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Interfacing unusual 4 digits 7 segments display

There is a common anode and common cathode of 7 segments display. But I got an unusual one.

There are 7 pins controlling 4 digits 7 segments. enter image description here

I find the pinout by a multimeter. Basically, each pin can be anode or cathode. enter image description here

My questions:

What is the exact name of this 7 segments display?
How to drive this display by Arduino, any website/sample code?
If I am using the 5V tolerant board, which resistor I need to use? And how is the schematic?

Answer*

1. I don't know. If I had to talk about it, I would say a LED segment display with a matrix layout with a complementary drive.

2. If a pin is used as a cathode, a LOW signal must be applied. If it is used as an anode, a HIGH signal must be applied. 

You could define constants for each segments anode and cathode that define a pin number. e.g :

 #define CAT_1A = 2 
 #define AN_1A = 1

and so on. Then if you whant to show segment 1A set CAT_1A to LOW and AN_1A to HIGH. 

Of course the whole process is more complicated. You have to define which segments must be light up to show a specific digit. You must show all four digits one by one in a fast loop and so on. But that's the same as you would do for an ordinary LED segment bar. The difference is that you have to configure pin pairs and not single pins.

3. Use your Multimeter to measure the Forward voltage of the segments. If they are red, it should be around 1.8 to 2.4 Volts. Substract this Voltage from 5 Volts. Then devide by 0.02 (20 mA) and you get the Resistor in Ohms. A standard LED is bright enough with about 20 mA.

Example:

 (5V - 1.8V) / 0.02A) = 160 Ohm

Use a resistor of say 100 Ohms at each pin. It is common available and as the current flows always through 2 pins, you have 200 Ohms together. 

It is also possible that the LED bar has internal resistors. Try to meassure the resistance beetween to pins.

**Edit**
When I said, "each digit must be shown in a fast loop", I forgot to clearify that, for each digit, the segments must be switched sequentially. 

You can do it even more complicated and show all Segments with a common cathode or a common annode at once. Then you need fewer cycles, but as you use resistors for all pins (see point 3.) this could dim the LEDs remarkably. Also, becaus technically, the forward voltage of different LED may differ, so the some could be brighter the others. 

There is also another thing, I did not think of, when I wrote the answer. If you power a segment, say from pin A (+5V) to pin B (GND), you have a resistor (100 Ohms), an LED (minus 1.8 Volts) and a second resitor (100 Ohms) in series. So in an ideal situation the voltage on pin B (also on the second resitor) would by around 1.6 Volts. Pin B is the cathode of the LED segment, but can also be the anode of another segment (S2). If the cathode of S2 is connected to GND there is a very small chance, that the segment lights up. If you don't see (ghost) lights, all is fine. But if some segments light up (but much darker than the right ones) you have to find a better solution.

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Re "You must schow all four digits one by one in a fast loop": That won't work. At any given time, you can only light segments that all share either the same anode or the same cathode. So, instead of looping over the digits, you will have to loop over the anodes (or over the cathodes).

Edgar Bonet
– Edgar Bonet

2020年12月07日 15:44:45 +00:00
Commented Dec 7, 2020 at 15:44
@EdgarBonet Showing Digit one by one is meant in addition to show each segment one by one. It seams that I did not express myself clearly. I'll edit the answer.

Peter Paul Kiefer
– Peter Paul Kiefer

2020年12月07日 16:45:01 +00:00
Commented Dec 7, 2020 at 16:45
Re "show all Segments with a common cathode [...] could dim the LEDs remarkably": no more than showing the segments one by one. If you want a brighter display, you have to drive the cathodes with transistors. Re "If the cathode of S2 is connected to GND there is a very small chance, that the segment lights up": that's why that cathode should not be pulled LOW (connected to GND): it should be left floating (INPUT mode, a.k.a. "high impedance").

Edgar Bonet
– Edgar Bonet

2020年12月07日 17:23:57 +00:00
Commented Dec 7, 2020 at 17:23
@EdgarBonnet As multiple LEDs with a common cathode or a common anode have a single source or drain with a resistor. The current through all LEDs together is the same as the current through that single resistor. So each LED sees only a part of the current. Say you have 5 LEDs with a common cathode then the current through the common resistor is 5V / (100Ohm+20Ohm) ~~ 41mA. The current through each LED is 1/5 of this ~ 8,3 mA. I think you would see the difference. Using high impedance state for unused pins is one of the better solutions I mentioned.

Peter Paul Kiefer
– Peter Paul Kiefer

2020年12月07日 18:07:33 +00:00
Commented Dec 7, 2020 at 18:07
You are right, of course, the display will be dim. But that's not my point. My point is that it will also be dim if you handle the segments one by one. Think duty cycle, and average the brightness over a full scan cycle.

Edgar Bonet
– Edgar Bonet

2020年12月07日 19:30:32 +00:00
Commented Dec 7, 2020 at 19:30

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