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I am just diving into the world of multiplexers, so my knowledge of them is not the greatest. I am trying to develop a circuit that can control 10 RGB LEDS (30 total LEDs) via 2 inputs per LED and one input that affects all LEDs (21 total inputs). I would like to achieve this with an EEPROM, so I was going to try a LED multiplexer on the output pin, but I am unsure if there exists a multiplexer that can pull in 21 inputs and condense them down to 8 inputs so that they are addressable on the input side.

I would like to avoid a microcontroller, because I want to see if it can be done with an EEPROM. Thanks for the help ahead of time!

Truth Table utilizing 7 EEPROM ICs: LED EEPROM Truth Table

I am sure that this is super inefficient cost-wise and space-wise, since it uses 7 EEPROMS for 13 RGB LEDs (39 total LEDs). The left table is the input into the 15 Address pins on the EEPROM, and the right table is the I/O output pins to the LEDs. The table with Blocks 1-8 control 12 RGB LEDs (36 LEDs), and the bottom table is a single EEPROM that controls a single RGB LED (3 LEDs). Let me know if this all makes since or if I can clarify anything.

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  • \$\begingroup\$ 21 inputs and condense them down to 8 inputs ... are you talking about 21 bit addressing, or are you talking about throwing away 13 inputs? .... what are you trying to accomplish with the LEDs? \$\endgroup\$
    – jsotola
    Oct 24, 2021 at 17:00
  • \$\begingroup\$ My goal is to use an EEPROM with 8 bit addressing to receive 21 inputs through some sort of multiplexing system. My understand of multiplexers isn’t that great, so I wasn’t sure if there was a version of a multiplexer or IC that could accomplish this. All 21 inputs would be utilized. The outputs would control 30 LEDs via a multiplexer. \$\endgroup\$
    – Grogglebob
    Oct 24, 2021 at 17:06
  • \$\begingroup\$ 21 inputs 10 LEDs each with 8 states (on/off) for 80 states total but no transfer function defined except 1 for all Off. Or did you mean 2 pins / LED and want some Charlieplex solution \$\endgroup\$ Oct 24, 2021 at 17:10
  • \$\begingroup\$ It would be 21 inputs, where 2 inputs control the three colors for 1 RGB LED output and 1 overall input to control all LEDS for a total of 21 inputs for a 10 LED system. Sorry if I’m not being clear, since this is a new topic for me. \$\endgroup\$
    – Grogglebob
    Oct 24, 2021 at 17:24
  • \$\begingroup\$ How did you want to address each LED and how often changed? and what pattern? 8 toggle switches? \$\endgroup\$ Oct 24, 2021 at 18:01

1 Answer 1

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If you only require the visual appearance of the LEDs being controlled, you could use (say) the lower 3 address inputs on the EEPROM and clock them at a relatively high frequency (kHz), then multiplex the LEDs in 8 banks, giving you up to 64 outputs.

If you have RGB LEDs with common anodes you would likely want to drive them in 5 pairs of 2 with two EEPROM outputs unused and 3 bytes of EEPROM unused (you could count to 5 rather than 8 to improve the duty cycle).

So you have a 3 bit counter and a 1 of 8 decoder, a clock oscillator and 6 sink drivers (eg. ULN2003A) and 5 high-side source drivers (could be a few dual MOSFETS) for the 10 LEDs. Plus 6 resistors. You might need to add a one-shot multivibrator to reduce ghosting.


Edit:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Hmm, I am having a difficult time visualizing this. Would you mind showing me a quick schematic or block diagram? \$\endgroup\$
    – Grogglebob
    Oct 25, 2021 at 0:58
  • \$\begingroup\$ Which part is hard to visualize? The LED matrix or the drivers? \$\endgroup\$ Oct 25, 2021 at 1:02
  • \$\begingroup\$ - What does clocking the EEPROM addresses at a higher frequency allow for vs a lower frequency in this instance? - I can't really visualize what you mean by paragraph two. \$\endgroup\$
    – Grogglebob
    Oct 25, 2021 at 1:10
  • \$\begingroup\$ You want to cycle through each of the 5 sets of LEDs faster than the human vision flicker fusion frequency so they appear to be on or off continuously. Something like a kHz or two is fast enough unless there is a lot of vibration, in which case you might want to go higher. This is the same principle used in multiplexed 7-segment displays. \$\endgroup\$ Oct 25, 2021 at 1:12
  • \$\begingroup\$ Ah okay I see. I'll play around with this idea a bit. \$\endgroup\$
    – Grogglebob
    Oct 25, 2021 at 1:18

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