We are developing a embedded system (a electronic door lock) that uses different LED's and a Buzzer for signaling different events to the user. There's a very limited opportunity to communicate to the user. A signal is just a sequence of LED's and Buzzer. Always only one at a time.

For example: Access OK: Green LED (100 ms) Buzzer (150ms) Access OK: Buzzer (200ms), Pause (100ms), Buzzer (200ms), Red LED (150ms)

Now the problem is that the timing is not very accurate. So we try to define the acceptable timing differences in the signal. We want to figure out how we could define some tolerance on the given times so the user doesn't see any difference.

The main questions are:

  • What is the minimum perceptible difference in the duration that an LED or a Buzzer is on?
  • What about pauses between them?
  • How long is it acceptable (for a responsive device) to wait to signal after a user action?

I know this is a very fuzzy topic. It's not realy measurable and has also to do with psychology and other factors.

But maybe someone has already done some research on the topic or there is some known related work.

  • Sorry, I'm not understanding the question. What does this mean? "We want to figure out how we could define some tolerance on the given times so the user don't see any difference." and "Now the problem is that the timing is not very accurate." – Paul Dessert Aug 21 '14 at 18:36
  • Sorry it's quite difficult to describe the situation. For example if I have a LED (or a buzzer) that should be active for 100 ms but it's active twice as long (200 ms) so the user will recoginze a difference. So how big is the tolerance so that the user can't see any significant difference. – woodtluk Aug 25 '14 at 7:15
  • That the timing is not accurate means that the LED on phase is often qute different than we wish it to be. Sometimes even twice as long. – woodtluk Aug 25 '14 at 7:16

Below are some brain reaction times, I found from the book of Jeff Johnson, Designing with the Mind in Mind :

  • Shortest time a visual stimulus can be shown and still affect us (perhaps unconsciously) : 5 ms
  • Maximum interval for visual fusion of successive images : 50 ms
  • Time lag between a visual event and our full perception of it : 100 ms
  • Maximum interval between events for perception that the one event caused the another : 140 ms
  • 1
    Thank you for the information. It's this kind of measurments that I'm looking for. – woodtluk Aug 25 '14 at 7:17

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