Blinky the one-eyed clock

In this tutorial you learn how to make a blinking clock with a difference!

Updated 18/03/2013

Followers of my website would realise that I tend to make too many clocks in those tutorials. Well, I like making clocks… so here is another one. However this time I have tried to make the most simple version possible. Usually projects will have many LEDs, or perhaps an LCD, buzzers, buttons, all sorts of things. Which looks great and will impress many. But the other day I thought to myself … “how few things do you need to show the time?”

So here is my answer to that question: Blinky the one-eyed clock …


It reminds me of the giant killer orb from The Prisoner… Using a minimal Arduino bootloader system, a DS1307 real time clock IC and an RGB diffused LED … we can make a clock that blinks the time, using the colours of the LED to note different numerical values. For example, if the time is 12:45, the clock will blink red 12 times, then show blue for a second (think of this as the colon on a digital clock) then blink four times in green (for forty minutes), then blink three times in red for the individual minutes. If there is a zero, blink blue quickly. Then the clock will not display anything for around forty seconds, then repeat the process. Here he (she, it?) is blinking the time:

Setting the clock is simple. It is set to start at 12:00 upon power up. So for the first use you have to wait until about five seconds before midday or midnight, then power it up. To save cost it doesn’t use a backup lithium battery on the real-time clock IC, but you could if you really wanted to. If you would like to follow my design process narrative, please read on. If you only want the sketch and schematic, :( head to the bottom of this article.

Design process narrative…

So let’s get started!

The first thing to do was test the RGB LED for brightness levels, so I just connected it to the digital output pins of my Eleven via suitable current-limiting resistors. Each LED is going to be different, so to ensure maximum brightness without causing any damage you need to calculate the appropriate resistor values. This is quite easy, the formula is: resistor (ohms) = voltage drop / LED current So if you have a 5 V supply, and LED that needs only 2 volts, and draws 20 milliamps (0.2 amps) , the calculation will be: resistor = (5-2)/0.02 = 150 ohms. To be safe I used a 180 ohm resistor. The LED was tested with this simple sketch:

It was interesting to alter the value of d, the delay variable, to get an idea for an appropriate blinking speed. Originally the plan was to have the LED in a photo frame, but it was decided to mount a ping-pong ball over the LED for a retro-style look.  Here is a short video of the result of the test:

If you are going to use a ping-pong ball, please be careful when cutting into it with a knife, initially it may require a lot of force, but once the knife cuts through it does so very quickly:


Now it was time to develop the sketch to convert time into blinks. The sketch itself is quite simple. Read the hours and minutes from the DS1307 timer IC; convert the hours to 12 hour time; then blink an LED for the number of hours, display another colour for the colon; divide the minutes by ten and blink that in another colour; then the modulus of minutes and ten to find the individual minutes, and blink those out. Here is the first sketch I came up with. Finally, the code was tested using the Eleven board and my DS1307 real time clock shield. It is best to use existing hardware while testing, before committing to purchasing new hardware and so on. So here it is on the breadboard:


And telling the time! In this example, the time is 3:45…

But perhaps that was a little bland. By using analogWrite() we can control the brightness of the LED segments. So now there are two more functions, whiteGlow() and blueGlow(); whose purpose is to make the display “glow” by increasing then decreasing the brightness. And scale back the amount of blinking, to increase battery life and make blinky less obvious. So now the display will glow white to announce the forthcoming display of time, wait a second, blink the time (with a blue glowing colon) then stay dark for ten seconds before repeating the process. Here is a quick demonstration of this display style:

Here is the sketch for the above demonstration, and the final one I will use with the hardware prototype. Once happy with the sketch, I put a fresh ATmega328 with Arduino bootloader in the board and programmed it with the blinky sketch, to be used in the final product.

Next was to build my own hardware. My last hardware unknown is the amount of current the circuit draws. Once I know this the correct voltage regulator and power supply can be decided upon. I had a fair idea it would be less than 100 milliamps, so I put a 6V battery onto supply duty via a 78L05 5V regulator (data sheet), and recorded the result:

So it varies, between 20.5 and 46 mA. As it only reaches 46 mA for a short time, we could consider the constant draw to be averaged out at 30 mA. I really want this to be able to run from a battery, but without having an external lead-acid battery lurking around, it will need a plug-pack with an output voltage greater than 7V DC. Another alternative would be to run it from a USB socket, a nice source of 5V. If doing so, there wouldn’t be a need for the 78L05 regulator. Which brings us to the  circuit diagram, which includes the power regulator:



It does not allow for programming in the circuit, so you will need to program the microcontroller on another Arduino or compatible board, then transfer it to the blinky circuit board as described above. At this stage I tested it again, but using a solderless breadboard. In doing so you can make final hardware checks, and  generally make sure everything works as it should. This is also a good stage to double-check you are happy with the display behaviour, default time and so on.


Time to solder up the circuit on some stripboard. Blank stripboard varies, but luckily I found this and a nice box to hold it in:


Stripboard does vary between retailers and so on, so you will need to work out the layout with your own board. In doing so, please double-check your work – follow the layout against the schematic and so on. Have a break, then check it again. There is nothing worse than soldering away to realise you are one strip too far over or something. My hand-eye coordination is not the best, therefore my soldering isn’t pretty, but it works:




One would say that there is a good argument for making your own PCBs… and I would start to agree with that. The LED is soldered to some short leads to give it a bit of play, and some heatshrink over the legs to keep them isolated:



And finally, to add a DC socket to feed blinky some power…



The last thing was to check the soldering once more under natural light, to check for bridges or shorts, then have a cup of tea. Upon my return I drilled out a hole in the enclosure lid for the LED, and one one the side for the DC socket, and fitted the lot together… and success! It worked :)

So there you have it. The journey from a daydream to a finished product… well a prototype anyway. But it works, and that is the best feeling of all. You can download the schematic from here. And here is the Arduino sketch:

I hope you enjoyed reading this post and hopefully felt inspired enough to make your own.

In the meanwhile have fun and keep checking into Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

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John Boxall

Founder, owner and managing editor of

19 Responses to “Blinky the one-eyed clock”

  1. Usng a png-pong ball as a dffuser s a great dea! (one that I shall lberate for a project I’m workng on)

  2. DJPhil says:

    Interestng! It’s spnnng off all sorts of deas n my head. I bet a four sphere verson usng a color to number converson would look neat. Add temperature, weather data, hmm.

    I’m gong to go make some coffee and thnk on ths.

    • John Boxall says:

      Sounds great… f you do please let me know

    • DJPhil says:

      I’ve got a long way to go myself before I can craft such a thng, as I’m currently completely unschooled n programmng and mcros. I’m workng on that though, as soon as I fnsh a half dozen or so analog projects I have backed up . .

      The coffee got me as far as wonderng f RGB LEDs could reproduce resstor color codes. I’d magne brown would be dffcult. It could gve you another dsplay method to play wth.

      P.S. I stopped to wrte the fellow who developed your wordpress theme. I knew I’d seen that name before, and then t ht me. He was a muscan n the late 90s trackng communty (mod, fasttracker, screamtracker, etc.), and very good at t. Small world!

      • John Boxall says:

        Crkey, small world sn’t t?
        Were you consderng a resstance meter that shows the colour codes? That would be awesome. You could always use a small colour LCD nstead, could be easer to make brown.
        have fun

    • DJPhil says:

      Oh, also, dd the png pong ball actually say ‘For External Use Only’ on t?!?

      D: Ykes!

  3. Joshua says:

    Normally a fan of 24-Hour tme….. But that could make readng the tme a much longer process….

  4. Carlo says:

    H John,
    when you make a clock, you always use a tmer c…
    Isn’t possble to use the arduno tme lbrary or the mlls() functon for keep track of tme?

    • John Boxall says:

      H Carlo
      Good queston. I use the DS1307 (and soon the NXP PCF8563) for accuracy, t s an actual real tme clock IC, plus wth an optonal 3V cell t can remember the tme when the crcut loses power. mlls() depends on the board beng on constantly, otherwse t resets to zero, and also resets after 50(?) days or so. The dateTme lbrary s good, but depends on the Arduno havng a crystal for accuracy, some do not. So f I am makng a clock, n my opnon a dedcated tmng chp wth back up s the way to go. But at the end of the day t s up to you! :)

  5. Collin says:

    That s REALLY cool! You are gong to cost me lots of money you know!

  6. Paul says:

    Hey John,

    Thanks for the fantastc clock dea. I thnk t s just great. I played around some wth your program and put the fnshed project up on Instructables:

    I gave you full credt n the Instructable and the vdeo ncluded n t for the dea behnd the project. Just gvng you a heads up. Thanks agan.

    As you say over there, Cheers mate!


  1. […] you are lookng for a less s more soluton to showng the tme then you need to have a look at the Blnky Clock that John Boxall created. It s beng run from a ATmega328 Mcrocontroller and a DS1307 Real Tme […]

  2. […] when prototypng my clock and tmer Arduno-based projects on a breadboard. For example, blnky, or the varous clock projects n the Arduno […]

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