Driving segmented LEDs?

[alienmeatsack]

I've recently acquired a few LEDs like these:


I want to use these on some bends for displaying letters/numbers, statically. Dynamic is a whole other ball of wax and would require a chip to handle it I'm sure.

Each digit is 7 segments, then the 3 dots are their own item. If you aren't familiar with how these work, there are 2 pins that drive each segment. You power the appropriate segments by connecting +/- to the right pins to form letters and numbers. For the unit I have, as an example... you put + to pin 1, - to pin 2, and it lights up the top left segment, etc.

In my case, for example, I'd like to spell out something like APC or whatever. And do so efficiently.

Currently, minus using a display driver IC, the only way I know how is to just wire up the segments as if they were a handful of LEDs in series and power them. They are pretty low power, but still, a standard letter could be up to 7 segments lit, aka 7 LEDs all at once. I was thinking if I did them in series and then added a resistor to keep their power use low, it might work. But I'm concerned it will use more battery than it's worth.

In the example of using the letters APC... I'd need to apply power to 1+2, 1+3, 1+6, 1+7, 1+10 and 1+11... That forms the A like thus:



I've not actually breadboarded up this to check the power usage, yet.

Is there some more efficient method for this? A trick or something one of you knows to do this better?

[jksuperstar]

Aside from cranking up the resistance (thus also dimming the LEDs), the only other thing I know, which some chip solutions provide, is using strobes to pulse each LED. Like PWM synthesis, this activates each LED for only a very short period of time, cyclically. The LED remains lit for more time after the pulse is gone, and your eyes fill in the rest. Thus, you use a very small amount of power, and the pulse width directly effects the brightness of the LED (this works for two reasons: A. the LED is only on, with current flowing, for say 10-25% of the time; and B. The inrush current of an LED is typically less than the current that would have been used the other 75% of the time).

[alienmeatsack]

Interesting on the pulse idea. I'd bet you could get the pulse to just the right frequency so it used the least power and the LEDs still appeared to be lit constantly...

Cool idea... I will have to look at how I can achieve it with the least amount of parts/space used!

[Danno Gee Ray]

Here are some things to look at.

http://www.romanblack.com/led_none.htm

http://www.allaboutcircuits.com/vol_6/chpt_7/9.html

http://www.melabs.com/resources/articles/ledart.htm

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[Sam_Zen]

[electri-fire]

[quote="Sam_Zen"]

[Sam_Zen]

You're right, I suppose. LED's have a voltage drop of 0.7 volt like any diode, so it represents a certain resistance.
So I guess LED's in series take the same power, but each one will be less bright than a single one.
2 LED's in parallel means half the resistance, so will use more power.

[Blue Hell]

Voltage drop over an LED is about 2.2V, depending on color, but for an LED not depending too much on the current flowing through it. Which also means that the resistance of an LED gets lower when more current flows through it.

Power is voltage times current ... double current or voltage and power will double if the other is kept constant.

Re. LEDs in series, total LED power will double when two LEDs are put in series if using the same current as for the single LED, meaning the series resistor has to be lowered, also meaning less power is dissipated in that resistor and also that the power supplied by the supply does not change.

Re. LEDs in parallel ... LED power will double too, if using the same voltage over the LEDs, meaning that the resistor has to be lowered, also meaning less power is dissipated in that resistor and also that the power supplied by the supply does not change.

[Sam_Zen]

Thanks for the correction.

[alienmeatsack]

Good info everyone. Thanks! I will take notes and all that and see what I can figure out!

[electri-fire]

I realise now, the parallel/serial discussion in this thread is irrelevant, as these LED displays have a common pin (anode or kathode depending on type), so these LEDs are parallel. There's no way to make them serial.

edit: then I read back and see that's more or less what Sam_Zen wrote Fri Apr 09, 2010 3:34 am.


(electri-fire apologises for all the blah he posted, but realises we all go through learning curves)