# Multiplexing led sticks

I'm using led sticks (those plastic capsules with 5 piranha leds, including resistors, powered by 12V, using 50mA) to make a big clock. Since its a 6 digit, 7 segment display clock, this totals 42 led sticks.

First I was thinking of controlling them independently using 6 shift registers, but when I faced the amount of cables in front of me, I started to think about multiplexing. That would meake me save on cable, components and add not much complexity to the software. I've seen some posts talking about the trouble that is multiplexing big 7-segment displays, but no one about using these led sticks, nor what the problem was. Electronically I plan to use two shift registers (one for anodes, one for cathodes), one ULN2003 for sinking current and six BC327 transistors (one for each digit). Does anyone have experience with them? Hope I can build something to test it during the weekend. Worst case scenario is that it flickers or light is to dim, right?

I would think that aside from the higher voltage and current the multiplexing would be the same as for any 7-segment display. Seven 'columns' for the seven segments and one 'row' per digit.

Thanks Johnwasser, itâ€™s the same I was supposing.

Adding to the original question, is it true that a multiplexed system draws less current that one in parallel?

Well, a multiplexed display does not have all segments lit at the same time so it will draw less average current, if you run the displays at the same voltage/current. This means that the lights will appear dimmer. If you light the display one digit at a time the digit drivers will have to deal with 7 times the current of a single segment and the segment drivers will only have to drive one segment at a time.

Segment drivers: 50mA at 12v Digit drivers: 350mA at 12v Power supply: 350mA at 12v

If you run the Arduino off the same power, add some current for that.

Considering that, the ideal solution would be to lower the values of the limiting resistors of the leds, BUT, since they are included into de led stick, this is not possible. Would increasing the supply voltage be a solution or just a way to burn lots of leds? :cold_sweat:

Try it at the nominal voltage first. If you don't get enough brightness you can slightly increase the voltage. That will increase the current, too, so make sure your drivers are rated for both the higher voltage and current. When you pick the parts you might want to allow for a 50% margin in both: get 18V 75mA or better for segments and 18V 555mA or better for digits.