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Recently I've got my hands on several 20x2 character displays which don't seem to work. This is what they look like: enter image description here

enter image description here

Apparently, they are HD44780-compatible. The three 16-pin connectors (single-row and double-row) are just parallel to one another. Pins 15 and 16 (typically used for backlight in LCDs) are not connected to anything.

I am able to push the data in using a microcontroller using both 4-bit and 8-bit HD44780 protocol, then read the data back, so the logic seems to be working. The logic is working at both 3.3V and 5V supply voltage. The display, however does not light up. I've tried voltages from 0 to 5V on the contrast input (pin 3) to no avail.

I've taken one of the displays apart, and this is what the matrix looks like:

Top

Bottom

This is what the board looks like with the matrix removed:

enter image description here

I'm pretty sure the driver for the matrix is not dead. In the disassembled screen I've measured the waveforms at rows and columns. There is a DC voltage at about 7.5V across all the rows, and different waveforms with about 1.5V amplitude and period of about 7ms at different columns. The waveforms look like they correspond to some pixels being on and off.

UPDATE: Amplitude of these waveforms is dependent on the brightness input (pin 3), going from 0V to 7.5V.

https://imgur.com/a/TNg1l

The devices were new in factory packaging, so my guess is either they are all defective (even though there is a QC stamp), or they have some design flaw.

I've tried applying different DC voltages across rows and columns of the matrix, but nothing happens.

What kind of matrix is this anyway? Doesn't look like an LCD, so can it be some kind of LED (PLED, OLED)? Is there a way to test this matrix separately (because the logic board seems to be fine)?

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    \$\begingroup\$ Just for giggles and laughs, try using a negative voltage on the contrast pin. Try voltage range of -4 through zero. Reason: many low-temperature LCD displays require negative contrast voltage. \$\endgroup\$ – Dwayne Reid Sep 10 '17 at 21:50
  • \$\begingroup\$ I don't think this is an LCD. \$\endgroup\$ – Beowulfenator Sep 10 '17 at 22:30
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    \$\begingroup\$ Not sure why you think it isn't an LCD, that glass with the barely-visible shadows is characteristic of polarized regions of a liquid-crystal display. This looks to me exactly like the glass part of an LCD, and it's also consistent with the multiplexing waveforms. \$\endgroup\$ – MarkU Sep 10 '17 at 23:02
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    \$\begingroup\$ Did anyone check the contrast adjust? \$\endgroup\$ – Transistor Sep 10 '17 at 23:13
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    \$\begingroup\$ crystalfontz.com/product/cfap2002ayecs \$\endgroup\$ – Bruce Abbott Sep 11 '17 at 4:10
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In my experience, these displays need at least 4-5V between the VCC pin and the contrast pin, sometimes more. Which means you need to either power them with 5V and adjust the contrast pin to 0..1V, or you can power them with 3.3V, but you'll need negative voltage on the contrast pin. The latter is preferred if your MCU is not 5V-tolerant. Perhaps your display needs a negative voltage even at 5V.

A similar display I've got actually has unpopulated pads for a MAX660 charge pump which generates reverse voltage for the contrast circuit: enter image description here

Depending on the matrix, it may also be impossible to see anything without backlight. Green displays with black characters are usually readable in ambient light, but black displays with blue or green characters need backlight to be readable.

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  • \$\begingroup\$ This screen has no backlight at all. I've tried bringing the contrast voltage all the way down to 0, and nothing works. Will try negative voltage, but i don't think that's the answer. Also, these displays have a MC34063 that generates about 7.5V positive voltage that goes into row terminals of the matrix. \$\endgroup\$ – Beowulfenator Sep 11 '17 at 12:12
  • \$\begingroup\$ @Beowulfenator Sorry, I could only see 1 picture in your question. I see your units have some sort of voltage-doubling circuit already. What is surprising is the lack of backlight. Maybe you could put a small white LED under the LCD (perhaps in the opening in the corner) and check if you get to see anything? \$\endgroup\$ – Dmitry Grigoryev Sep 11 '17 at 12:35
  • \$\begingroup\$ Done that before I disassembled one of the units. There was an opaque black tape at the bottom of the glass plate. Also a polarizer on top. What I don't understand is how unresponsive the matrix is to different voltages applied across any of its terminals. I'll probably disassemble the matrix further just to see what's inside. \$\endgroup\$ – Beowulfenator Sep 11 '17 at 14:09
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I have used several of this type of display, from various sources. All my displays had a backlight and were powered from a USB connection. My experience is that they are very sensitive to supply voltage; a change from 4.95V to 5.08V in one case resulted in the screen appearing totally blank and unresponsive. In most cases it was possible to restore the display to visibility by adjusting the contrast voltage, but even this was not perfect if the supply voltage was significantly different from 5.00V.

I would try varying the supply voltage in small steps from 4.80V to 5.20V. Incidentally, if there is no backlight you may have a reflective display, in which case you need reasonable light on the front of the display to see any change.

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  • \$\begingroup\$ Thanks for your response. By now I'm pretty confident all of those displays are broken. The logic is fine, but the actual display matrix is defective. And I've tried every combination of supply and contrast voltages I could think of. Also, these have no backlight for sure. \$\endgroup\$ – Beowulfenator Dec 12 '17 at 17:50

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