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Thanks for that. I've been using the Arduino forums for a while but I guess my question is not Arduino related enough.. But I asked there anyway...

I had not heard of EDAboards. I've now registered and even found a thread about almost exactly the problem I'm trying to solve...

So thanks again ! ;-)

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Give us a clue yesyes. I'm sure there are a few who can help here.

I was mainly asking in general as I have recently started my electronics hobby again after 15 years break.

But I do have a current problem. Wasn't sure if I should ask here as it's not really astro related. (although the project is a fancy clock for my yet to be built observatory) ;-)

But you asked for it... :(

I'm looking for a solution to drive 3 inch 7-segment common cathode LED displays with a MAX7219 LED driver IC.

The displays require 9V 20mA per segment. The 20mA is well within the MAX7219's max segment current of 40mA.

However, the MAX only provides 5V to the segments. I've tried this and the only segment that comes on is the decimal point (through a zener diode) as that requires only 4V.

I've done some searches on the internet and I've found several ways to approach this problem but they all don't really apply to my situation.

I've found a MAXIM application note that describes driving displays with higher voltage but they require common anode displays. I have already bought the common cathode displays and wouldn't want to spend more money on common anode displays.

Using the MAX7219/7221 to Drive Higher Voltage or Current - Maxim

The other thing I found is in the MAX7219 datasheet on page 12. It uses the MAX394 quad analogue switch IC to drive a MOSFET per digit. 2 problems with that solution are that a) it seems to be for higher current, not higher voltage (supply voltage still at 5V) and :) I can only find the MAX394 as SMD and I don't have SMD soldering equipment.


I've been thinking about this for a while but just can't work it out. If this was logic outputs I'd probably find a way. But since both anode and cathode of each segment are connected to the MAX directly in a matrix kind of way I just can't get my head around this.

The best thing I could come up with is to use a matrix of 64 opto-isolators. But that would be slightly exaggerated, I hope.

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The problem, as you have found is that these devices are TTL, hence the 5V threshold. I think the only way you'll drive the display is using some form of mosfet / transistor driver to switch a separate supply voltage.

A quick google came up with this project

Electronics 7-segment LED Clock - Page 7 - bit-tech.net Forums

this clock uses TD62873 chip to drive the LED's, this chip seems to run from a 22v supply ! Whilst this may not be ideal for your project, (I haven't read through the complete thread ) I hope it might help with possible alternative approach if you follwo my drift

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Thanks for that. I did a lot of googling before even asking. I don't think I came across this one but quite a few similar ones. The problem is that most circuits are for common anode displays. I have common cathode. The other thing is that the MAX7219 is multiplexing the displays. So both sides of the segments/diodes go to the MAX.

I've got some answers on the other forums I tried. Looks like the TD62873 / UDN2981 (they seem to be the same thing) is the way to go. Schematics of how I think it might work attached.

MAX7219 high voltage LEDs (Schematic).pdf

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  • 2 weeks later...

Oh, by the way, I did get this working the other day. Attached is the schematic of the circuit that actually works.

I'm just posting this for completeness. I hate threads that describe your problem exactly, then there is some discussion and then the thread ends. No clue of whether the poster had got this working in the end.

So I'm rounding this off and post the solution.

MAX7219 high voltage LEDs (Schematic)_final.pdf

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On one of the electronics forums mentioned here I have been recommended to add drivers to the cathode side of the displays in addition to the anode side drivers that were already present in the first schematic I posted. Since the added drivers are inverting I needed to use 2 to achieve a double inversion (i.e. back to original).

The driver integrated circuits are 8 darlington transistor arrays in one chip.

I don't mind discussing more details. But I don't want to go too much off topic. The only relation this has to astronomy is that, in the end, it will be a fancy clock for my obs.

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