# LED Matrix Question

Discussion in 'LEDs and Optoelectronics' started by xevious809, Jun 21, 2013.

1. ### xevious809

6
0
Jun 21, 2013
I'm going to be making a 30x20 LED matrix (30 long and 20 high) and I don't know what Ohm of resistance to use for the anode columns and cathode rows. I'm going to be using this for an audio spectrum analyzer.

5V input

LED: 5mm blue
3.0-3.4v / 24mA max / 13,000 mcd

Whatever makes it shine the brightest and won't burn out is ideal.

Thanks!

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
It depends on how you're going to drive it.

Presumably you will only power a single LED in each row or column at a time.

Whatever direction this is (let's call it the columns), that 's the one with the resistor. The value you choose is the one which allows the current to be the average current you require multiplied by the number of rows (the other direction).

So assuming 30 x 20, it makes sense to have only a single LED in each 20 LED column lit at once (since that is the shortest dimension). If you want an average 5mA, then the resistor needs to limit the current to 5 * 20 = 100mA per column.

If the voltage is 5V and the LEDs are red, then this might be a value like (5 - 2.3)/0.1 = 27 ohms.

The column drivers will need to be capable of supplying 100mA each.

the row drivers will need to be able to supply a max of 100mA * 30 = 3A each.

If your row or column drivers are bipolar transistors then you may need to recalculate the resistor above to allow for the voltage drop across them. This is unlikely to be necessary for most mosfets (if it is you can simply subtract Rds of both mosfets from the resistance calculated).

Note that the LED current (100mA) is likely to exceed their max continuous current, so you *MUST* ensure that your scanning of the display doesn't freeze up. You can check the datasheet for the max peak current, but even for small LEDs it is often around 1A for short pulses. Note that you will have to scan the display at least 20 (preferably more) times per second.

Depending on your circumstances, it may be easier to create a number of 30x8 displays and stack them (one of them being only 30x4) so that you are effectively creating a 90 x 8 display.

3. ### xevious809

6
0
Jun 21, 2013
LED "Audio Spectrum Analyzer" HELP Needed

I'm slightly new to circuitry and I'm designing a 30x20 (30 Wide, 20 High) LED matrix for this project and I really have no clue what materials I need to make this possible.
I will obviously be multiplexing the lights and am not sure if I should, and by what dimensions, break up the matrix to make it easier to apply to my arduino. (Someone suggested 30x8, 30x8 and 30x4)
I don't know what circuitry components I need, or the resistance needed for the cathode and anode ends of the LED Matrix (LED info down at bottom).
Any direction or guidance for the matrix and even the end product, the audio spectrum analyzer, would be greatly appreciated.

5v Input ~>

LED: 5mm blue
3.0-3.4v / 24mA max / 13,000 mcd

Whatever makes it the brightest without burning up is preferable.

Thanks

11,513
2,651
Nov 17, 2011
5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
You could multiplex the LEDs at up to about 190mA (assuming they are multiplexed into groups of 8). However you may find that you don't need them this bright (that will give approx the same brightness as running them continuously at 24mA).

First determine the constant current which makes them the appropriate brightness, then multiply this by the number of groups, and that's your answer.

(And don't start new threads for the same topic)