do not put a resistor in the circuit, theoretically your circuit will still have resistance as all wires carry resistance.



CAPACITORS
Now suppose you want to control how the current in your circuit changes (or not changes) over time. Now why would you? Well radio signals require very fast current changes. Robot motors cause current fluctuations in your circuit which you need to control. What do you do when batteries cannot supply current as fast as you circuit drains them? How do you prevent sudden current spikes that could fry your robot circuitry? The solution to this is capacitors.

Capacitors are somewhat complex in theory, but most people can get by on the basics which I will explain here. Capacitors are like electron storage banks. If your circuit is running low, it will deliver electrons to your circuit. If your circuit is in access (such as when your robot motors are turned off), it will store electrons. In our water analogy, think of this as a water tank with water always flowing in, but with drainage valves opening and closing. Since capacitors take time to charge, and time to discharge, they can also be used for timing circuits. Timing circuits can be used to generate signals such as PWM or be used to turn on/off motors in solar powered BEAM robots.
Quick note, some capacitors are polarized, meaning current can only flow one direction through them. If a capacitor has a lead that is longer than the other, assume the longer lead must always connect to positive.

How do capicitors charge over time? This Capacitor Charge Curve Chart should help. The discharge rate would be the direct inverse. Theoretically (as made obvious by the graph) a capacitor can never be fully charged or discharged, but in reality this is never the case.


So how can you use capacitors in your robot?
Power surge/drainage management.
The problem with using robot components that drain a large amount of power is sometimes your battery cannot handle the high drain rate. Motors and servos being perfect examples. This would cause a system wide voltage drop, often reseting your microcontroller, or at least causing it to not work properly. Just a side note, it is bad to use the same power source for both your batteries and your motors. So don't do it.
Or suppose your robot motors are not operating at it's full potential because the battery cannot supply enough current, the capacitor will make up for it. The solution is to place a large electrolytic capacitor between the source and ground of your power source. Get a capacitor that is rated at least twice the voltage you expect to go through it. Have it rated at 1mF-10mF for every amp required. For example, if your 20V motors will use 3 amps, use a 3mF-30mF 50V rated capacitor. Exactly how much will depend on how often you expect your motor to change speed and direction, as well as momentum of what you are actuating. Just note that if your capacitor is too large, it make take a long time to charge up when you first turn your robot on. If it is too small, it will drain of electrons and your circuit will be left with a deficit. It is also bad to allow a large capacitor to remain fully charged when you turn off your robot. Things could accidently short and fry, such as curious kitties that get too close. So use a simple power on LED in your motor circuit to drain the capacitor after your robot is turned off. If your capacitor is not rated properly for voltage, then can explode with smoke. Fortunately they do not overheat if given excessive amounts of current. So just make sure your capacitor is rated higher than your highest expected.

Capacitors can also be used to prevent power spikes that could potentially fry circuitry. Next to any on/off switch or anything that that could affect power suddenly should have a capacitor across it.

Capacitors can eliminate switch bouncing. When you flip a mechanical switch, the switch actually bounces several times within a microsecond range. Normally this is too small of a time for anyone to care (or even notice), but note that a microcontroller can take hundreds of readings in a single microsecond. So if your robot was counting the number of times a switch is flipped, a single flip can count as dozens. So how do you stop this? Use a small ceramic capacitor! Just experiment until you find the power capacitance value.

Capacitors can improve efficiency and longevity of electric motors up to 100%. Place a small ceramic capacitor of like 10uF across the two leads of your motor. This works really well with el-cheap-o motors. Not much effect with high-end expensive motors however. These capacitors will also signficantly reduce EMI (Electro Magnetic Interference) and system noise too.


TRANSISTORS
Transistors are probably the post important electrical component created. After all computers tend to have billions of them. But not just computers, almost everything today that is remotely electrical probably has them. Your car, your washing machine, your microwave. The list goes on.

But transistors are somewhat complicated in theory, and especially calculation. Plus it is my firm opinion that they are outdated technology. So for robotics what you should use is something similar to a transistor called a MOSFET, or Metal Oxide Semiconducting Field-Effect Transistor.


MOSFETs
The operational theory required to use a MOSFET is not much different from a simple mechanical on/off switch. But instead of flipping a switch, you apply a binary signal to it straight from your microcontroller. Apply a 0 (for zero volts) to the Gate (or Base) and it is turned off, apply a 1 (for usually 5V or more) and it is turned on. It is somewhat like a water spicket. By turning a small valve, you can control huge flows of water through a pipe.

If a Transistor is turned on, current will flow from the Collector to the Emitter. If the MOSFET is turned on, current will flow from the Drain to the Source. Yes, the names are counter intuitive. What makes transistors more complex is that Collector to Emitter current is not just dependent on Base voltage but also on Base current. All quite complicated . . . then you got Darlington Pairs which are just crazy and stuff . . . MOSFET's are just simple on and off. There is however MOSFETS designed for various applications. But usually a PWM optimized MOSFET would be what you want.

Another note on MOSFETS, they are more efficient with higher Gate voltages. So you probably do not want to just apply a simple binary 5V. Instead you should amplify it with say an operational amplifier or even better, a MOSFET Driver IC. All are quite cheap and easy to implement. I highly recommend amplifying the Gate voltage because it would save on battery power, and significantly reduce overheating. Which reminds me of another point . . .

DONT FORGET TO PUT A HEAT SINK ON ALL POWER MOSFETS!!!!!!!
They heat up pretty quick and burn you when you touch them. A typical MOSFET is rated for about 300F, but its quite easy for them to reach that temperature.

So what can you use a transistor for? Well it is required for an H-Bridge. Also, the concept works for Photoresistors and DC Motor Braking.

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DIODES
--diode tutorial coming soon!--

--zener diodes coming soon too--


RELAYS
--relay tutorial coming soon!--