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Raspberry Pi has several output voltages, GPIO outputs theoretically giving exactly 0V for False and exactly 3.3V for True, 3.3V line and 5V line. However, how precise are these outputs? Can the output voltage vary on the total load on the particular pin or total loads on all pins?

In particular, I need very precise ON-OFF 3.3V or 5V voltage. If GPIO output is not precise enough I intend to use 3.3V or 5V line via operator amplifier to get a stable and reliable voltage output as shown below.

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What is your opinion or advice on that? I tried to Google some useful information on that but found none.

  • This Q&A has relevant info on the GPIO pins: raspberrypi.stackexchange.com/q/60218/19949 In short, do not expect HIGH to be exactly 3.3V that's just not how logic outputs work. They are specified to be above not at a certain level. – Ghanima Feb 3 '18 at 13:53
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    Just to reiterate this a third time: "GPIO outputs theoretically giving exactly 0V for False and exactly 3.3V for True" -> Nope, that is not the theory. And this is the reality: en.wikipedia.org/wiki/Logic_level#Logic_voltage_levels Note the very large discrepancy in relation to "exactly" anything. The 3.3V and 5V power pins will be much better if you want a reference but still, I think, prone to at least 5% variance depending on load. raspberrypise.tumblr.com/post/144555785379/… – goldilocks Feb 3 '18 at 14:08
  • @goldilocks Thanks, this was helpful. I still have to know if using circuit like above would output more stable on/off voltage. If I undestand the second document right, 3.3V line is more stable than 5V and should be use to power the operator amplifier in the above circuit? – Pygmalion Feb 3 '18 at 14:58
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    I believe the 3.3V line is internally regulated whereas the 5V is not, and perhaps the latter is more prone to fluctuations depending on the quality of the external supply. But that may be an incorrect assumption of mine about the extent to which a buck converter can "absorb" a variation in supply voltage (I'm no electrician). You are probably better off asking that question, and the one about the op amp, on our larger sibling site, Electrical Engineering. – goldilocks Feb 3 '18 at 15:07
  • @goldilocks I have posted question here electronics.stackexchange.com/questions/353854/…. Please feel free to suggest improvements to the question. – Pygmalion Feb 3 '18 at 15:49
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The Pi's GPIO are to set logic levels (on or off). They are not designed to be a voltage reference.

The 3V3 rail is regulated. You'd have to see the specs for your Pi model to find the actual regulator used. I doubt if it is calibrated or particularly stable.

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  • I understand that it is not regulated. But are they any data about the voltage divergence? It is not the same if divergence is +-0.1V or +-0.03V. What specs should I look after (RPi3)? – Pygmalion Feb 3 '18 at 13:08
  • It certainly won't be as precise as +0.1V. Again, as has already been stated, that's not how digital logic works and the GPIO pins are not intended to be voltage references and will not work well for such a purpose. – goldilocks Feb 3 '18 at 14:02

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