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I'm using a CD4094 shift register to power my LEDs along with a ULN2003A darlington transistor array. I want to hook up the LEDs to an external power supply of 12V (common anode).

The cathodes of little groups of LEDs will be connected to the ULN2003A which is connected to the shift register which will get its data from the arduino (the digital pins which i assume work at 5V).

My question is, if I connect the Vdd pin of the shift register to the 12V external power supply, will it still function as intended through the Arduino?

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No. CD4k devices reference their high and low voltages to a (mostly) fixed ratio of Vdd, so there will be no way for the Arduino to operate the device.

Consider using a TPIC6C595 instead, which combines the shift register and the high-voltage driver into a single device.

  • Is that TPIC drop-in compatible for a HEF4094? Basically I'm trying to repair a display that uses these chips. My first goal is use the chip (acquired an extra from ebay) to drive ~8 LEDs off 1. I realize this isn't the best way to learn (I'm trying to fix something, by first replicating parts of the circuit) and the biggest impediment is finding good examples to mimic and learn from. – Scott Prive Jun 4 '18 at 17:16
  • @Crossfit_and_Beer: It's drop-in compatible with a 74HC595 (except for the whole OD bit), but I'm not sure it would substitute for something in the 4k series. – Ignacio Vazquez-Abrams Jun 4 '18 at 17:37
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    @Crossfit_and_Beer: Having looked at the datasheets now, you'd have to put it on an adapter to use it, and it still expects a 5Vish supply. – Ignacio Vazquez-Abrams Jun 4 '18 at 17:44
  • Good to know. I'll add (since there's a dearth of examples for this chip) that the "BP" chip version accepts a VDD (VCC) of up to 15V. Sadly while I have good reputation on other SE sites, I don't have rep "here" and cannot post questions yet, so a good definition of my problem will have to wait... – Scott Prive Jun 19 '18 at 4:48

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