The Greenest Wall-Powered Clock


Some of the most inefficient appliances in the home are AC mains-powered clocks. You can’t exactly turn them off and they use a whole lot of energy considering how often they’re looked at. [t3andy] came up with a great low power AC Mains clock that is only on 3% of the time. As a neat bonus, it also looks really, really cool.

[t3andy] is using a Teensy 3 as the brains of this clock, and the serial interface on the board provides a relatively easy means of setting the time without having to use buttons or tact switches. The clock face consists of 13 neopixels, with two red pixels showing the hour and a single green pixel showing the minutes. The time is measured with a DS3232 I2C real time clock with a battery backup.

The design is remarkably efficient since the LEDs are off 97% of the time, only being lit at the top of the minute. There are provisions for IR control and a PIR sensor to display the time whenever it’s needed, but that would obviously mean a hit to the energy efficiency.

51 thoughts on “The Greenest Wall-Powered Clock

    1. Many people just want to know the time.. Like, how long till my show starts, or I have to leave, etc.. It’s not a stopwatch, it’s not a timer, it’s a clock. So use it to tell the time, not count your heart rate.
      And who knows, maybe someone will add 60 led’s for you seconds needers.

      1. You really don’t need a second hand, but it sounds like this only powers up the display for a moment each minute. So there’s a 97% chance of the clock being turned off and totally useless when you glance at it, even if you only want to know the hour.

    2. I agree. At the very least you pulse the LEDs at a 1-5% duty cycle and be able to know the current time while saving power. However, it would be much easier to either use ePaper like the Kindle or replace the LEDs with a mechanical servo to always know what time it is. A clock that only tells the time 1/60th of the time is not very useful.

      If the author really wants to stick with this design, perhaps consider putting a capacitor or inductor in series with the LED to have it stay on longer. I’m not sure if this is possible, its been a while since I have done circuit design. If this isn’t possible, could you put a thin layer of glow in the dark plastic over each LED so it will stay lit for longer. I don’t know if 1 second is enough time to even light up, but I’m just throwing out ideas.

    1. So…this clock only illuminates the LEDs for 1.8 seconds every minute?
      How do I tell time on this contraption; Sit around for an average of ~~30 seconds every time I look at it?
      No thanks!

  1. > Some of the most inefficient appliances in the home are AC mains-powered clocks.

    How do you make this sentence meaningful? Given that a clock’s job is to tell time (and not to convert energy), how do you compute an efficiency figure that can be compared to other appliances?

        1. I’m with Angus here.
          AC digital clocks use very little power. I had trouble finding a figure (If someone has a meter and could test the current draw that would be awesome) but it would seem to typically be in the range of about 2W. I’m sure a more dim more energy efficent digital clock would use even less power.
          However that’s a digital AC clock. If you really want low power you can always go for an analog clock (which is what is emulated here). An analog clock can easly run on a single battery for many months if not years (and that’s with a second hand!). Some watches will run on a single SR416 (8mAh) battery for months. You can also get plug in analog clocks if you really want. So it’s a nice hack but trying to sell it on “energy efficent” is just plain maddness. You can have a far more practical clocks which lets you tell the time to a much higher accuracy that still uses less power.

          1. My watch battery cr 2016 (90 mAh) last years. It last so long that I forgot when I last changed it. that’s with one of those old timex digital / analogue Combo watches. then again the thing that chews though the most battery life on a watch is the backlight if you use it and how often you use it. I almost never use my indiglo backlight.

          2. You’d be surprised.

            5-10 watts is pretty common.

            Over a year 10 watts is about 87kwh of power, at my current price of 30 cents per kwh that’s $25 a year. (30 cents is Australia – yeah I was surprised to see that too.)

            A replacement clock is still going to take a couple of years to pay off (or throw it out and use your phone like the cool kids do now.)

            This high power is because most clocks have transformers rather than switch-mode power supplies, they waste that power. My microwave oven (with clock) use 10 watts just sitting there, my washing machine manages to use 7 watts while off!! The display is off, but the transformer is still powered up. On old Sony CD player did the same thing, I guess someone felt the cockroaches didn’t have enough places to keep warm.

            That’s why the energy star stuff got going, wasting 5-10 watts is pretty insignificant but multiply that by a dozen items per household and it add up.

  2. This is an art project, not a good way to save energy. As pointed out in the design PDF, you probably already have a few always-on digital LED clocks, and a watch or a phone, and you don’t need this clock at all.

    Now, there’s nothing wrong with building a unique expensive low-power clock. But combining it with comments about how other clocks represent the “American way of life” and making implausible claims about bankrupting electricity utilities is silly.

    1. Oh and if you never push the button to see the time, then you could claim that it is nearly 100% more efficient than other clocks made in America or anywhere else for that matter.

      1. Make a clock where you push the button, it turns on, receives the time from GPS or WWVB or something, displays it, then turns off.

        Sure, it takes 30 seconds or a minute to receive and display the time, but when it’s turned off it needs no power at all!

  3. There are many low power displays that would get far better performance being on all the time than this would being on 3% of the time … hell i think even an LP 8 segment LCD displays can do better

  4. considering a regular clock is a persistent display, having one with no second hand would mean it only “powers up” every minute yet can be viewed at any time. would fit the criteria here rather well. but i cant talk my house is lit up like a christmas tree 24/7 and i go out my way to use incandescents as i hate CFL bulbs lol. mains powers cheap.

  5. I see what looks like a Barrel jack feeding power into this circuit. Presumably thats DC power being supplied by an AC-DC “wall wart” adapter. I would wager the energy inefficiency of that power supply is dwarfing whatever power efficiency they’ve designed into this circuit.

    1. agreed. if you must have your led clock, try solar, and super caps, or something that is actualy green . i’ll stick with my westminster it’s always visible. it may only anounce every half hour but it runs on pasta, and coke.

  6. From the forum where the project is posted;”We went down to get a burger at McDonalds and within 1 hr. our NeoPixel T3 clock project views jumped by 300 – impossible.
    There is some wrong with the new server?”
    “Now they jumped by another 200 ?
    Something or somebody does not know how to count!”
    LOL the Hackaday effect? I’m not going to register just to suggest that possibility, that’s Brian’s job.

    Certainly something I wouldn’t thought of. Not really sure how a public used to see the time the moment they look at a clock would like it. At my current effective electrical rate this would save 7 cents per kilowatt hour. No more power mains connected clock uses uses, it may take a long time for this to pay out. An interesting and fun for awhile novelty.

  7. Triaxial counter display driven by one second pulses of suitable accuracy, otherwise known as a clock with a gravity powered human reset power source pendulum with thermal compensation. Substitute choice of modern timing, drive static display.

  8. Are AC powered wall clocks even made any more? I’ve not seen one for years and I’d challenge the justification for this project. If your only goal is to replace a AC powered wall clock then replace it with one of many single AA cell powered wall clocks on the market today.

    Regardless I really like this project and it should stand on it’s own with no other justification!

  9. If you’re going for extreme power efficiency, there are a significant number of poor choices going on here. First… LED is not an efficient display medium compared to other simple-to-use technologies such as LCD. Think of how long a digital wristwatch runs on a tiny button cell battery – mine is going on six years. And that’s running full-time, 24 hours a day. The second issue here is the particular LEDs used. These are not high-efficiency units, they’re high-intensity RGB pixels with integrated communication circuitry. And they are run with a processor hammering along at 24 effing MHz, why? Then the processor runs at 3.3V but the Teensy is fed with nominal 5, so there’s a LDO onboard eating the rest of that. Thankfully the LEDs themselves run straight from 5, but the RTC runs off a second LDO for more loss. But of course the greatest loss is in the conversion of AC mains to 5V DC, which will almost certainly eclipse the consumption of this board and probably by several times, even if it is a “high efficiency” switcher.

    For a really efficient setup still built DIY, check out the experiments done on MSP430s driving LCDs while powered from supercapacitors, posted here some time ago: … combine such a display system with a more efficient power source (solar is an oft-used example but there are others, such as ambient RF harvesting), or even leech off the usage of another appliance using a few loops of wire around the AC feed. There are better ways to do low-power…

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