What you need to know about resistors and resistance.
Resistors are a key component to every electronic project. They limit current flow and can also be used to lower voltage levels within circuits. Resistance is measured in Ohm's(Ω) Unlike diodes, resistors are not directional. They can be placed in a circuit in any direction. When designing circuits and drawing schematics we use a jagged line to represent a resistor. Every resistor has coloured bands on it. These bands represent the resistance value of the resistor. If you are confused about which direction to read them from, there is usually a gold, red, brown or silver band that indicates the tolerance. This side is not the side you want to read from. You want to start on the opposite side of the tolerance band. If a resistor has three bands, not including the tolerance band, then the third band is known as the multiplier band. If we have four bands, not including the tolerance band then the fourth band represents the multiplier value. Here's a list of the band colours and what they represent.
If your resistor only has three bands minus the tolerance one then discount the band 3 column. Let's look at an example. If I have a resistor with the colour code bands, red, green, red and gold. We have three bands minus the tolerance which means we only use columns one and two, multiplier and tolerance. I have 25 ohms x 100 ohms = 2500 ohms with a tolerance of 5%. What if I have four bands? Let's pretend I have red, brown, black, black and silver. We are going to use all columns. I have 210 ohms x 1 ohms = 210ohm with a tolerance of 10%.
Now we know how to find out how much resistance a resistor has, but how do we find out how much resistance we need for a particular circuit. We have an equation by Mr. Ohm himself.
R = V / I
R represents our final resistance value. V is our voltage drop. For example if we have 9 volts and want to drop it to 1v we take 1 from 9 and get 8. 8 will be our V figure. Now we need I. I represents the amount of amps we need. If we have an Led that only uses 40 mila-Amps then .04 is our I figure. If we slot these in to the formula we get:
R = 8 / 0.04 = 200 ohms
There is one more important part when calculating resistance. We have 9 volts and drop it to 1v. Those 8 volts don't disappear, they get turned into heat. Our resistor also has to be able to take the heat and not explode or vaporize itself. This is called resistor wattage. We use another of Mr. Ohms laws to calculate it.
P = I²R
P represents the power in watts, I represents current in Amps and R represents our resistance value. We need .04 amps for our circuit. .04 squared is .0016. If we multiply .0016 by 200 ohms we get .32 watts. We need a resistor that can withstand .32 watts.
There are many websites on the internet that have resistance calculators and colour band decoders for you to use for free which definitely helps speed things up than the old pen and paper way.