General > General Technical Chat

Low cost, low power consumption (<30ma) 2 digit 7 segment displays?

<< < (2/2)

NiHaoMike:
The eye is actually most sensitive around green. Most blue LEDs are based on a GaN process, which gives them a higher efficiency than normal green LEDs and thus appear brighter despite being further away from the peak. (There are GaN green LEDs, but I don't have much experience with them.)

Red is quite far from the peak, but it does not affect night vision as much as other colors.

EEVblog:
My latest project uses one of these 3 digit LED displays:
http://au.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMvkC18yXH9iIodegacuwFE2SXUTz8B28EM=

Even with each segment at under 3mA, it's still quite a visible display. In multiplex mode that's only 24mA max peak current if all segments are on. An even lower average current when you take the PWM into account.

So a 2 bright 2 digit display under 30mA is easily possible.

That first display you linked is speced for relative brightness of 1 at 10mA, so half that brightness at 5mA. That's 40mA for all segments on in a multiplexed display, and lower than that on average.

The key is not to drive them direct and static, you have to multiplex the display.

Dave.

mikeselectricstuff:
If there is any advantage in muliplexing, then the same could be achieved with static drive by pulsing on & off. I doubt there's a big difference in practice, and in battery apps, the higher peak current may reduce the effective battery end-of-life point.

comox:
Everyone,

Thank you for your valuable comments.   The display will be for indoor use in the office or home, and one option for the LCD would be to illuminate the bezel with am LED.   Thank you for the suggestions regarding PWM and multiplexing as well.  Makes sense.

As mentioned, the display will require an I2C interface, and there are a couple of NXP drivers that I will be evaluating.  The first one is the NXP SAA1064 (somewhat ancient), which can drive multiplexed LED displays, however has no PWM capability.  The second NXP LED driver is the PCA9532, which can dim the LEDs via PWM. 

Yes, the amber LEDs appear to output a higher luminosity per ma, as demonstrated in the following spec sheet http://www.farnell.com/datasheets/78144.pdf  May consider this route as well.

I believe I may have become a victim of "datasheet fatigue", which occurs when searching for a solution by reading datasheets only, when actual experimentation would have been a better approach.  The data in the datasheets (e.g. continuous forward current) discouraged me early on.  I have the NXP drivers mentioned above on hand, with a selection of various Avago single and dual digit SMD 7-segment displays (albeit all green).  What I need to do now is perform some good ol' fashioned empirical analysis of a) the current consumption of the display when showing 88, and b) the acceptability of the brightness of the display, in combination of various PWM frequencies.

Give me a week to wire something up and test (e.g. the NXP PCA9532 and 2 Avago single digit 7-seg displays HDSM-281H) and I will post my findings here. 

Thanks again.

Comox

NiHaoMike:

--- Quote from: mikeselectricstuff on June 10, 2010, 08:23:10 am ---If there is any advantage in muliplexing, then the same could be achieved with static drive by pulsing on & off. I doubt there's a big difference in practice, and in battery apps, the higher peak current may reduce the effective battery end-of-life point.

--- End quote ---
http://members.misty.com/don/ledp.html
There is an advantage of pulsing LEDs.

Navigation

[0] Message Index

[*] Previous page