Introduction: Arduino Clock Using Standard Clock Display

About: Hi There! I'm a mobile app developer (iPhone and Android) in British Columbia, Canada. I also write firmware, and enjoy dabbling with electronic circuits and micro controllers.

This is a relatively simple clock to build, in terms of the hardware required and in terms of hooking up all the wires. The complexity lies in the software, which I've conveniently included as part of this instructable :)

This instructable illustrates a few things:
* Keeping relatively accurate time on an Arduino without using a real time clock (RTC)
* Using a basic clock display - the type you see on most alarm clocks. In this case an LTC-617.
* Multiplexing a display to make it appear all digits are shown at once, even though the display only display one digit at a time.
* Controlling the brightness of individual digits (more LEDs on = dimmer digit)

I originally built this for a different platform, and decided it was time to port it to Arduino!

I hope you enjoy this instructable and find it useful - it makes a great start for so many evil project possibilities!

Step 1: Parts

For this Instructable, you will need:

1 Arduino (I used an Arduino-nano)
1 LTC-617 clock display (you might need to solder male headers onto it)
many jumper wires
1 button
1 resistor: 10k or close (to prevent short between gnd and 5v on button press)

For me this was a no-cost project as I already had all the parts.
The clock display was part of a grab bag I bought long ago. I imagine they should be very inexpensive and might even be salvageable from old clock radios.
Little buttons and resistors are also salvageable from old electronics (reset buttons from old computers, for example), and also very inexpensive to purchase.
The Arduino starts at around $15 on eBay, though I would expect anyone would be unlikely to ever use it for only just this one project!

I used a breadboard for easy connections, but that's not necessary. 

Step 2: About the LTC-617 Clock Display

I received several of these displays in a grab bag of electronics a few years ago, so I decided to put one to use.

This particular display does not have any smarts added to it. All of the pins go directly to the LEDs in the display. That means the smarts have to come from the software that controls the display.

You'll notice from the attached picture that each of the 4 digits is controlled by a single IO line, and all digits share the IO lines for the individual 7 segments. Which means that you cannot have two different digits shown at the same time.
To get around that limitation, we use the Arduino to show each digit for just a fraction of a second. When this is done repeatedly and fast enough, our eyes just see one steady display. In the code you can change the timing to make it blink each digit more slowly, if you wish to see how it works.

You could save some I/O pins and software complexity by using a 4511 binary (BCD) to 7-segment decoder, which allows you to use 3 Arduino pins to send a binary number to the decoder chip, which then does all the work of figuring out which LEDs in the digit to light up.
Here is a good link explaining how the decoder chip works:
That link also contains the logic table that shows which segments need to be on for any given number. I used something similar to this when I was building my software.

So what was the point of all that? My point is just that different displays will behave differently and the code would need to be modified accordingly.

Step 3: Make the Connections

There are quite a few wires that need to be hooked up, but if you go carefully and double check each line as you hook it up, it should be pretty straightforward.

I'm assuming you already know the basics of getting your Arduino going, so I won't get into those details. If not, there are a lot of nice examples online to get you started (I like the ones at

Here are the connections that need to be made. On the left is the pin of the LTC, on the right is the Arduino pin. For example, LTC pin 4 is connected to Arduino pin digital7. nc means that pin of the LTC is not connected to anything.

The button is connected in the usual buttony way using a pull-down resistor which connects the pin to gnd for LOW, and the button when pressed connects it to 5v for HIGH. As a side effect, pressing the button also connects 5v to gnd, which is why we need that 10k resistor to keep it from being a short.

LTC - Arduino
1 - nc
2 - nc
3 - nc
4 - d7
5 - d3
6 - d2
7 - d11
8 - d10
9 - d4
10 - gnd
22 - d9
23 - d5
24 - d6
25 - A0  // because d13 already has a built-in LED getting in the way
26 - d8
27 - d12
28 - A1
29 - gnd

5v - button - A4 - 10k resistor - gnd

I didn't have space near the Arduino, so in the picture you'll see that the button is connected through a few extra jumper wires.

Note that pin 1 on the clock display is at the bottom left (the pins are under the display area), and every hole counts, even the ones that don't have any connections. On this LTC display, "pins" 11 to 21 are just holes.

Step 4: Install (upload) the Software

The final step is to download the attached software sketch, and upload it to your Arduino.

Viola - we have a clock!

You can press the button to set the time. Hold the button down to set it quickly. If you hold it for longer than 5 seconds, it will go 10 times as fast.

The centre hour:minute colon blinks for the seconds.

The software has some comments to help you figure it out, but essentially it keeps track of time by checking the value of millis(), and every time 1000 millis have gone by it increments 1 second. Millis() returns the number of milliseconds since startup.

Obviously this won't be as accurate as using a real time clock (RTC), but it's good enough for plenty of fun projects involving time.

You'll notice that I remove 1,000 from the time tracking variable, rather than resetting to the current millis() - this helps it remain quite accurate. The code that updates the time will take some time to run, and the clock would lose a few milliseconds every time we do that. This way keeps it compared to the original start time and the accuracy depends on the accuracy of millis(), which I would guess is fairly decent.
Also, using a delay() to count to the next second would block the chip from other things, like detecting the button press.

ps, for the button code I heavily borrowed from the basic button example that comes with the Arduino software. My use is slightly different, but I debounce it in the same way.

I also noticed that different digits had different brightnesses - for example, the digit one has 2 LEDs lit, while 8 has 7 LEDs - lighting 7 LEDs at the same time makes it a fair bit dimmer. To compensate, I simply keep each digit on for a set amount of time multiplied by the number of segments in that digit.


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