Resistors perform two basic functions in electronics:
These two Resistor features of current limiting and voltage setting are implemented in various ways in electronics as listed below:
The figure below shows how a resistor is used as a current limiter:
The figure below demonstrates how a resistor is used to create a voltage divider capable of providing a dc voltage that is a fraction of the input.
A Photoresistor replacing one of the voltage divider resistors acts as a variable resistor whose resistance decreases with light intensity. When this resistance decreases, the voltage present at the microcontroller’s input increases, eventually reaching a logic HIGH level. Once a logic HIGH is set up, it is then up to the microcontroller’s program to determine the next course of action.
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Each resistor has coloured bands on it, which enable us to see what value of resistance it has. The most common labelling scheme uses four bands: the first band represents the first digit, the second band the second digit, the third band the multiplier (as an exponent of 10) and the fourth band the tolerance (if there is no fourth band, the tolerance is 20 %).
Table: Resistor Colour Codes
| Colour | Significant Figure | Decimal Multiplier | Tolerance (%) |
| Black | 0 | 1 | – |
| Brown | 1 | 10 | ± 1 |
| Red | 2 | 100 | ± 2 |
| Orange | 3 | 1000 | – |
| Yellow | 4 | 10000 | – |
| Green | 5 | 100000 | ± 0.5 |
| Blue | 6 | 1000000 | ± 0.25 |
| Violet | 7 | 10000000 | ± 0.1 |
| Grey | 8 | 100000000 | – |
| White | 9 | 1000000000 | – |
| Gold | – | 0.1 | ± 5 |
| Silver | – | 0.01 | ± 10 |
| No colour | – | – | ± 20 |
Note that, the body colour of the resistor doesn’t carry much meaning except in some cases it may specify the temperature coefficient. But if you find resistors within a circuit that are white/grey or blue in colour, they may be non-flammable or fusible resistors. Care must be taken when doing the replacement of such resistors, don’t substitute ordinary resistors in their place.
On precision resistors, you will find five bands: the first 3 bands are used as significant figures, the fourth band is the multiplier, while a space between the fourth and fifth band that is wider than the others is used to identify the fifth tolerance band.
Another five band labelling scheme that is typically reserved for the military specified resistors has a fifth band reserved for reliability level. The reliability band tells you the percentage change in resistance over time interval e.g. 1000 hours, brown = 1%, red = 0.1%, orange = 0.01%, Yellow = 0.001% .
Surface-mount resistors use either a three-digit or four-digit label. In the three-digit scheme, the first two digits represent significant figures, and the last digit is the multiplier.
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