I am working on a project concerning the creation of a LED matrix (4 x 6) but without using a micro-controller. It will be powered by a 13V battery and the leds will be controlled by a switch. My question is about the resistors. Should I use a resistor for each led or a resistor for each line and if the latter should it be placed at the start or the end of each line ?
If you are controlling each LED independently then each should have its own resistor. If you are controlling a whole line/string at a time then using a single resistor per line may work out well enough.
Position does not matter as long as the resistor is in series with each LED or string. The only issue that might change this is if there is a chance you could short an LED line to ground and cause an over current condition, in that case you might want to place the resistor(s) on the + supply side.