I had the chance to purchase some 14x21 flip dot displays, and as a total nerd for things like this, I couldn’t pass it up. At €85 for each module, they were a little pricy, I didn’t really know what do do with them, but they’re so seldom on ebay these days that I couldn’t pass it up.

They’ve been sitting on my shelf for months. I have no idea where to start, and whlist I have a pretty well stocked electronics bench, handling undocumented boards with 4× 2×7 IDC headers, was new to me. Expensive, delicate, and overwhelming. Not a great start.

ChatGPT helped me work through it though, wiring at 3V, current limited to 100mA I could begin poking around. Of course the boards are undocumented, and I don’t know how they work, so it took a while.

module-14x21.png

The arrangement of the IDC headers (did you know that’s the proper name for ribbon cable headers?) holds come clues, the module is 14x21 and there’s a header with 14 pins, and 3 headers with, effectively two rows 1x21 of pins.

Some hapless probing with my meter on diode mode showed that there was a path between H1’s pins, and the “first row” (i.e 1..7) on the other headers, and in reverse there was a pathway back (i.e from 8..14 back to the header pins in the first header block). Hapless how, exactly?

module-headers.png

Well probing with positive on any pin in H1, any of the pins in the “top” (as viewed) here row shows a forward voltage of 5v, that implies there’s an “allowed” path across this diode matrix between those pins.

H1 seems to be some kind of “row” select, and the “top” row of pins on H2..4 seems to be some kind of column select.

Lots of hand-waving quoting “top” and “bottom” just won’t do, so time to get it straight: an IDC connector canonically has pin 1 on the upper right side of the connector, where the “upper” is defined as the side with the key slot:

idc-connector.png

With that in mind,lets actually mark out what pins we have on the board, it would strongly appear to be this:

module-correct-pinout.png

So, from ROW SELECT to even numbered pins on the COL SELECT headers has a forward voltage, and from odd numbered pins on COL SELECT there’s a pathway back to the pins on ROW SELECT.

So, I think it’s fair to say that this is how the board is laid out:

module-set-unset.png

Toggling a pixel on, we supply positive voltage on H1 (which ever pin/s we like), and we make sure the SET pins are pulled low.

Trial and error with a benchtop power supply in CV (Constant Voltage) mode, showed that for a short time (as judged by “as long as it took to the dot to flip when messing with crocodile clips”) at 5V the coil draws only about 100mA.

I measured coil resistance (i.e between H1, Pin 1, and H2, Pin 2) at about 500Ω later, and found out that my benchtop power supply shenanigans didn’t make sense. ChatGPT told me that more like 10mA current would be better, and a 10-15ms pulse to drive it.

Inductive loads like this can damage sensitive GPIO pins on microcontrollers, so it’s wise to use some kind of h-bridge, or transistor pair, or something, this much I knew, but which?