Big 7 seg LED display

[shahidali55]

Hi everyone,
I made a 4 digit LED clock using 4, 7segment common anode displays of 1 inch size.
Now i want to make a bigger one using 2.3inch displays.
But these 2.3 inch displays require more than 9.6v to function.
Since i am multiplexing the 4 displays, i need to control the 8 cathodes (which are in parallel) and the common anodes. But these anodes require 9.6 volts.
But the pic can supply only 5 volts. How should i make the circuit so that it supplies 9.6 volts to the displays???
Can anyone please help me?

Hardware :-
4 common anode 9.6v displays (2.3 inch)
pic16f84A

[mister_e]

Do a little search for Level Translator.

You will find some Transistor, CMOS, etc etc circuits for that.

[sayzer]

A concept as below could work, I think.

It is just a concept though. Can be modified for anode / cathode path.
You will need 8-transistors + 4 for the common ones.
ULN type of drivers could be handy here, but you need to make use of them with respect to anode/cathode issue.



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[mat janssen]

This will not work.
The transistor will work as a emittorfolower and the voltage on the emittor will not come over 4,3 volt.

[sayzer]

While I was editing the concept, mat posted.

How is it now?

[mister_e]

Nope ! or i have some doubt...

[mat janssen]

The schena above is still not ok.
Her an example how it should work.

From Darrel:
Mat, the file caused an error in vBulletin due the the filename "div. voltages.GIF". Can you try again?

[shahidali55]

Thanks for your replies everyone..
Is there any logic level convertion IC that can do this???

Sayzer, your idea for the a,b,c,etc segments is using NPN transistors is what i had in mind.
If i feed the output of the pic to a 4011 (AND gate with its both inputs made 1 common),
and then give it to a PNP transistor, will it work???
I think the AND gate will do the level conversion, i'm not sure ? ! ?

[mat janssen]

Here another try for the diagram.

[keithdoxey]

Thanks for your replies everyone..
Is there any logic level convertion IC that can do this???

If i feed the output of the pic to a 4011 (AND gate with its both inputs made 1 common),
and then give it to a PNP transistor, will it work???
I think the AND gate will do the level conversion, i'm not sure ? ! ?

No. The 4011 will still be connected to the 5v supply so the PNP will always be driven as its base will not be allowed to go to 9.6v to turn off.

You could use a chip with open collector or open drain outputs. Another ULN2803 would do the trick but would occupy about the same space as the three resistors and NPN transistors it replaced.

[sayzer]

shahidali55,

As Keith said, you will most likey have to use two ULN type of drivers.

Thats seems to be the way.

Also, why not consider mat's schematic?

Or if you have limitations, why not try finding Big Led displays with 5V, if there is any?

[shahidali55]

Big displays of 5v are not available...
Thanks for the schematic Mat.
Will your circuit work for 12v displays also???

[shahidali55]

Will this work ? ? ?

[mat janssen]

Will also work on 12 volt, 15 volt.
I cannot open your schema. There is something wrong with the server I think. Because I could also not upload my diagram's like that, so I post them somewhere else, and gave a link to this forum.

[Dave]

shahidali55, Why not just use a UDN2981 for the sourcing driver as it is a direct connect to the PIC?

Dave Purola,
N8NTA

[keithdoxey]

Why not just use a UDN2981 for the sourcing driver as it is a direct connect to the PIC?

Excellent find Dave. Have downloaded the datasheet and will almost certainly be using this chip in the future.

[mat janssen]

That is indeed a nice ic, also for this solution as the CA driver. But you will lose about 2 volts over the internal output transistor.

[sayzer]

Hi Dave,

Thanks for the info.

If the circuit below is a correct operation, then this chip is indeed nice.

Is it correct?



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[shahidali55]

If the UDN2981 is available here, that'll be my first choice...
Thanks Dave for the nice chip find and Sayzer for the schematic...

[shahidali55]

I found another cheap alternative...
<img src="http://www.winpicprog.co.uk/graphics/lamp_source2.gif" border="0" alt="" /></div>

[shahidali55]

..........

[sayzer]

I believe UDN2981 would be cheaper since you would have to use many transistors.


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[shahidali55]

Hi everyone, thanks for all your help.
I made my LED clock using transistors cause i couldn't get the UDN2981.
It works great. I added an hourly animation and a battery battery backup also ( i cant belive i'm getting more than 20 hours out of a 60mAh rechargeble battery {with display off}).
I've attached a picture of it...

[sayzer]

Good to see something is working!

FYI: Upon the chip info above, I checked the prices.
The price of UDN2981 is around US$1.60-1.70 (for 1000s lots), which is a lot more expensive then traditional ULN2003 types.


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[Dave]

sayzer, It probably is but I don't think you can source current with a ULN2003.

[sayzer]

Yes, of course Dave.

I had no info about this UDN2981 before, but now I will consider it when the project allows me to use it (cost & hardware etc.).


Thanks for the info.


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[Zhao Hui]

Hi everyone, thanks for all your help.
I made my LED clock using transistors cause i couldn't get the UDN2981.
It works great. I added an hourly animation and a battery battery backup also ( i cant belive i'm getting more than 20 hours out of a 60mAh rechargeble battery {with display off}).
I've attached a picture of it...

shahidali55,
May I get your schem about the final Common Anoda display 7 segment (12 V) to me, couse i've same problem with you before.
thanks for helping ...

[email protected]

[shahidali55]

Hi Zhao Hui.
Check post #20.
That schematic will drive the high current consumption at the common anode pin.
The individual cathodes can be driven by a single NPN trasistor buffer each.
Check the schematic below (for seperate cathode drive).
This circuit will invert the PIC inputs so dont forget to correct this in software.

[shahidali55]

Zhao Hui and mdaweb, here is the schematic.
The display should be a common anode display.
The 9.6v input shown in the schematic should not exceed the display max segment voltage.
I have shown the connection only for the 'a' segment. The patern just has to be repeated for the rest of the segments.

I have been using this circuit for some time now. It works great.

But the operating logic is a bit different.
Logic 1 on both p.1 and p.2 will turn segment 'a' on.
p.1=1 and p.2=1 --> seg'a' on
p.1=1 and p.2=0 --> seg'a' off

[Zhao Hui]

shahidali55 and all you guys in this forum......... thank's alot for technical information my job almost done now...
once again thaaaaankkkkkkkkk's

pic 16F628 with 4digits (10 inc) 7seg 12 Volt



zhao hui

[Zhao Hui]

Here the final Schem that work fine, no software modification on PIC ..... just use the CMOS Inverter (4049 - Hex Inverter) to invert the logic.

[shahidali55]

I'm having certain 'strange' errors in my digital clock.
Processor = 16F84A @ 4 Mhz
4 inch , 12 volt LED Display
The schematic is almost the same as the one in the previous post.
(A battery backup has been added)

Problem 1:-
The clock sometimes just 'hangs'. I think the oscillator stops or something.
The display goes blank or most of the time only one digit on . . .

Problem 2:-
Timer0 is used for timekeeping (Darrel Taylor's Instant Interrupts).
The clock suddenly stops incrementing time.
i.e. the display multiplex routine keeps working as it can be seen that the all the digits on the display are displaying time.
The thing is that the time never increments.
I have programmed the colon between the digits to blink at 0.5 Hz.
This blinking is controller by the time keeping registers mid byte which is kept updated by timer0.
Since the digits keep showing a frozen time, and the colon stops blinking,
Could it be that timer0 has somehow accidentally been disabled ? ? ?


Attached Files :-

clock.pbp - main file
TMR0_int(ASM).pbp - interrupt handler

Could someone please help me out ?