Hi,

In the old days, the "E12" resistor series used to be called the "Preferred" resistance range, which is quite an appropriate name. Each value is about 20% larger than the previous, i.e. 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100 etc. and can continue through many decades (i.e. up to 1 Mohm and above).

It dates from when resistors were made by mixing up a concoction with carbon powder, moulded, and baked in an oven. Then the value of each "resistor" was measured and put in a "bucket" appropriate to its value (give or take 10%). Now, with Carbon (or Metal) Film technology it's possible to manufacture whatever value is required to much higher accuracy, but the basic principle of "percentages" is still very relevant. Generally, you are likely to find that, for example, a 1.2 ohm resistor (which your switching box wouldn't have) to be more useful than 99 ohms (if it has 100 ohms anyway).

Now, there is the E24 series which "fills in the gaps", i.e. 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91 and even the E96 series of "1% tolerance" values. A "resistor box" which provides the Exx series is likely to be more useful than a linear range. Perhaps as many as E96 in the middle of the range (say 100 ohms - 100 kohms), but there are practical limitations with "accurate" values below 10 ohms or above 1 Mohms, so E12 or E6 values may be sufficient in those regions. Even the E3 series (10, 22, 47, 100) can be sufficient for capacitors and inductors, etc..

Of course you don't need to buy one resistor of every value, as hippy indicated earlier, it's possible to combine resistors in series or parallel (not quite as easy to calculate) for example 20k = 10k + 10k and 11k = 22k in parallel with 22k, etc. A good way to use the computational capabilities of a PICaxe.

Cheers, Alan.

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