CIE Instructor answers a student's question.
Q. I have a seven-segment display with nothing to tell me what the pins are connected to. I’m not even sure if it is common-anode or common-cathode. Is there any way I can identify the pins?
A. Take a 1000-ohm resistor and insert it in your bread boarding socket. Then install the display in the socket near by. Connect +5V to the left end of the resistor. To the other end of the resistor, connect a piece of hookup wire you can use as a probe. Connect it to pin 1 on the display. Then connect one end of another jumper to ground. Use the other end of the wire as a probe, too.
Touch it to each of the remaining pins on the display. Note which segments light, if any. Repeat this procedure with the positive probe connected to pin 2, then pin 3, and so on. If you have a common-anode display, when the positive probe is on an anode pin, all the segments will light one at a time when you “sweep” through the other pins.
You can determine which pin is which segment simply by noting which segment lights when you ground a given pin. If it is a common-cathode display, this pattern will emerge if you repeat the procedure, but with the positive and negative probes exchanged.
The 1000 ohm resistor is important. It limits the current to no more than 5 mA. Without it, the current would be too high, and would bum out the segments. You will probably want to make yourself a diagram of the pinout for future reference.