7 segment, 12V, 4 digits, low light :(

Hi!

I have a problem… I assembled a circuit what works well, except its low brightness. I attached the schematics. Used 3 optocouplers to isolate Arduino’s 5V from 12V power supply for the LEDs. Those are four Kingbright SC23-12EWA-s. The resistors are 130 Ohms. Everything goes well, but the more segments emits light the less brightness it has. Maybe I have to stabilize the voltage, but I don’t know how. Before or after the OC-s? And what with? Transistor? Which?

Thanks for the help and sorry for the schematics, I don’t have enough practice in it.

I'm not familiar with the driver you have shown but I see it uses an external 12 volt supply. Are you connecting the ground of this external supply to the 0v of the Arduino?

(it isn't clear in the diagram).

No. Two of the OC’s switch the 7 segments and the point (anodes, +12V), and the remaining one (under the display on the drawing) switches the common cathodes (12V power supply GND). I need this because of multiplexing the four displays and to isolate the two circuits. I only signed one of the cathodes on the schematic, the remaining pins of the optocoupler connect to the Aruino’s digital pins 11, 12, 13.

post links or datasheets for the ic's you are using and if you post schematics that don't have values , post the resistor values for the resistors in the posted schematic. Is this your design or something you found ?

i'm sorry, forgot abut it. All of the resistors are 130 Ohms. The datasheet of the optos: http://pccomponents.com/datasheets/VISH-K847P.pdf I modified an existing design.

Thanks

Looking at the datasheet for your 7 segment display I find this

Which clearly shows pin 7 as the anode for segment A

If we trace that on your schematic, we find it connected to pin 3 of your chosen opto isolator.

Looking at the datasheet you provided I find its pinout like this

So just from tracing this very first line, I’m wondering what it’s doing going into the input side of an opto isolator. It should be on the output side of the chip.

I understand what you’re trying to achieve. I’ll see if I can work out a more logical schematic for you.

Thank you very much! My logic was: I tried to use the opto isolator like a switch which opens/closes the "wire".

So just from tracing this very first line, I'm wondering what it's doing going into the input side of an opto isolator. It should be on the output side of the chip.

I understand what you're trying to achieve. I'll see if I can work out a more logical schematic for you.

My logic was: I tried to use the opto isolator like a switch which opens/closes the "wire".

If that's the case you are not even close to having it connected correctly. As pointed out , it should be connected to OUTPUT side of the opto NOT input.

You schematic and choice of words suggests you have no electronics experience.

I modified an existing design.

Based on what reference information ? Can you post any links to designs anywhere that use the INPUT side of an opto to switch a led (not a "wire"), which if you think about it , completely defeats the purpose of using an opto because you lose the isolation.

I suggest you scrap your design and go back to the drawing board. Post the specs for the devices you are using . Show us that you know what the voltage and current ratings are for BOTH sides of the opto. (and the leds for that matter).

I've knocked up a slightly better schematic of what I think you're trying to achieve. In principle it could work but there are serious problems.

Naturally you realise that you can't turn on all four displays at the same time. There are two reasons for this. Frstly, since all four displays share a common databus, they would all show an identical output. Secondly, it would overload the opto issolators that are driving them. (the opto issolators output are rated to an absolute maximum of 60 ma. each SEGMENT of the display consumes 20ma 3 bars and you're out)

The way you can work around this is by time multiplexing ie first turn on one digit, then turn it off and turn on the next digit. If this is done at around 50 times per second, to the human eye it looks like all four digits are being displayed.

The problem comes with the opto-issolator that is driving the cathodes of each digit. This will have to handle the current for all 7 segments (and the decimal point) so if you ever wanted to show a floating number in the range of 8. something, you'd have 160ma being pushed through an opto-issolator that is only rated at 60ma

With a few mods this could work but it would need quite a bit of redesigning

Although I've drawn it a bit neater, i firmly believe this circuit will fail in it's current form.

Thanks to all for the useful advices! I will try these and dig deeper into basic electronics :)

belatoth30: Used 3 optocouplers to isolate Arduino's 5V from 12V power supply for the LEDs.

OK, there are applications such as automotive circuits where opto-isolators are desirable, but in this situation where the opto-isolators are preventing the circuit from working in a useful manner, I have to ask why any necessity occurred to you to isolate the 12V and 5V power supplies?

(In most cases, it would be appropriate to use a regulator to derive the 5V for the Arduino from the 12V supply, best by using a switchmode regulator.)

Aah, I've studied my own drawing, and recognized, I drawn 180' rotated the optos.... But in the real life, I connected the pins correctly. Mea culpa.