total time so far: 17 hours

May 27th:

Ok so this is something I've wanted to make for a while: a kind of super-clock that shows not just the time but also other relevant information. Initially I wanted to use flip-dot displays, but those are expensive and hard to find, so I'm pivoting to an LED matrix. I want to put a bunch of these cheap 8x8 displays together, right now I'm thinking 30x30cm, which is 15x15=225 displays or 120x120=1440 pixels. The raw cost of those displays would be ~$80. I'm trying to figure out driver ICs. Ideally I'd like to solder everything myself, with no tiny SMD parts. One option is a bunch (30-450) of 74HC595s, depending on how many displays I want to be able to control at once. Let's take a second to do the math of how many LEDs/second I need to update to get, say, 10fps: numbat let target = 10 fps unit leds let num_leds = 15*15*64leds let leds_per_second = num_leds/frame*target = 144_000 leds/s Okay. So say I go with the minimum of 30 74HC595s, and I update one column of the screen at a time. At 3V3 the max clock frequency is something like 25MHz, so let's see if 1MHz is enough: numbat let clock_freq = 1 MHz let columns_per_second = clock_freq/120 = 8333 columns/s let frames_per_second = columns_per_second/120 ~70 fps That's actually very good! And if I can actually hit 10MHz then I could get 700fps which might even be enough to do some PWM (BCM?) dimming, and that's assuming I have all the shift registers daisy-chained together and am not driving them in paralell. And another reason why that's great is that it makes my current needs low, if I'm only lighting a max of 120 leds at a time. So that would put my current needs at: numbat let leds_per_column = 120 leds let current_per_led = 12mA/led let current_per_column = leds_per_column*current_per_led = 1.44 A And since I'm only using one column at a time, that power is very manageable! I'll probably need to put a transistor on each column and row, because the 74HC595s can only source 6mA per pin, and I'd like to hit more like 12 or 14. So I'll need 240 transistors and 120 resistors, which won't be fun to solder, but shouldn't be too bad.

...

okay, i've done some more research, and it looks like it might make sense to switch to something like the TPIC6B595 for the low side, since it can sink enough current without needing a transistor.

And then to fit it within the JLC PCB size I'll make a bunch of smaller boards that hook together with like right-angle duponts.

OK so here's what the major components are looking like right now: Part|Quantity|Price/unit|Total price|# pins/unit|# pins total|Link ---|---|---|---|---|---|--- 788AS 8x8 LED Matrix|225|$0.35|$80.00|16|3600|https://www.aliexpress.us/item/2251832771187101.html 74HC595 Shift Register|15|$0.22|$4.00|16|240|https://www.aliexpress.us/item/3256807421796895.html TPIC6B595 Shift Register|15|$0.50|$7.50|16|240|https://www.aliexpress.us/item/3256806981485001.html IRF9520 MOSFET|120|$0.40|$48.00|3|360|https://www.aliexpress.us/item/3256806868594722.html 220Ω Resistor|120|$0.02|$2.80|2|240|https://www.aliexpress.us/item/2251832766343175.html 5V 3A Power Supply|1|$4.00|$4.00|2|2|https://www.aliexpress.us/item/3256805577151044.html Matrix PCB|10|$2.20|$22.00|0|0|JLCPCB Total|||$164||4700|

Okay, $164 isn't terrible. What I'm more concerned about is 4700 joints, that'll take a while. Rounding up to 5000 for the connectors and stuff, if I can do 40 joints per minute (optimistic), that'll take: numbat unit joints 5000 joints / (40 joints/minute) = 125 minutes ...that's not as bad as I thought. It'll probably end up being twice that, but I was afraid it'd be something like 40 hours. This project seems feasible!

total time spent (estimate): 2.5 hours:

May 27th, part 2:

OK i'm going to sit down and try to work out a schematic. I also realized that I needed to switch to a P-channel MOSFET for the high side, which I changed in the BOM.

[much later]

ok so there's a schematic now. i decided im not going to have a separate PCB for the control board, i'll just do that on perfboard. (again, bom has been updated). so now the plan is to make 9 (well, 10, because of JLC order sizing) boards to almost the schematic. each board will be populated with its 25 displays and the 4 40 pin connectors, and then one board in each row/column will have that row/colum's driving circuitry. Then the board with driving circuitry will be connected to the control board, which will have a pi pico w on it. next up: routing this monstrosity. i'll have to make sure i don't make impossible sandwhiches, but other than that it shouldn't be difficult, just tedious.

screenshots

close-up on one matrix and drivers

total time spent (estimate): 3 hours

May 27th, part 3:

start time: 11pm

ok i'm going to start on the pcb layout. I'm not ghoing to try to do routing tonight, but i do want to at least get the rough positions of things figured out.

ok, i'm about 30 mins in, and its becoming clear that i have to rotate the top and bottom rows of displays 90 degrees so i can fit the row connectors in. this will cause some headaches when doing the firmware, but i don't think i have a choice. this is what it's looking like now:

ok now for the (hopefully easier!) task of shoving all the ics and mosfets under the displays.

...and i'm realizing that there's no way to do this without 4 layers. fortunately it's not that much mroe expensive.

30 more minutes later, these mosfets are just way too big. i'm going to switch to the IRFU9024NPBF, which is actually cheaper than my original option, and it's in the smaller TO-251 package.

...it's been another 30 minutes, and i've gotten everything placed, thanks to the smaller mosfets. here's what it's looking like:

and that may not look terrible, but keep in mind that almost every empty pad is going to have an led matrix in it from the other side, and that's not even considering the traces between the near-1000 pads on this board. if this is possible, it's possible by the slimmest of margins.

total time spent: 1.5 hours total time on May 27th: 7 hours

May 28th:

Start time: 9am Okay I'm starting to route the rows to each other. I'm changing the footprint of the mOSTFETS slightly to allow a trace to fit between their pads.

[one hour later]

so i've gotten the middle rows and the rght column and a half routed. it's difficult but not impossible. the top and bottom rows are going to be a massive pain since they're rotated.

[another hour later]

Ok, i have all of the matrices routed, using just the internal layers. I haven't routed the connectors or drivers yet, but hopefully the outer layers are enough room for that. here's what it looks like now:

total time spent: 2 hours

May 28th, part 2:

Start time: 2:45pm I'm going to start routing the connectors. Hopefully I can stay on the interal layers, but I'm okay to use the outer ones sparingly.

[one hour later]

okay, one connector and some of the mosfets are routed. i've needed the outer layers 3 times so far - not as bad as I was expecting!

[another hour later]

ok, another connector and the rest of the mosfets' column connections are routed. next up: the row connectors.

...this is really annoying routing.

...ok i've routed the left row connector. i'll do the right one later.

total time spent: 2.5 hour

May 28th, part 3:

Start time: 8:30pm ok time to do the last connector and then hopefully the last part won't be that bad.

[30 minutes later]

i've just realized that i set up the connectors wrong in the schematic, so row 1 would connect to row 2 and vice versa, row3<->row4, etc, same with columns. either i can give up the past 3 hours of work, or i can spend a similar amount of time fixing that in software. let me think for a second about that: ok so the way this would manifest is just in the display, it wont cause any electrical issues fortunately. it'd just be that the middle row/column of matrices would have their column pairs swapped with each other. i wonder how i can efficiently represent that in software.

well i'm going to have to have a preprocessing step for every frame i want to display. and i'll leave it up to later me to figure out what the best way to swap the columns and rotate the displays is there, i'd rather spend 3 hours programming than 3 hours rerouting stuff i've already done once, and it'll hopefully take less than 3 hours to fix. it's unfortunate, but oh well.

anyways back to routing the last of the right connector, yipee!

[another 30 minutes later]

ok, all connectors complete! now the only thing left is just hooking up the shift registers, and i have almost all of the outer layers to make it happen. here's how it's looking now:

ok now i'm going to hook the mosfets up to the shift registers

[30 minutes later] ok the mosfets are connected to the low-shift registers. now i need to connect the rows to the low-side shift registers, connect the shift register data lines to each other, and give power to everything.

[15 mins later]

all shift registers are connected to their matrices/mosfets. next: data lines.

[15 mins later] ok the low-side data lines were pretty quick, because there's patterns between the chips. i've done the data lines on 1 high-side driver and they were super annoying - they're long and there's no patterns from chip to chip. i'm going to call it a night.

total time spent: 2 hours total time on May 28th: 6.5 hours

May 29th:

Start time: 9am Time to finish up the data lines.

[45 minutes later]

That wasn't so bad. Now my only remaining unrouted nets are power and ground!

[30 minutes later]

ok, power and ground are routed, and I learned that fills don't need pre-existing traces in order to automatically connect to nets, so that saved me a lot of work. Time to run DRC and see how bad everything is.

...413+ errors. oh boy.

okay, so ~200 of those were because i'd input the JLC via size constraints (confused via diameter with annular width), so those went away. Most of the rest were because of me putting pads under the displays, which was by design. Then there were a few which were about short traces being unconnected, which I'm ignoring, and a bunch of silkscreen overlap, which I will now go fix.

WAIT I RAN THE SCHEMATIC PARITY CHECK AND SOMEWHERE ALONG THE WAY I FULLY DELETED A DISPLAY ok time to trawl thru the git history and find when that happened

ok it was in the last commit so hopefully i can just put it back and things will be fine

yeah it turned out fine. i must have deleted it right before running drc. phew.

anyways time to go mess with silkscreens

[30 minutes later]

ok, I think the board is complete! Here are some screenshots: || ---|---|--- || ||

going to export the gerbers and get a real price!

ok so it's $9 for the boards (with a coupon), plus $16 shipping. i can't believe how cheap JLC is.

and just going to do a revised bill of materials here: (for some parts I'm getting spares since they'd be a pain to source if some turn out to be dead on arrival. if this can't be included in the grant i'll happily cover the spares myself)

Part	Quantity	Price/unit	Total price	# pins/unit	# pins total	Note	Link
788AS 8x8 LED Matrix	230	$0.35	$82.00	16	3600	not going to count on 0% DoA rate here	https://www.aliexpress.us/item/2251832771187101.html
74HC595 Shift Register	15	$0.22	$4.00	16	240	2x20 is cheaper than 3x5	https://www.aliexpress.us/item/3256807421796895.html
TPIC6B595 Shift Register	20	$0.50	$10	16	240	again, not counting on no DoA	https://www.aliexpress.us/item/3256806981485001.html
IRFU9024NPBF MOSFET	130	$0.40	$50.00	3	360		https://www.aliexpress.us/item/3256806868594722.html
220Ω Resistor	120

wait

waiittttttt

i forgot the current-limiting resistors.

time to go back to routing hell (:

ok wait can i get away without them
so the mosfets will source up to like 1.5A/col
and the tpic6b595s will sink 150mA/row
and the max current per led is 20mA
yeahhhhh

ok next idea: can i avoid putting the resistors on the board using the third dimension
like can i directly solder one leg to the drain pin of the tpic, and then solder the other leg to the pad where that drain pin is meant to go
the datasheet says the legs extend 1/8th inch below the body, so... maybe? uhh let me try this with a different DIP part

ok after experimenting a bit i dont think that's feasible. but maybe i could solder a resistor to the chip, then solder the other leg to somewhere else where the net is connected (i.e. onto one of the matrix legs).
looking at the pcb, the farthest distance that would be is 50mm, and my resistor is 60mm long. so that should work! it'll be really, really ugly, but it should work.

ok crisis mostly averted. i'm going to go add silkscreen to where i experct resistors to be and then i'll finish the bom.

ok silkscreen added and board reexported. that was annoying, and it'll be more annoying in the future, but i don't think i could reasonably add the resistors to the board without spending at least another 5 hours routing.

anyways, BOM:

Part	Quantity	Price/unit	Total price	# pins/unit	# pins total	Note	Link
788AS 8x8 LED Matrix	230	$0.35	$82.00	16	3600	not going to count on 0% DoA rate here	https://www.aliexpress.us/item/2251832771187101.html
74HC595 Shift Register	15	$0.22	$4.00	16	240	2x20 is cheaper than 3x5	https://www.aliexpress.us/item/3256807421796895.html
TPIC6B595 Shift Register	20	$0.50	$10	16	240	again, not counting on no DoA	https://www.aliexpress.us/item/3256806981485001.html
IRFU9024NPBF MOSFET	125	$0.40	$40.00	3	360		https://www.aliexpress.us/item/3256808251284284.html
220Ω Resistor	120	$0.02	$2.80	2	240	2x100 is cheaper than 12x10	https://www.aliexpress.us/item/2251832766343175.html
2x20 right-angle header female	2	$2.00	$4.00	40	80		https://www.aliexpress.us/item/3256805899201197.html
2x20 right-angle header male	2	$2.50	$5.00	40	80		https://www.aliexpress.us/item/3256804718416281.html
5V 3A Power Supply	1	$4.00	$4.00	2	2		https://www.aliexpress.us/item/3256805577151044.html
Matrix PCB	10	$2.5	$25.00	0	0	i'll buy this separately from the grant so i can combine it with another order	JLCPCB
Total			$173		4850	total w/o PCB: $148 :tada:

i think this is almost ready to ship! total time spent: 3.5 hours