Complete Idiot

[Brass]

Complete Idiot HELP NEEDED

Hello,
I am a complete idiot when it comes to electronics so please bare with me,

I have a computer which needs an additional circuit for a diagnostics "P80P" display, its a 2 digit 7 segment led display, I believe that the segment timing control is done by the PC itself and that all i need is to get things hooked up with power from the PC floppy connector to run the two 7 segment leds. the computer connection looks like this.

LODIG NC
SEGA SEGB
SEGC SEGD
SEGE SEGF
SEGG HIDIG

I am learning some basic electronic stuff at the moment but have nowhere enough knowledge to even contemplate calculating whats required - and by the time I know enough I probably wont need it anymore.

Can someone help me please.

Note: I can operate a Multimeter and weild a soldering iron with some competence.

Adjusted title for better result

 

[Optikon]

This description does not lend itself very to well to receiving help. If you can try and work out a schematic showing how you think it should hookup and point out where you are not sure. Then post that schematic for all to see. You'll get better suggestions that way.

 

[Brass]

I shall do my best

Ill do my best to do that i found a circuit that I can use to hack up to make the drawing side easier.

May the gods of Electronics please forgive me for what shall appear in my next post.

 

[Brass]

My Circuit - help needed.

Ive attached a gif file

Things I dont know

Will the 5v Rail from the PC floppy overload this?

When digit Hi is selected output should go to the Hi LED when low is selected it should go to the Lo LED, how do I acheive this and will it result in one digit going out when the other is lit?

Do I need a LED line driver to do most of this work for me ?

Is it appropriate to take the -5 volts output from the LEDS and run that to the ground on the floppy connector.

I want to ensure that none of the voltage / current from the floppy power gets to the 10 pin data connector as I beleive its CMOS 3.3v if im using the 10 pin connector data lines to feed the signals on the LEDS will they receive any voltage or current from the 5v floppy connector and if so what can i do about it ?

How do I ensure that only the appropriate LED is getting the data ?

Thats about it I think.

If theres something ive forgotten please let me know.

Cheers in advance.

 

[philba]

ok, a little googling helped me understand what you are doing. more context would have helped a lot. p80p is an onboard diagnostic 2 digit LED display driven by the bios (POST code, I dunno). I assume that you are planning to take the p80p pins and connect them to the display. what motherboard are you using?

so, that said, your diagram is wrong. you don't connect the display to voltage or gnd. the segment pins (or hidig/lodig) have all the driving voltage. forget the floppy cable. Hidig and lodig are the common pins for the coresponding digit. the segA-G will be connected to the same segment on both digits. the BIOS (or ?) will select the digit via the hidig/lodig pin and then power the desired segments. it will alternate rapidly between the two digits so they will both appear to be illuminated at the same time.

You need to understand whether the p80p port expects a common cathode or common anode display. do you have an actual 2 digit display in hand?? can you point to where you are getting this information?

I think, from what i read, that the p80p is driven with 3.3V. so you will need to calculate the resistors based on that and forward voltage of the display segments. this will be in the datasheet. If you don't have a datasheet, for red, use 2V and green or yellow, use 2.2V. the resistor is used to limit the current. I'd probably pick 20 mA. the resistor is calculated via the formula R = Vf/I = (for red) (3.3-2)/.02 = 1.3/.02 = 260. choose the next higher standard value resistor. 270 will be fine for red. don't use blue LEDs - they require more than 3.3V. note also that some super bright leds take higher voltage and/or current. I'd would not use them here since the current capacity of the p80p circuits isn't clear.

also, next time try using a title that is discriptive of the problem, not the poster.

 

[Brass]

P80p

The board is a Chaintech 7NJS0 Ultra

Is there any way to safely and acurately measure the voltages coming of the motherboard lines?

Up until now Ive been using my multimeter between the pins and the -5v line on the floppy connector is this appropriate ?

 

[philba]

safe? yes. accurate, no - the cpu is scanning the 2 digits so if you put an ocilloscope on the pins you would see a pulse train. It all depends on what your meter does with that sort of thing.

measure between the pins and gnd, not -5V.

look here for a description of driving 7 segment displays including common cathode vs common anode. https://www.melabs.com/resources/articles/ledart.htm

 

[Brass]

Keep it simple

I'd like to thank you guys for your help , my circuit is working now and I found what the major hassle was.

The data sheet for my 7 seg leds states : TOP
A B C D E

F G H I J

but what i found out is that this is not the pin assignments from looking at it from a top down view i.e being able to see the leds, all they are actualy indicating is what is the top of the device and all the pinouts are actualy structured from the bottom thus what I wired to the spec above should have been E D C B A

J I H G F

I suppose im realy very luck that C and H were both commons.

What gave it away ? the decimal point kept comming on and i hadn't wired it in. cheers all ill be back when i work on the next thing to break