I am in the process of ordering bits to display numbers for my current project, but my knowledge of electronics is a bit limited and I need a bit of help choosing parts.
I am going to use 4x 4 digit white LED displays that I plan on driving with two of these MAX7219CNG chips to lower the number of pins I need to use on my arduino board
The chip says common cathode. The display I want is this type https://www.sparkfun.com/products/10931 which is common anode. Does this mean that they will work together (cathode hooks up to anode and vice versa for the other pins) or do I have to get a “common anode” driver instead ? I am confused !
The 7219 chips are an excellent way to drive lots of LEDs. But you do have to have a common cathode array of LEDs for them to work… I’ll draw a sketch in a minute to explain it.
The 7219s are pretty easy to use, but you could also get a whole display board like https://www.sparkfun.com/products/11646 which has a driver chip on board You can also find board with 7219 on them
Thanks Jonathan. I’ll have a look at some of other displays. I saw the mounted ones but they are a bit too expensive for this project. I can get 4 displays for the same price as one of those if I wire them myself ! it says each 7219 can drive 8 digits (so two displays for each). The displays on their own only around £1.50 each
On the other hand, if you have that well-known mother of invention driving … you can do it if you have to: specifically if the LED array has nothing except LEDs (specifically, no resistors in it). Basically you consider the common anode display to be a differently shaped common cathode display:
Just showing 3 LEDs per ‘digit’ … you can see that an N-digit M-segment common cathode LED array is just the same as an M-digit N-segment common anode display. (In drawing, M=2, N=3, top right drawn as 2 digit 3 segment CA, bottom drawn as 3 digit 2 segment CC.) If all the LEDs are the same (like on an LED wall), it just shows your image inverted. If the LEDs are different (like a 7-sgement array), then you get a scrambled image. If the bundles are RGB triples, you get wrong colours.
The 7219 can display “as digits” where you send 4 bits to drive a digit of 7 segments; it can also use a mode where it drives them “as pixels” where you send 1 bit per LED to switch it on and off. If you use this second mode you’ll get it to work. Here’s someone who wrote it up with good explanation [x]
Note that for an LED wall there are some subtle effects which show as the ‘italics’ effect which you might care about if you have many. For 4 digits of 7-segment this is immaterial.
Hope that’s helpful too.
All best,
Jonathan.
PS: just to directly answer your question: “cathode hooks up to anode and vice versa?” – categorically no, that won’t light up. You put cathodes where cathodes should be, anodes where anodes should be. You’ll get a scrambled display. Then work on your software until it shows the right thing.
Ok thanks, that makes sense. I have found this display which is bigger but that might actually make things easier for me. https://www.sparkfun.com/products/11409. Plan is to drill or laser etch an array of tiny holes into the front plate and have the display sit behind it so the panel appears seamless (a bit like the pulsing power light on the aluminium macbooks). A bigger display would be easier to manage if I go for drilling.
I’d rather have non-italics but couldn’t find any white ones unfortunately
… sounds elegant, hope to see it when it’s made. Jonathan.
PS the red ones are only $1.50!
PPS. In case you didn’t see it, when using multiple 7219 it’s usual to have them cascaded so they behave like a single bigger display, datasheet gives good diagram.
PPS one way to make a nice looking stainless steel thing with lots of tiny holes would be to get a solder paste stencil made, or a custom Gobo filter.
I have a GPS clock built on Vero with 6 x 1" displays that uses the 7219, great chip and really flexible, especially the ability to dim the display as its a great way to limit the current used (setBrightness)…
Will try to remember it on Thursday so you can have a look / play
