Re-purposing a LED panel from an Equity Alarm Clock

Hello!

I am working on a timer project - specifically for timing pick-up basketball games (countdown from 12 mins). Too many arguments and we need to speed games up. Anyway, I needed a decent size display so I purchased an Equity alarm clock (model 30037) and took it apart. Now, I’m so new at this it isn’t funny but my head is spinning on all this anode/cathode/segment stuff.

Anyway, here is a pic of the display.

Using Arduino_Pin 10 (my black wire w/ 220 resistor) and Arduino_Pin 2 (my red wire), I can get each segment to light up if I touch the right combination

For instance

Arduino_Pin10 → LED_Pin1 (which is marked as A)
Arduino_Pin2 → LED_Pin21

Will light up the “:”

Arduino_Pin10 → LED_Pin3 (which is marked as B)
Arduino_Pin2 → LED_Pin10

Will light up the “left-upper” of Digit 2 (from the left)

When I mapped it all out (since I could not find a datasheet), I was able to determine which pins belong to which segment. (See attachment)

Now, based on the combinations, I have 24 combinations. LED_Pin3 and LED_Pin4 are common between.

My initial set of questions are:

  1. What kind of display is this? (I should note that when lighted, there appears to be 1 LED driving each segment.)
  2. Can I assume that LED_Pin3 (shown as B) and LED_Pin4 (shown as C) are my ground?
  3. Because of the 24 combinations, I’m assuming that I need to do multiplexing. If so, can I handle all of that within code or will it require something additional like shift registers? (This really goes back to question 1, because if I know what kind of display, I can research the “how” better)

Any help that you guys can provide would be very much appreciated. I’m new to this stuff (although I can handle a soldering iron and know how to program) and I have a lot to learn with Arduinio. Also, sorry for my “schematic” - just did what made sense. I’m sure it will make an electrical engineer cringe though! :slight_smile:

Stephen

By '24 combinations' do you mean there are 12 Anode (+) pins and two Cathode (-) pins (B and C). If so you can light up any combination of segments by connecting all 14 pins to Arduino pins. You can use the Analog Input pins as Digital pins.

An Arduino pin can't directly sink the current for 12 LEDs so you should probably put a small NPN transistor between each Cathode and Ground. Google "Arduino transistor" to find out more.

The display code will look something like this:

loop(){
    // Turn off both Cathodes
    // Turn on the Anodes for Cathode B
    // Turn on Cathode B
    delay(5);
    // Turn off Cathode B
    // Turn on the Anodes for Cathode C
    // Turn on Cathode C
    delay(5);
    // Turn off Cathode C
}

Previously discussed here.

Paul__B: Previously discussed here.

Yes, this is one of the threads that has helped me get to where I am right now. Going to go through it again; I'm sure it will make more sense since I read it 24 hours ago. Thanks!

johnwasser: By '24 combinations' do you mean there are 12 Anode (+) pins and two Cathode (-) pins (B and C). If so you can light up any combination of segments by connecting all 14 pins to Arduino pins. You can use the Analog Input pins as Digital pins.

An Arduino pin can't directly sink the current for 12 LEDs so you should probably put a small NPN transistor between each Cathode and Ground. Google "Arduino transistor" to find out more.

The display code will look something like this:

loop(){
    // Turn off both Cathodes
    // Turn on the Anodes for Cathode B
    // Turn on Cathode B
    delay(5);
    // Turn off Cathode B
    // Turn on the Anodes for Cathode C
    // Turn on Cathode C
    delay(5);
    // Turn off Cathode C
}

Thank you! The way you rephrased the question helped me immensely. I'm counting 14 anodes (+) and 3 cathodes (A,B,C) total, but it appears to only use 12 anodes and 2 cathodes for the digits. I've been doing more research on shift registers and I think that is the way for me to go (using multiplexing to help reduce the amount of energy required). Just ordered a few of the 74HC595 chips.

Appreciate the input and suggestion on the transistor too; going to research that right now. Would like to understand how that may work as well.

You MUST use multiplexing. If you enable two cathodes at the same time, two segments will light up for each anode enabled. Since you can't have more than on cathode enabled at a time you muse switch between them fast enough for the switching to be invisible. That is the definition of multiplexing.