Electronic Circuits and Projects Forum

Thanks for both your replies. My futurlec order just arrived and I've got some 74HC595s and a matrix. I assume I can use these for the anodes fine.

For the cathodes, the datasheet suggests they can sink or source 25mA, so could I use another 74HC595 as a column driver? Or would transistors be a better solution?

I'm working on a project with an LED matrix that requires alot of power (each matrix element is 4 LEDs). I am doing exactly what 3v0 suggested... 74HC595s. They are chainable to a disgusting number of outputs and fairly fast (~20MHz, IIRC).

If you need to switch alot of power, throw in some Darlington Transistor Arrays - ULN2068B.

Originally Posted by edeca

I'm sure I've seen somewhere an LED matrix with minimum pins. I was going to pick up one of these:

LED Matrix - LEDM88G

Which looks perfect for use with Nigel's tutorial. However I'm sure I've seen a few really good circuits that use less port pins by implementing row/column driver ICs.

I've also seen Mike's excellent PWM version of a similar thing, but I assume that would need all the pins too.

Can anybody suggest what other components/ICs I should buy at the same time so that I can try it?

This may be of use:

http://www.opencircuits.com/PointLess_LED_Array

uses a high and low side driver, but high pinout pic, so no port expansion.
Think it is the same led array you just got.

Thanks, they are two useful links. However the PointLess one sure uses a lot of pins, surely the PIC could drive the array without the external ICs!

I guess my question really was: are the 74HC595s suitable for sinking & sourcing which would be a 6 pin interface to the 8x8 array.. Or am I going to have to use one 74HC595 to drive and 8 transistors which would use 11 pins?

I'm slightly confused how the 74HC595 sinks current rather than sources it, i.e. how it becomes an "input" rather than an "output".

Originally Posted by edeca

Thanks, they are two useful links. However the PointLess one sure uses a lot of pins, surely the PIC could drive the array without the external ICs!

I guess my question really was: are the 74HC595s suitable for sinking & sourcing which would be a 6 pin interface to the 8x8 array.. Or am I going to have to use one 74HC595 to drive and 8 transistors which would use 11 pins?

I'm slightly confused how the 74HC595 sinks current rather than sources it, i.e. how it becomes an "input" rather than an "output".

the pointless does use a lot of pins, but current output is still limited. the dirvers are I think good for 500 ma per pin, with a total device current of a couple of times that. it is a very conservative design, but could be made very bright

Originally Posted by edeca

I'm slightly confused how the 74HC595 sinks current rather than sources it, i.e. how it becomes an "input" rather than an "output".

Ah, so it appears the OE pin on the 74HC595 controls this. I'll have a play and see if I can get normal output onto 8 LEDs first (via sinking & sourcing), then play with the 8x8 matrix.

Originally Posted by edeca

I guess my question really was: are the 74HC595s suitable for sinking & sourcing which would be a 6 pin interface to the 8x8 array.. Or am I going to have to use one 74HC595 to drive and 8 transistors which would use 11 pins?

Directly driving columns and rows from the 74HC595 outputs at a 1/8th or 12.5% duty cycle will not provide enough 'peak' current for a full brightness display.

If 10 ma forward current through an individual LED provides full brightness then you would need approximately 80 ma forward current or 'peak' current at your 1/8th duty cycle to provide 10 ma 'average' current and to perceive the same brightness level.

You need to provide 80 ma 'peak' current per LED at 1/8th duty cycle and you need to provide for up to 8 LEDs lighted at one time in a column or row, depending on which way you're multiplexing the display. That's 640 ma 'peak' current on your column or row drivers and that's why I use N-FET (sinking) or P-FET (sourcing) drivers on the rows in my design examples.

Good luck with your project.

Mike