MAX7219 not quite man enough!

[yesyes]

Ah, yes, of course! Thanks for the correction.
I ordered a few commoned resistor networks with different values and will try this out as soon as everything has arrived. The pin-out of the 2803s and 2981s makes it real easy to add these resistors on a breadboard.

I'll keep this thread updated.
It might take a while cause I have ordered some of the components from China for cost reasons.

 

[yesyes]

OK, I'm happy to report that this works. :lol:

I only had to change the limiting resistors for the LEDs. I hadn't taken into account the voltage drop through the 2803s and 2981. So these are now 22R and 560R for the decimal point. But you would want to recalculate these for your displays anyway.

Also, the load resistors between the MAX and the 2981 were not required but it worked just as well with them.

I have amended the schematics. This is how it actually works.

I took a few pictures of my testing. First few show the circuit on breadboard. Then half the display in action. Then I soldered everything on strip board. The first board is the one with the MAX that I had already made. The second one I made today is the "driver board". On this board you can see why it was a lot easier to use another 2803 as inverter. Only one wire on the whole board. ;-))

**broken link removed**

 

[betwixt]

Very nice! Well done !

I can beat you on size though, my biggest digital clock was just over 4 metres high !

Brian.

 

[yesyes]

Thanks!
They usually say size doesn't matter. But it did matter a lot for this project.. ;-)

4 metres you say? The room where my display will end up isn't even 4 metres in any direction... ;-)
Mine will have 8 numerical digits and a 8x64 LED matrix for text (another 8 MAX7219) ;-))

 

[betwixt]

The big clock was alphanumeric, made from 6' fluorescent tubes. The tube heaters were left running at low power so it would start instantly and the mains was switched to the tubes with triacs. It was for a well known car retailer who thankfully had a short name, the display alternated between the time and "L*X FOR CARS".

Brian.

 

[abidr]

Here is a small trick, invert the digit 3 (ten minutes one) so that Dp is now on top left. Now connect this digit with rest as follows:
Digit 2 (hours) Digit 3 (minutes ten)
d to a
e to b
f to c
a to d
b to e
c to f
g to g
Dp to Dp
now you could blink Dp on digit 2 and 3, and it will look like professional one.

 

[Jasonnp]

I am using a MAX7219 to run an 8x8 matrix but I need a bit more oomph. I tried the method on this page but didn't see much of an increase in brightness, even when running 9VDC in I only see 2.2VDC across the LEDs, any sugestions?

Thanks!

 

[betwixt]

LEDs are essentially constant voltage devices, thats why you don't see it increase. It's the current and the muliplexing time ratio that you need to increase to make them brighter.

Brian.

 

[mvs sarma]

voltage across LED is not the criteria for brightness. increase the ON time and reduce the off time. while the voltage would still be same, brightness would increase.

 

[Jasonnp]

Thanks I understand that, the forward voltage is suposed to be 3 VCD on the LEDs, but besides that is it even possible to increase the on time with the MAX7219, I don't see anything in it's data sheet for setting a scan time or anything similar, I already have the chip set for maximum brightness and I'm using a UDN2981 to drive the LEDs, I'm not using 2803s since I'm still running at 5VDC and the chip can sink more then it can drive, running 8 LEDs is less current then the max current for the chip.

 

[mvs sarma]

Can you please indicate the no of LEDs per segment as a chain,in series ,and number of such chains in parallel. this combination decides the current needed.
now the intensity word has 4 bits
and is given in a table
it is a control on digit drive, the ratio changes from 1/322 ( low brightness ) to 31/32 ( high brightness). This info is in table7 on page 9 of rev3/97 of the MAX7219 datasheet, that i could lay hands ON.

Perhaps the number of digits regidster is already known and you might have implemented.
Like wise
the Iset resistor.
Appears the schematics using 47K is OK as they used 2981 chip for working at higher voltages. as you know the series resistor is calculated after deducting the overall voltage dropped across a set of series LEDs per segment from the input voltage let it be 9V or 12V. The current demand i-- if you think that 20mA is needed, it is for a series cahin.
suppose that your segment has has more such chains in parallel, perhaps we need to multiply 20mA by number of such chains.
for example 2 chains of 34 LEDs
each and these are RED LEDs
we consider 1.4*4=5.6V and suppose only one chain.
the current is 20mA.
the resistor value for a 12V supply could be = (12-5.6)/.02 =6.4/.02

but unless you set the intensity word to full, this calculation may not be valid.


hope my reply helps to resolve. all the best.

 

[Jasonnp]

It is an 8x8 Matrix, the intensity is set for 31/32 the LEDs have a continuous current of 24ma, and peak of 74ma. I have tried Iset resistors as low as 10K, which I know is less then recommended with no change on brightness, I am currently using a 22K

 

[betwixt]

If the 7-segment displays have maxed out and simply can't produce more light, the other option is to use strings of individual super-bright LEDs. They need more voltage than integrated units but that is available anyway. They will produce many times more light level but at the expense of more wiring.

Brian.

 

[mvs sarma]

why not suspect the quality of LEDs used?

 

[betwixt]

I don't think it's an issue of LED quality. Seven-segment LED displays are normally used as panel indicators where the ambient light level is usually low enough that high brightness isn't necessary. In bright light conditions it would be more normal to use an LCD. There are some circumstances though when there is no option but to actually produce more light so LEDs are the only viable option.

I'm working on a project at the moment where an 8 digit display has to be read in less than 20mS and possibly under bright lighting conditions, the display has to be captured in one frame of video so multiplexing may not be possible. The only option is super-bright LEDs chained to make segments of digits. As far as I know there are no super bright 7-segment modules on the market at the moment, at least at affordable prices.

Brian.

 

[stonnedd]

dude , i think this is really a good job. im working in something similar and it would be very helpful for me if you upload the schematic again, i could not download it

thanks

Yonadab

 

[yesyes]

dude , i think this is really a good job. im working in something similar and it would be very helpful for me if you upload the schematic again, i could not download it

thanks

Yonadab

Hi. I'm assuming you mean my schematics.... Here they are.

 

[stonnedd]

dude again me, i got a problem im trying to make works a 512 matrix leds, but when i connect the 7th segment i had a decay in clock signal and all breaks, i made many tests in my circuit, i look when i connect just a wire in the out clock signal in the 6th segment the matrix becomes crazy, i saw your project pictures and i have shorter wires so i cant found the problem, some idea?

 

[yesyes]

Hi. I'm not quite sure what you mean. I'm guessing you are trying to cascade 8 MAX7219, each with a 8x8 LED matrix? By "segment" you mean one MAX?

- - - Updated - - -

One thing comes to mind.... make sure you connected those 100nF decouple capacitors as close as possible to the MAX ICs and that you use one for each MAX, not added them up and put them somewhere else.... ;-)

 

[stonnedd]

scheme shows 2 capacitos do i need the both of them per max or just one?!