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12-Digit LED "no parts" display driver!

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Mr RB

Well-Known Member
Hi I just finished this project, driving a 12-digit LED 7-segment "calculator" style display using no parts, no ressitors, no other chips, just a PIC chip.

Since there's been heaps of posts lately about driving 7-segment number displays I thought it was worth mentioning here.

The displays are dirt cheap, about a buck each and less if you buy a heap at once, they are brand new and perfect condition (gold plated, no corrosion) you can get them from Oatley Electronics in Australia and they will ship international.
(No I am not affiliated with Oatley).

My web site has a PCB artwork for toner transfer and I have provided source code for a few projects including;
:) LED clock (with hundredths of seconds)
:D LED stopwatch (with hundredths of seconds)
:eek: 12 digit display with 19200 baud serial control (only needs 1 wire!)

Here is the project's web page;
12 digit 7 segment LED no-parts driver

It might be worth buying a heap of these beautiful little NEW "retro" displays from Oatley before they sell out, I did! :D
Remember they just don't make these things anymore. :(
 

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Boncuk

New Member
The displays are dirt cheap, about a buck each and less if you buy a heap at once, they are brand new and perfect condition (gold plated, no corrosion) you can get them from Oatley Electronics in Australia and they will ship international.:(

Not quite :(

Thailand isn't on their list.
 

audioguru

Well-Known Member
Most Helpful Member
The max allowed output current from each PIC output is only 25mA.
What limits the current on your circuit?
 

Sceadwian

Banned
Check out the website audioguru, it's multiplexed.

[FONT=Arial,Helvetica,Verdana,Geneva,sans-serif][SIZE=-1]Now for the safety. I drove the segments with a duty cycle of 1:24 so that only 4 segments can ever be lit at one time. So it multiplexes the top 4 segments, x 12 digits, then the bottom 4 segments for 12 digits again. Multiplexing is at 256uS (using TMR0 interrupt) which runs each LED at 162Hz. There is no deadtime, so peak (instantaneous) current is 55 segments / 24 multiplex which is the equivalent of 2.29 segments on all the time, so that is 51mA / 2.29 = 22.3mA per lit segment peak instantaneous current.

22mA per PIC pin is well within safe specs, and the most segments that can be lit at once is 4, so the max sourced by PORTB in total is 4x 22mA or 88mA. PORTB is specced at 200mA so that is fine. The max current sunk into PORTA or PORTC pin is also 88mA, which is technically above spec but can never be more than 1:12 duty cycle and must always have the long cool down cycle (11 periods) after any time it is used. Since the pin pull-down drivers are NFETs this should be comparable to any pulsed NFET with a low duty cycle, which is the average of 88mA / 12 or 7.3mA sunk per PIC pin is the worst case.
[/SIZE][/FONT]
 

tcmtech

Banned
Most Helpful Member
So other than up or down counting whats it good for?

Can you make a reliable and accurate volt or amp meter with .000,000,000,001 Resolution with a pic?
You know, a 1000 volt digital volt meter with nano volts resolution. 999.999999999 +- .000000005 %
 

shimniok

Member
So other than up or down counting whats it good for?

Can you make a reliable and accurate volt or amp meter with .000,000,000,001 Resolution with a pic?
You know, a 1000 volt digital volt meter with nano volts resolution. 999.999999999 +- .000000005 %

You're right. There's no humanly possible way to make use of a 12 digit 7 segment display for any conceivable project in the known universe. :rolleyes:

I'm thinking of using mine for a cool retro text display probably to go on my diy record cleaning machine which I want to have a vintage aesthetic. Altho I guess I would be better off using a VFD... well, whatever... Or else I will make a stylus hour meter ... I'll come up with something.

Michael
 

Mr RB

Well-Known Member
The LED display is for people who like the old retro old and who like to quickly kill batteries.

Well I wouldn't call 40mA to drive the whole display a "battery killer"... I've seen people use 40mA just to drive 2 LEDs.

Likewise the last 2x20 LCD project I did used 350mA just to drive the backlight!! And of course you cant see LCD in the dark without a backlight. ;)

Call me old-fashioned but there is a very visually attractive "cool" factor to these displays, coupled with the unhappy fact that they will never be manufactured again. If you understand why a nixie clock is cool then you get it.

For Boncuk and Dick Cappels, I cant speak for Oatley and what countries they choose to sell to, but I know the banks in Australia advise businesses not to accept credit cards from Thailand due to the history of frauds. If you still want to buy you could email Oatley and ask about payment options and paying in advance.

If that fails I may be able to arrange something from my personal stocks for you.

Hey TCM, I think I was "doing one of the common things in life in an uncommon way" heheh. ;)
 
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tcmtech

Banned
Most Helpful Member
I tend to associate numerical displays over 5 digits as good for counter or calculator readouts.
So I was just curious as to what else a pic can do with that many numbers since A/D signal conversion has a apparent resolution limit for practicality and accuracy.

Really I was rather hoping you would say a 12 digit digital volt meter was simple and cheap to make! ;)

As far as old retro I have four 7 segment filament type tubes. (3 good, 1 broke)The are mounted on a circuit board and each filament runs off 5 volt logic level power! Pre LED display tech. solid state circuitry with vacuum tube display outputs!

Dont know what the exact name for them is or if they are even made any more but they make a good conversation piece as is. If I had four good tubes I would rework them as the display for a digital clock readout.

Maybe if I ever build another vacuum tube amp I might integrate them into the design as a power meter of some sort.

Hey TCM, I think I was "doing one of the common things in life in an uncommon way" heheh. ;)

Care full though, you will get hell for it if it gets out! :eek::D
 

shimniok

Member
I tend to associate numerical displays over 5 digits as good for counter or calculator readouts.
So I was just curious as to what else a pic can do with that many numbers since A/D signal conversion has a apparent resolution limit for practicality and accuracy.

I was thinking along the lines of a text display... that webpage in the orig post has an example...

But I guess you could use it for displaying multiple readouts...
_###_###_### or _##_##_##_##, etc.

Cool re: the tube displays. Seems like they'd be perfect for a tube amp too. So these aren't really nixie tubes (where there's 1 filament per number) but rather 7 segments? Wild... no idea where you'd find more.

Michael
 

Dick Cappels

Active Member
Thank you, Mr RB, for your suggestions. I might stop by Bahnmo (the area in Bangkok with lots of electronics surplus stores) later this month and see what they have. This discussion has renewed my interest in the old 7 segment displays.

Those who are old enough, you might remember 7 segment displays being used to display hex numbers. Kind of kludgy (to use an expression of the day) but it worked.
 

Mr RB

Well-Known Member
TCM, you could make a 12 digit voltmeter with a PIC, but since the PIC has only 1024 resolution points it wont be much good after the first few digits. But the PIC has 5 ADC inputs, just with 2 of them you could make this;
12.75v 1.04A
which fits fine in 12 digits even including the 'v' and 'A' chars. You can also program it to do overvoltage indicators or errors etc, something you can't implement on a typical digital panel meter.

Re your 7-seg filament display, those are incredibly rare. You might want to google for the "calculator museum" a cool site, he lists just about every type of digit display there, but I don't think he shows your filament type.

Shimniok, sorry i missed your first post. I think one of these displays would look great as a "retro" text display, 'Start' 'finished' "cLean' 'Error' etc. Sweet project! :)

Dick, yeah they can display all the 6 hex characters and the 0-9 numerals. I glad that it renewed your interest, it did the same for me I guess. These displays (the Oatley ones) are nice in they look to be state of art as the last era of segment displays, National Semiconductor brand, gold plated, they are crystal clear, high brightness on low power even with low duty cycle. They are also very "sharp" as the clear bezel sits right over the led chip itself.

If you compare to a typical red LED alarm clock which are dull and murky, these are razor sharp with a beautiful intense red glow to the tiny led chips themselves. It REALLY doesn't come through in the photos... :(

(edit) I googled for caculator museum, there is more of them than I remember! But this is the page I was talking about;
https://www.vintagecalculators.com/html/calculator_displays.html
 
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audioguru

Well-Known Member
Most Helpful Member
If you compare to a typical red LED alarm clock which are dull and murky, these are razor sharp with a beautiful intense red glow to the tiny led chips themselves. It REALLY doesn't come through in the photos... :(
my clock radio is about 25 years old and its LED display has dimmed due to the high number of hours.

I still have an old calculator with an LED display. It eats batteries and is not very bright.
 
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