I have this level shifter that I'm trying to feed a 5V power source and then hook up a 3V device.
I've got the 5V source hooked up fine (on the top pinouts), but having trouble figuring what the "power out" would be that's 3V.
Here's the data sheet.
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That's because it can be used to shift 3V to 5V, not 5V to 3V.– Ignacio Vazquez-AbramsCommented Nov 1, 2016 at 22:37
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2 Answers
It's not a level shifter. It's a buffer chip that can be used to level shift in one direction only.
It is good for shifting from a low voltage logic signal (3.3v, say) up to a 5V logic signal. It can't shift down from 5V to 3.3V.
Basically it works by having a fixed voltage that it considers to be a logic HIGH, and any voltage above that will be output as 5V. Any voltage below will be output as 0V. This is the "TTL Logic Level" specification in the datasheet.
So if you have a sensor that emits a 3V logic signal the HIGH logic value will be above its fixed threshold and will trigger a 5V HIGH on the output.
In contrast "CMOS" logic uses a percentage of the supply voltage to set the thresholds (usually > 0.6 x Vcc), and a 3V signal is less than that (3.3V is about bang on it, so is "iffy"), so won't work (or not reliably).
The datasheet would let you think it cannot shift in that direction, but it's the chip recommended by Adafruit to do so : https://www.adafruit.com/product/1787
The "trick" is to feed it 3V power. It seems it will then happily accept 5V input and produce 3v outputs.
