Driving an electromagnetic flip dot display with ULN2803's? - Page 1
Forum section: electronics / projects

author: aberco
author_rank: Contributor
timestamp: November 06, 2017, 06:39:16 am
content: Dear all!I have recently been working on a project where I have tried to reverse-engineer a Luminator/Gulton flip dot matrix display that I have acquired from eBay.To make things easier, I tried to reuse their solenoid driver design, which appears to be using four ULN2803. The problem is that all the IC marking have been erased, but the pinout seems to correspond to an ULN2803.On the original Luminator board, I have wired the ULN2803 HV side to +12V, and soldered wires directly to the inputs which I have been driving with a PIC MCU (5V).Everything seems to work well and the corresponding dot is flipping nicely.http://www.paleoserver.com/aberco/flipdot/IMG_5373.MOVNow, I have tried to replicate the same design but with my own PCBs, using ULN2003s (as I only have 7 rows), but the design does not work. The ULN2003 that is wired "backwards" heats up and shorts the power supply. The same happens when I place the original diver ICs from the Luminator boards on my PCB. Here is the schematics that I have replicated, as traced from the original Luminator board:I am not sure to understand how a low-side driver can be used to build a H-bridge, but it seems that this is what they did. I would appreciate any help regarding this, thanks in advance!

author: ebclr
author_rank: Super Contributor
timestamp: November 06, 2017, 11:17:55 am
content: UDN2981

author: aberco
author_rank: Contributor
timestamp: November 06, 2017, 04:06:55 pm
content: Ah nice, I was not aware of a similar high side driver, that could fix my problem.However, the original ICs used in the Luminator module are all the same. They all have an inverting input, and all short my new design... so it seems they used ULN2803s only.Is there any way to do that? I am struggling to understand how they used only low side drivers.I will decap one of the ICs to look at the die and hopefully find markings.

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2017, 05:13:01 pm
content: Something has to source current to the coils.They are not all ULN2803's (18 pin); you have 16 pin IC's too. I would look at the schematic but not sure why you have two pictures- is this one board?The only other way to maybe do this is using 12VAC so the coils get pulsating DC of either polarity when switched.For a modern driver I would suggestTPIC6B595(like UCN5821) on the low side, or A6818.

author: aberco
author_rank: Contributor
timestamp: November 06, 2017, 05:46:51 pm
content: There's 2 pictures as I have 2 boards, the master board has the row drivers and the column drivers.The slave board is chained to the master board and only the ICs for the column drivers are fitted.I have included both photos as it is possible to see the traces that run under the ICs on the slave board.The boards are quite big so I have only pictured the column driver section.The 16 pin ICs are likely not part of the coil driver, they are regular 74 series logic.On the video I posted, I severed the copper tracks that goes from the 16 pin ICs to the 18 pin ICs and drive the input directly with a PIC MCU.I used 12VDC on the drivers, and I alternate the polarity on the bridge to flip the dot.I agree with you that something has to source current and I'm still puzzled by the laminator design. I will try to further trace the PCB.Here's my re-spin, hopefully I can get it to work without redesigning a new PCB to use new drivers.

author: stj
author_rank: Super Contributor
timestamp: November 08, 2017, 02:26:54 am
content: give the original ic's a good scrub with IPA,more than 50% of the time a chip looks like the top has been rubbed, it's actually been painted-over with black ink.

author: aberco
author_rank: Contributor
timestamp: November 13, 2017, 01:54:59 pm
content: It is possible that they have been painted over, I have been able to read the marking on the chips that are connected to the drivers, which are 4051 mux.Unfortunately not for the drivers... so I uncapped it and the mystery of the high side driver is solved.It's a UDN2580A, which, unlike the UDN2981 that ebclr pointed out earlier, has inverting input.I do not know about the low side driver yet, but I guess it's a standard ULN2803.Nonetheless, I'll have to modify my project accordingly, hopefully with minimal modifications!

author: aberco
author_rank: Contributor
timestamp: November 13, 2017, 02:01:44 pm
content: And an update of the schematics, which now makes sense

author: stj
author_rank: Super Contributor
timestamp: November 13, 2017, 04:42:26 pm
content: btw, do you know what this is from??i'v always liked these type of display, but the smallest i'v seen was for the front of a bus!!

author: amyk
author_rank: Super Contributor
timestamp: November 13, 2017, 05:11:43 pm
content: The full display assemblies are large but are usually composed of individual pixel modules.