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

[comox]

Hi y'all,
I've been lurking on the EEVBlog site and forum for some time now, and I am now seeking advice/recommendations for a display requirement I am trying to satisfy.

I am developing a timer product and I need to display 2 digits to the user.   A dual-digit 7-segment display would satisfy the requirement; however, I am trying to keep power consumption and cost to a minimum.   Specifically, the 2-digit display should not consume more than 20ma to 30ma.  Normally I would consider a small 2-digit 7-segment LED display, but the power consumption of a bog-standard 7-segment LED display would greatly exceed the power restrictions.    To give you an idea of a 7-segment LED display that would satisfy the display requirement, check out: http://www.avagotech.com/pages/en/led_displays/surface_mount_seven_segment_displays/0.28_dual_digit_series/

I am also looking at 2 or 2.5 digit LCD displays, including one from Lumex http://www.lumex.com/products/detail/25_digit2 and another from Varitronix (VI-201) http://www.varitronix-hk.com/index.php?pid=2  I am planning to drive the LCD with an NXP PCF8566 which satisfies another requirement, in that I need to control the display via I2C.

Clearly the LCD display will operate with minimum power consumption; however, I would prefer the bright intensity of an LED display.    I have already placed an order with Digikey for the above NXP display driver and the LCD modules, and I have various 1 and 2 digit 7-segment displays on hand already.

Does anyone have any suggestions for an inexpensive 2-digit display that consumes a minimum amount of power?

Comox

[alm]

If you can live with lower brightness, you might be able lower the current, although I doubt that regular LED 7-segment display will emit much light with all segments enabled at 30mA. You should use a switching/PWM-type constant current source for the LED's, not simple resistors. Depending on your input voltage, this might allow you to deliver a lot more current. The visibility of LCD sucks in the dark (unless you have a backlight), the visibility of LED sucks in bright light. So it depends where your product will be used. Is a backlit LCD module an option? Backlight should be possible below 20mA if it's LED.

[NiHaoMike]

Most blue LEDs are very bright for a given current. (Even after accounting for the higher voltage drop.) The blue LED in my homemade iPod cradle is too bright to look at directly at just 5mA. ( http://ecorenovator.org/forum/appliances-gadgets/984-allie-moore-ihybrid-40-next-generation-ti-hybrid-digital.html ) Finding a 7 segment display that uses them, however, is a little difficult.

[tecman]

If you can live with a passive (no light source) LCDs can run under 1 mA.  If you need an LED, most new displays with high efficiency LEDs will give a reasonable output under 5 mA.  Mux'ing the display digits keeps the draw low with LEDs.  Reds have the highest output for a given current, but abler is more visible/readable compared to red or green.  Just how the human eye works.

Your request is not difficult.

paul

[mikeselectricstuff]

Low-current HE red displays can be readable at around 2-3ma per segment. All depends on your ambient light level. Best brightness/current is probably white but not cheap.
Obviously smaller displays tend to have better visibility at lower currents. Best bet is to order a selection and test them. Remember you can also often improve visibility with a tinted display filter to improve contrast.
20-30mA avarage for 2 digits should be easily doable.

[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]

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.

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