Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.
Yet another useful package from Conrad (NL #418714) – a set of 390 resistors from 10 Ω through 1 MΩ:
Resistors come in specific values and are based on a logarithmic range, i.e. you’ll see them organized as “E6″, “E12″, or “E24″, meaning that they are split up into 6, 12, or 24 values per decade, respectively.
Here’s some info about what’s in that above box:
This is actually mostly a subset of the E6 range (which is 10, 15, 22, 33, 47, 68) – see this Wikipedia article about preferred numbers for how and why things are organized that way.
The point is that you can never have enough resistors, which can probably be considered to be the most elementary components in electronics. Whether for limiting the current through a LED or creating a voltage divider, these things just tend to get used all over the place.
But what if you need a different value? Well, that’s often trivial: by using two resistors, either in series or in parallel, it’s often possible to get real close to the value you’re after.
The formula for two resistors in series is simply the sum of their values:
Rseries = R1 + R2
The formula for two resistors in parallel is slightly more complicated:
Rparallel = (R1 x R2) / (R1 + R2)
(this can easily be explained using Ohm’s law, I’ll be happy to write a post about this if you’re interested)
Here’s an online calculator which will find the proper values – although I recommend doing the math yourself, at least initially, because it will help you get a good grasp of how resistors work together.