For a school project I need to have a timer counting down from 1 minute and I have a 4 digit seven segment display that was taken from an alarm clock. I understand how seven segment displays work and the Arduino kit I have comes with a smaller 4 digit 7 segment display that has 12 pins, but my problem lies with the alarm clock display only having 9 pins. I have been able to power individual segments by applying power and ground to random pins but the segments powered seem to be completely random, and I have discovered that using the first two pins do nothing, and certain pin combos light up several segments very dimly( I assumed those certain combos are special and not intended to be run alone.) I am using 220 ohm resistors but haven’t figured out how to translate what I know about the 12 pin system to this 9 pin system. An example code would definitely help. The pins on the alarm clock display are all in a line and I have the ribbon cable cut and connected to a bread board for easier access.
Have you found it to be common cathode or common anode?
Maybe seven pins for segments, then a binary value on two pins to indicate which digit.
Welcome, Figuring out that display could be a lot more work then your entire project. A common display is a 16x2 1602 LCD Display Module. Here is a link the last ones I purchased. Delivery was a few weeks but I had time. I got it with the backpack so the I2C (4 wires) was all I needed to connect it to my Arduino.
https://www.aliexpress.us/item/3256805954359569.html?spm=a2g0o.productlist.main.
Have you been given this special display as part of the project? In other words, is the analysis of the display an integral part of the assignment?
Special about a clock is that in the first digit position only a 0,1 or 2 are shown. Also normally no decimal points on each digit, only a central colon.
Interesting to know would be if you find a combination of 2 pins which light a segment or group of segments , then if reversing the polarity lights a different set of segments.
My thoughts also. The display could be Charlieplexed. Also I remember reading somewhere about early led alarm clocks using low voltage AC to run the display. I can't remember how those work
@vulcanginger11 There may be no other option but to reverse engineer the display by testing every combination of pins methodically and drawing a schematic from what you find. Testing every combination means connecting +V to pin 1 and -V to pins 2 to 9 in turn, and connecting -V to pin 1 and +V to pins 2 to 9 in turn. Then repeat that for pin 2 and so on. You are correct to always use a current limiting resistor for testing.
Is any part number/code printed anywhere on the display?
I could imagine something like this which uses 8 pins. Maybe there is a special colon or an AM/PM indicator in addition.
EDIT
This fails. See below.
No, I don't think that would work. If 1 was high and 5 was low, 2 segments would light.
I did once build a 4 digit 7 Seg display, Charlieplexed using 9 pins.
OK. Back to the drawing board . . .
Very nice demo. Maybe then this where matching segment groups go on the same pin. The digit pins are 1 to 4 .
The multiplexing strategy would then to light all digits where segment A should appear, then all digits where segment B should appear etc. Momentarily unused digits are set to high impedance.
I will have some additional information later today, but I do remember that reversing the polarity on the same pins lights a different segment. There is a decimal point between the first and second digit and I believe the dp of the 3 digit was moved to be the hour and minute separating colon with digit twos dp.
I’ll have a picture of the display and my current wiring setup later on today. I only need to use the first three digits and the colon as I am counting down from one( 1:00 and 0:59-0:00). Reversing polarity of two similar pins changes which segment lights. Additionally the segments lit by any combo of pins aren’t always the same digit.
It is likely that once you get a working combination, then the same segment will be lit in all digit positions. The colon may be different. Anyway, for this reverse engineering, you'll simply have to systematically record all tested pairs of pins and the results as suggested here :
Use the standard A to G scheme for specifying the segments lit :
More details on that circuit I built a decade ago.
That is very interesting. I hadn't looked at Charlieplexing before. You've used a normal 4 digit seven segment display (exposing 12 pins) . Do you think it could work for a display such as the one the OP appears to have which exposes only 9 pins ?.
I can see how to do it, assuming the schematic in post #10 is close to reality, using segment by segment multiplexing where the segments are stepped through and, for each segment, the digits are lit which share that segment. In that case, the current limiting resistors go to the digit pins.
I have everything connected via a bread board in the same orientation. Is there a better way to wire this? Also, the first two pins only run a buzzer that is on the back of the display, so only seven of the pins are used for the display.
That looks fine. Are you sure the buzzer does not share a common with something else ? Is there any chip or visible circuit on the other side of the display ?
No, look closely: I used 4x single-digit displays.
I could not have done it with a typical 4-digit display, because the internal wiring in those is not suitable for Charlieplexing. But with 4x single digit displays, you have the flexibility to wire up for Charlieplexing.
But @vulcanginger11 's display may have been designed for Charlieplexing. If so, it's internal wiring will be similar, but almost certainly not identical, to the schematic I posted. There are many ways they might have chosen to wire up the segments for Charlieplexing.
Ah, interesting....
If there was a driver chip, I would not have expected @vulcanginger11's attempts to have successfully lit any segments at all.
Is that a product code?