Multiplexed Display
You must have noticed this in movies or any other recorded videos - if the recorded video displays seven segment displays they always flicker.
Have you ever wondered why? The answer to that is multiplexed display. And if you would like to know more, please read ahead.
First some basics. A seven segment display (SSD), as the name indicates it has 7 segments, like below. For our understanding, I’ve marked them with alphabets. Each segment is a diode.
(a)
-----
| |
(f) | (g) | (b)
-----
| |
(e) | | (c)
-----
(d)
Now, you might ask, what is the problem. To understand the problem, lets learn a tad bit of a diode.
A diode, as we know, passes current in one direction - and this is the feature that we find most useful in many of the circuits we see today. A diode is symbolized as below.
|\ |
anode (+) | \ | cathode (-)
___________| \|__________
| /|
| / |
|/ |
To make a LED ON, we connect the positive terminal of the power supply to the anode and the negative supply to the cathode - remember the reverse polarity wont make the LED ON. At the same time, the LED will not be damaged if we reverse the polarity, because we just learnt, on how a diode works.
A SSD will have 8 pins, 7 anodes, one for each diode and 1 common cathode.
So, if we have to make a 1, then we connect the anodes of b and c to the positive terminal (indicated as Vcc) and connect the only one common cathode to the negative terminal (Gnd) of the power supply.
Circuit
(a)
_____/ ______|\|_______
| |/| |
| |
| (b) |
|_____--______|\|_______|
| |/| |
| |
| (c) | Output
|_____--______|\|_______|
| |/| | |
| | | (b)
| (d) |
_________|_____/ ______|\|_______|_______ |
Vcc | |/| | | | (c)
(+) | | -----
| (e) | --- Gnd
|_____/ ______|\|_______| - (-)
| |/| |
| |
| (f) |
|_____/ ______|\|_______|
| |/| |
| |
| (g) |
|_____/ ______|\|_______|
|/|
Similarly, for a 2, we connect the anodes of a, b, g, e and d to Vcc and then connect the cathode to Gnd.
Circuit
(a)
_____--______|\|_______
| |/| |
| |
| (b) |
|_____--______|\|_______|
| |/| |
| |
| (c) | Output
|_____/ ______|\|_______| (a)
| |/| | -----
| | |
| (d) | (g) | (b)
_________|_____--______|\|_______|_______ -----
Vcc | |/| | | |
(+) | | ----- (e) |
| (e) | --- Gnd -----
|_____--______|\|_______| - (-) (d)
| |/| |
| |
| (f) |
|_____/ ______|\|_______|
| |/| |
| |
| (g) |
|_____--______|\|_______|
|/|
And so on.
So, what is the problem, you ask? Well, there isn’t any. Yes, there is no problem, when we deal with one SSD. But most often we see SSD’s always grouped - we need at least 4 SSDs to display time, for example.
Let us take a 2 SSDs together to explain the problem - it would be something like below:
(a) (a)
----- -----
| | | |
(f) | (g) | (b) (f) | (g) | (b)
----- -----
| | | |
(e) | | (c) (e) | | (c)
----- -----
(d) (d)
digit - 1 digit - 2
In this case, the anodes of a of both the digits are interconnected. Similarly, every segment of each digit is interconnected to the corresponding segment of the other digit. But each digit has its own cathode. This is called a Common Cathode SSD. It would be something like below.
(a)
_____/ ______________________________|\|_______
| | |/| |
| | |
| | (b) |
|_____/ __c___________________________|\|_______|
| | | |/| |
| | | |
| | | (c) |
|_____/ __c___c_______________________|\|_______|
| | | | |/| |
| | | | |
| | | | (d) |
_________|_____/ __c___c___c___________________|\|_______|_______
Vcc | | | | | |/| | |
(+) | | | | | | -----
| | | | | (e) | --- Gnd
|_____/ __c___c___c___c_______________|\|_______| - (-)
| | | | | | |/| |
| | | | | | |
| | | | | | (f) |
|_____/ __c___c___c___c___c___________|\|_______|
| | | | | | | |/| |
| | | | | | | |
| | | | | | | (g) |
|_____/ __c___c___c___c___c___c_______|\|_______|
| | | | | | | |/|
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | | (a)
|___c___c___c___c___c___c___|\|_______
| | | | | | |/| |
| | | | | | |
| | | | | | (b) |
|___c___c___c___c___c___|\|_______|
| | | | | |/| |
| | | | | |
| | | | | (c) |
|___c___c___c___c___|\|_______|
| | | | |/| |
| | | | |
| | | | (d) |
|___c___c___c___|\|_______|_______
| | | |/| | |
| | | | -----
| | | (e) | --- Gnd
|___c___c___|\|_______| - (-)
| | |/| |
| | |
| | (f) |
|___c___|\|_______|
| |/| |
| |
| (g) |
|___|\|_______|
|/|
Ok, we are close to the problem now. Let us say, we want to display 21, then we need to connect the anodes of a, b, g, e and d of the digit - 1 to Vccc and the anodes of b and c of digit - 2 to Vcc. However, the anodes corresponding segment of both the digits are already interconnected. So the only possibility is to show 22 or 11. In short, we will be able to show the same content in both the digits. You get the problem? Yeah, that, precisely is!
I’d recommend you to think of the potential solution. And if you find the solution, please do let me know.
I will publish the solution in my next post.
Thanks.