(44-54) 74HC595 / ULN2803 PCB(s) with 5vdc power and 12vdc LED modules ?
T&E Engineer
Posts: 1,396
I have 44 PCB that are connected with RJ45 cables for the Clock, Data, Latch, 12vdc, 5vdc, and ground connections from an sx48 proto board. The (44*8 outputs=352) 12vdc 840mW LED modules are powered with multiple 12vdc 16a supplies and produce correct LED bright displays. I had to use a stronger 5vdc supply for the 595s for solid 5vdc across each board and the sx48 has its own power. Grounds are connected between each board and power supplies.
When I run a simple shiftout sxb routine, I can get a single LED to come on and sequence through all 352 LED modules correctly but heres the problem... I see some LED modules randomly flicker and want to know what I can do to prevent this. I saw an arduino cascaded circuit for 595s that used a 1uF cap between the Latch pin and ground but that made it worse. I thought I saw an SX circuit somewheres that showed using a resistor between Latch and Ground or maybe it was pulled up yo 5vdc. Any ideas about that?
As far as arrays for shifting out data with an sx48, I cant see more than 180 or so before it runs out of ram. Any way around this for having 352 outputs from these 44 pcb? I also need to build a slightly larger LED chaser with 54 pcb. I hope I dont have to use an Arduino because I really want to stick with SX/B.
Thanks for your help.
When I run a simple shiftout sxb routine, I can get a single LED to come on and sequence through all 352 LED modules correctly but heres the problem... I see some LED modules randomly flicker and want to know what I can do to prevent this. I saw an arduino cascaded circuit for 595s that used a 1uF cap between the Latch pin and ground but that made it worse. I thought I saw an SX circuit somewheres that showed using a resistor between Latch and Ground or maybe it was pulled up yo 5vdc. Any ideas about that?
As far as arrays for shifting out data with an sx48, I cant see more than 180 or so before it runs out of ram. Any way around this for having 352 outputs from these 44 pcb? I also need to build a slightly larger LED chaser with 54 pcb. I hope I dont have to use an Arduino because I really want to stick with SX/B.
Thanks for your help.
Comments
You should only need 44 bytes of ram to control 352 outputs.
You probably want a RC circuit to the latch. That would be a resistor between the SX pin and the latch pin, and a cap from the latch pin to ground. You might try a 1K resistor and a 1uF cap. This will filter out any really quick pulses and prevent it from latching on noise. You will have to make sure that the actual latch pulse is long enough to get through the filter though.
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Thank you for the RC circuit idea. That's right the resistor was inline. That seemed to help but I think there is a much deeper issue with power from 5vdc and 12vdc over these 44 boards.
Currently there is (1) 5vdc 50A supply with (2) connections being run into boards (1) and (40) of the 43 total. There is also (2) 12vdc 16.6A supplies with (3) connections for each supply run into boards (1), (8), (16), (24), (31) and (38). The sx48 has its own power from the wall transformer. All grounds between supplies are connected together as well.
Looking at the attached spreadsheet you will see in bold where the power supplies are attached and what the strange voltage readings are. When the SX48 sends data through the cascaded 74HC595 / uln2803 modules and all the LEDs are lit up (except for 1 LED that is sequencing through the 352 LEDs as being in an off state), the voltages drop significantly. I haven't checked the voltages·in the reverse condition (1 LED "on"·sequencing through 352 LEDs) either.
Could this be a power issue? Each module·has optional connections and jumpers to accept 5vdc and 12vdc instead of running them via the RJ45 cable from board to board. Currently the power supplies tap into the above mentioned connections and only use the RJ45 cable run for power between each power·connection of either 5vdc or 12vdc as appropriate.
When a single·LED is sequencing through the 352 LEDs,·it works correctly. When inverting the SHIFTOUT statement with a ~, (e.g. all LEDs are one and only 1 is off while sequencing), the voltages are wacked out of place and it works except until the last few lights where double LEDs are off while sequencing and it skips and jumps over some of the LEDs.
Anything I can try here?
Thank you.