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I have to select a low-power display for a solar-powered digital clock. I have looked into numerous technologies, including e-ink.

However, I was still unable to compare between them due to the lack of online information regarding average power consumption.

I am aware of the existence of solar-powered pocket calculators, which use some kind of 7-segment displays, so I know that it should be possible to build a time-tracking device that is fully powered by light.

I can accept refresh rates as low as 1 frame per minute, but preferably I would like to have 1 frame per second.

The most stringent specification is power which, ideally, should be 100% supplied by a small solar panel.

Does anyone have a good suggestion for a good low-power display part that seems suitable for these specifications?

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"E-Ink" or "E-Paper" will only consume power during the refreshing, contrary to LCD, although some manufacturers have competing technologies with LCD. So you need to find how much power some device needs to refresh, and based on your refresh rate compare with LCD. I have seen a datasheet of one of these "E-Ink" or "E-paper", if I find I will post. –  Diego C Nascimento Jul 23 at 13:39
    
Yes, I am aware of the technological differences between them. My main point was that there is a lack of quantitative information online regarding the average power consumption of LCDs and Eink displays. I imagine that as refresh rates get lower, eink displays will naturally perform better in terms of power consumptions, but I need a clear view of that turnaround point to help me decide between those two solutions. –  user2088046 Jul 23 at 13:46
    
A link for one of then. 10uA at stand-by. There's information on refresh cycle so you can compare. –  Diego C Nascimento Jul 23 at 13:47
    
@user2088046 Could you rephrase your question and remove the request for part recommendation? Requests for parts recommendations are off-topic on EE.SE. A request for information about the types of display with low power consumption would be on-topic. Overall, this is a nice question, except for the last line. –  Nick Alexeev Jul 23 at 16:21

4 Answers 4

Have a look at old Sharp LCDs from calculators. You won't find lower power 7 segment LCDs. Consider using a microcontroller with a built-in LCD driver, as that will probably be lower power than anything you can do by simply twiddling port pins.

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So you believe there is no hope in trying to find a <10uA LCD display part out there? –  user2088046 Jul 23 at 14:35
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The LCDs used in solar powered calculators can manage 10uA I think. The problem is that they are all proprietary designs and not widely available. Most come from Japanese manufacturers like Sharp, or from China, and they tend to only be interested in selling hundreds of thousands to other calculator manufacturers. Having said that, they are not very complex and you can probably just recover one from a cheap calculator and figure out the pinout. The hard part will be finding a suitable display that has some kind of solderable connector, as many will be ultra low-cost zebra pads. –  MoJo Jul 24 at 8:43

It sounds like you want a LCD display. These can be very low power, and is why they are used in watches and many battery operated handheld devices.

The driving electronics for a LCD will take some power too, so you really have to look at the whole system. For a do-it-yourself low power clock, I'd probably start by looking at some of the very low power PICs with LCD drivers built in. The trick to lowest power will be very carefully written firmware that uses the LCD driver effectively and keeps the processor in sleep mode to the exent possible. For you PIC, you want to use the timer 1 oscillator driving a 32,768 Hz watch crystal.

Managing the power and storing it to keep the clock running when the sun isn't shining will also be a major design issue. A supercap is probably necessary, with a separate circuit that charges up the cap and only runs when there is power from the solar panel available.

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I already have a low-power oscillator and I will try to find a good LCD driver module. Any reason for staying away from eink displays? Or it is basically a price barrier? I need to avoid batteries as much as possible but I have also concluded that I will eventually need some kind of capacitance for energy storage... Thank you for the input! –  user2088046 Jul 23 at 13:40
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@user: E-ink displays are completely static and don't require any power to maintain the display. However, they do take power to change the display. I think they can be quite low power overall too, but I don't have experience with them. They are still proprietary and therefore relatively expensive and hard to find one-offs on the open market. But, go look at a e-ink datasheet. That should tell you how much power it takes. There is no point speculating. –  Olin Lathrop Jul 23 at 13:46
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If you take a look at this eink display datasheet, for instance, they provide a profile of the Current consumption during the refresh time in page 15. From the drawing I would estimate a ~10-20 mA of average current consumption during refresh time (although there is no time scale). But all the datasheets I have looked into are with this poor level of detail or even worse... –  user2088046 Jul 23 at 13:57
    
@user2088046 the datasheet I presented to you has the waveforms that you probably can get the time. Anyway time ago I have found one that clearly says their update power consumption. Anyway I think getpebble.com being a watch and so having 1 sec update at least can give you a market idea. –  Diego C Nascimento Jul 23 at 14:00

Lowest power consumption for a constantly changing display like a clock is probably a low-voltage LCD display with static drive (that means one common and many segment wires- one per segment), and of minimal area (obvious trade-off here that you'd like to be big enough to read). Low voltage also limits your ambient temperature range, but static displays are much better than multiplexed ones for temperature change to begin with. Alternatively, the off-the-shelf e-ink displays claim a typical current during the change of state (which might last 0.5-2 seconds) of 500nA/cm^2.

The static LCD displays draw continuous power that is related to the area, the square of the voltage, and the refresh frequency (usually something like 50Hz). Multiplexed LCD displays require a voltage divider to get the different voltage levels so they tend to draw more power at a system level.

You can drive static LCD displays directly with a microcontroller if it has enough pins (you have to generate symmetric waveforms that have no more than bout 50mV of DC component or the display can be damaged by electrochemical action, or some low power micros such as PIC and MSP430 have built-in LCD controllers (just write a bit to the register to turn a segment on).

You can drive e-ink displays at 5V directly from a suitable microcontroller (not a 3.3V type), or to full contrast they need 15V and a high-voltage driver. They also require DC balance on the drive but that simply amounts to pulsing the segments 'off' for as long as pulsing them 'on', and keeping track of the current state so you don't write them on more than once without turning them off first. You will also need n+1 pins to drive an n-segment e-ink display.

Chances are either type of display would work for you and would not represent the largest current draw in your system. Static LCDs are more familiar and would change faster from 5V so I think I'd tend that way for a clock display.

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Thank you for the help. Do you have any references for the 500nA/cm^2 eink displays? And also for the low-power LCD static displays? I was aiming at a 10-20 uW average power consumption for the display. –  user2088046 Jul 23 at 15:39
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Link added to guide (above). For static displays, look at Varitronix displays (eg. Digikey). Part numbers VIM-xxx are multiplexed, VI-xx are static. –  Spehro Pefhany Jul 23 at 15:46

The segmented LCD VI-602-DP-RC-S and the E-Ink display SC005221 both fit your needs. You'll have to carefully manage power consumption for the microcontroller that drives them, but they are both capable of running off a solar cell.

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You are probably right! I have already came across both of those displays..Still, I was unable to find the detailed datasheets of any one of those display parts..Can you point me to some documentation featuring the power consumption of either one? –  user2088046 Jul 24 at 10:27
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They don't publish power consumption because it depends on the controller and driving method. The LCD can be idlers as a capacitor per segment which you charge and discharge 30-60 times a second. The capacitance is tiny, but it does consume power. Talk to an applications engineer for lumex, varitronix, or another LCD supplier for calculations and notes on how to estimate power consumption and other lower power options. –  Adam Davis Jul 24 at 12:05
    
I will give it a try after getting those contacts! Thank you –  user2088046 Jul 25 at 14:46

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