Big 7-segment display schematic

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hakha4

New Member
Hi!
Does anyone have experience interfacing with 2.3" 7-segment display. I've googled and there are a lot of suggestions but mostly inconclusive. I've bought 2.3" displays (Common cathode) with forward voltage 7.5 volts. Can this be done with Maxim 7219 and Uln 2003 to drive the displays? Anyone have a working schematic? Any help would be appreciated
Regards Hucke
 
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MartinM57

Moderator
What part number do the displays have ... 7.5v forward voltage is very high...

You wouldn't normally need a ULN2xxx when using a MAX7219, and a MAX7219 won't power LEDs with 7.5v forward voltage directly. Maybe there's some hack you could do by taking the digit and segment drives from the 7219 and using them for your own purposes, but I've never seen it done.
 
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hakha4

New Member
Hi and thanks for answering. Every segment have four LED's in serie so I think 7.5 volt are needed for getting enough brightness but the tricky part is to provide this voltage.
Hucke
 
fernando_g

fernando_g

Senior Member
To drive common cathode LED's you'll require PNP or P-chan transistors.
Since the emitters (or sources) of those transistors will be at a voltage of at least 8 or 8.5 volts, (to take into account ballasting resistor drops and transistor saturation issues), this means that the bases (gates) cannot be driven directly from the PICAXE, but rather will have to have another set of NPN or N-chan (whose emitters or drains are ground-referred) preceding them.

I vaguely recall there are LED drive ICs that are capable of diving CC LEDs at voltages higher than 5 volts, but don't remember their p/n.
 
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Andrew Cowan

Senior Member
Maxim's LED display drivers all stop at 6v.

If you just have one display, you could use PNP transistors (driven by NPN transistors).

A UDN2981A or TD62783 would be fine if you don't like wiring lots of transistors (similar to an ULN2803, but pnp).

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hakha4

New Member
Hi and thanks for reply
I'll be needing 4 displays. Could I then use 7219 (saves pin and makes programming easier)and connect seg outputs from 7219 to input TD62783 -> 7-seg displays segment and cathode_x to dig_x on 7219 ?
Regards Hucke
 
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MartinM57

Moderator
What we are second-guessing exactly how the DIG and SEG outputs work, especially for higher currents (you still haven't said what displays you are using, so we have no idea what Iforward is) and how display brightness works (I thinks it's PWM, so inherently digital, so probably doesn't need to be taken into account).

The data sheet doesn't show a schematic of the DIG and SEG circuitry so we can't be sure that just sending the SEG outputs to a high side switch would work.

But the (proper) answer is in the data sheet anyway :) ... figure 2 shows a circuit for driving 2.3" displays with forward voltages around 8v :)

Figure 2 is significantly more complex than just a TD62783 though - including needing a -5V supply. If a TD62783 was all that was needed I think Maxim would have sussed that out and proposed it.

You could always give the TD62783 a go though..maybe google will turn up someone who has done it..but you'd be on your own mostly...
 
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hakha4

New Member
Hi and thanks for replying
Sorry I haven't given more info.
Common Cathode
Size: 2.3'' (69.70mm*47.80mm*12mm)
Forward Voltage: 7.5V
Forward Current: 5-10mA
Thats all info I been able to find about the displays
You are refering to fig2, where do I find that figure?
I've googled all night yesterday and also looked in several electronic forums but it seems impossible to find a straight answer. I can't be the first to try interface this!. Could You point me to the schematics you talk about?

Regards Hucke
 

Attachments

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Andrew Cowan

Senior Member
Page 12.

Nice idea - it uses a -5v supply to get around the limit of the 7219.

That schematic is designed for a 2.3 inch CC display - so it will definitly work.

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hakha4

New Member
I've read the schematic and I'll try this approach,I 've ordered samples of Max 394 from Maxim and get back to report how it works in case someone else want's do to the same. Anyway thanks all sofar
Regards Hucke
 
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