I would like to make a timer.

The timer will have a 7 segment digital display and will work with a microcontroller of some sort.

I would like to be able to put in a time (e.g. anywhere between 0 - 10 minutes) and have it countdown from the input time to zero. I would like to display down to the millisecond resolution as well, but this is not crucial.

I want the timer to be relatively accurate (up to +- 1 second in ~20 minutes).

What kind of circuitry do I need? Will a 555 timer circuit be able to handle this? Need some direction to get started! Please let me know what other information is required! Thank you.

So I got:

  • Arduino
  • HS420361K-32 x2
  • MAX7219 backing thingy
  • 16-Key Keypad

I am assuming all of this will work together well. Am I missing anything here?

I will be fiddling with the Arduino software for a while until I figure out how I can/want to do this.

Any tips would be most welcome =)

  • \$\begingroup\$ That accuracy will need a resonator at least. \$\endgroup\$ May 5, 2015 at 0:22
  • \$\begingroup\$ Why wouldn't you use internal timers on microcontroller? It is what they are meant for. Timing stuff. \$\endgroup\$ May 5, 2015 at 1:15
  • \$\begingroup\$ I was wondering if the internal timers on microcontrollers would be good enough... How accurate are they usually? Will the accuracy of the internal timer be affected in any way as I use/program the microcontroller? \$\endgroup\$ May 5, 2015 at 16:00
  • \$\begingroup\$ The internal timer is usually calibrated for 25degC and a specific voltage. Any deviation from those specifications will affect the accuracy. \$\endgroup\$ May 5, 2015 at 16:04
  • \$\begingroup\$ If I plan on keeping this question active and editing it as I go until I finish this small project, should I not accept any answers until the end or how should I handle this? i wanted to ask this on Meta but not enough rep ='( \$\endgroup\$ Jun 18, 2015 at 23:35

1 Answer 1


There are so many possible ways to accomplish this task.

You can build from discrete logic chips. Because you want a down-counter, this generally requires two chips per digit for the counting / display section. In addition, you will need several other chips to divide your timebase oscillator down to the count frequency that you want as well as the start - stop circuit.

It can be done - it's not hard. It's not even all that tedious.

Do note that using discrete logic chips means that you would most likely want to use LED 7-segment displays. You could drive bare LCD glass but that requires lots of logic gates to get the AC drive to the display segments.

You can also to this task using a small microcontroller. This type of project would normally require only the microcontroller (one chip) plus whatever timebase oscillator you want to use.

Using a small microcontroller allows you to choose whether you want to use LED displays or a small character-based LCD display.

The downside of using a microcontroller is that you have to write the firmware that does the application that you want. Also note that this is an upside - you can easily add features that would not be easy (or even possible) with a discrete-logic design.

Decide which way you want to go and modify your question accordingly. We'll help you get to the end of the project.

  • \$\begingroup\$ The display will be a 7 segment display and I will be using a microcontroller of some sort. Programming microcontrollers is fine with me, but the electrical portion is where I have the most trouble... So when you say I can use a microcontroller with a timebase oscillator, what is a timebase oscillator?? \$\endgroup\$ May 5, 2015 at 16:05
  • \$\begingroup\$ Your timebase oscillator is whatever is providing the timebase for your system. If this were built with discrete logic chips, I'd be looking at an oscillator that uses a 32,768 Hz watch crystal and then dividing that down. Because you are going to use a microcontroller, the timebase can be the same crystal or resonator that provides the clock frequency for your microcontroller. If you need more accuracy than that, many microcontrollers allow you to use a separate crystal connected to a different internal oscillator connected to one of the timers inside the micro. \$\endgroup\$ May 5, 2015 at 16:17

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