I've taken this seven-segment display with 4 digits from a broken micro wave. Any ideas how to find out how it works? Finding an appropriate data sheet would be great.
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I have found a slightly more accurate (although very similar to @Wouter's find) diagram:
Also found here (PDF file)
To make this work, you need to connect a current limiting resistor between each one of the pins 11, 7, 4, 2, 1, 10, 3 and the HIGH level of your circuit. Correct use of these pins will allow you to light up the correct number character (notice A,B,C,D,E,F,G,DP LED's).
On the other side, control if each digit is on via pins 12, 9, 8, 6. Note that connecting these pins to HIGH will turn off the respective LED's.
NOTE - Since this display is IC compatible (see linked PDF), then HIGH = 5V, LOW = 0V. In that case, 330 Ω is a good value for the current limiting resistor.
NOTE 2 - This whole solution assumes that the 4x 7 segment display is of the A type (see image). If it is in fact of the B type, you will need to switch LOW and HIGH positions. Do a test on one of the LED's to discover what type it is.
Get a power source (5 .. 12V DC) and connect a 1k resistor to the +. With the two wires, connected to gound and the (other side of) the resistor you can probe various pin combinations. When you have a combination that lights up a segment, start trying other pins while holding one pin fixed.
These are LED displays, so they are direction sensitive. In most cases one side of all segments of one digit are bundled together, and the other sides of all same-position segments of all digits are also bundled. How the two points are wired varies. Check any LED display datasheet for an example, for instance this one :
Using a display like this requires either a chip that is designed for this purpose or an appropriately programmed micro controller.