Can't figure out how this 4 digit display works
October 20, 2010
Sorry this isn't really a robot related question, just an electronics one.
I was cruising through a local electronics shop. They have lots of salvaged parts. They had a whole box of old 4 digit LED clock displays. Since they were only about $3, I picked one up. My son wanted to build a clock.
After working through all the connections, here's the schematic.
If you check carefully, you'll see that there are a lot of shared pins. The problem is that there are combinations that are needed that also light unwanted LEDs.
For example, if I want the display to write 12:00, I need to light the following LEDs:
- Digit 1: 1b, 1c
- Digit 2: 2a, 2b, 2g, 2e, 2d
- Colon: dp3, dp4
- Digit 3: 3a, 3b, 3c, 3d, 3e, 3f
- Digit 4: 4a, 4b, 4c, 4d, 4e, 4f
For Digit 1, I need a logical low on pin 2, and a logical high on pins 6 and 9.
For Digit 2, I need a logical low on BOTH pins 1 and 2, and a logical high on pins 13, 10, 9, and 12.
Already, we are in trouble. Because both pins 1 and 2 need to be low, but now setting pin 13 high will also turn on LED 2f, which we don't want when we are writing the number 2. The situation gets worse as you proceed through each digit, but I won't bore you with the details. Try it your self if you are interested.
So my question is, how the heck did this display ever work? There are several pins that are not connected to anything, but there are also two pins (7 and 8) that are connected but don't seem to do anything. Maybe they provide some sort of control?
Or maybe the clock that ran this display cycles pins 1 and 2 on and off in time with the enable pins for the LEDs, so that you don't have pin 2 set low when pin 13 is high to make a '2' on Digit 2. That's the only way I can see. Any other ideas?