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I'm attempting to use an DHT22 at 3.3V for a very low power application - remote sensor unit with a very long battery life. Currently it's on a naked breadboard with a 328p.

At 5V in a spare arduino uno the DHT22 works fine. At 3.3V... it doesn't. Things I've tried:

  • Shorter cables (down to ~1 inch or less)
  • Multiple cables (drive the overall resistance down further)
  • Longer waits (because I was being hopeful)
  • More capacitors (because maybe the voltage is sagging on power up?)
  • Removing other, high power components that might somehow be interacting with it (xbee)
  • Varied pull up resistor values (3.3k, 5k-ish, 10k)

None of that has worked. I've ruled out wiring mistakes, faulty component, bad code.

What else can I try to resolve this? I've had a bad night, I'm not thinking clearly and all I can think of is throwing it all in the bin or buying a 3.6V regulator, in the hope that it's high enough to appease the DHT22 and not fry the xbee (limit 3.6V).

I'm also confused because on one hand, the datasheet says 3.3V is fine. On the other - the internet says that sometimes 3.3V isn't enough. Sometimes?!

Please give me more options.

DHT22 datasheet https://cdn-shop.adafruit.com/datasheets/Digital+humidity+and+temperature+sensor+AM2302.pdf

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  • \$\begingroup\$ Is the MCU itself still powered by 5V supply, or 3.3V like the DHT22? \$\endgroup\$
    – Justme
    Commented May 3, 2023 at 8:34
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    \$\begingroup\$ Added datasheet \$\endgroup\$
    – user2702772
    Commented May 3, 2023 at 8:49
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    \$\begingroup\$ The DHT22 datasheet says that 3.3V is the lowest it'll run at. Meaning at 3.2999V it won't work. Where is you 3V3 coming from? What is the impact of the votlage drop along the cable to the DHT22? How stable is the 3.3V rail? In simple terms: you're complaining it doesn't work when you're operating it right at the lower limit. \$\endgroup\$
    – Puffafish
    Commented May 3, 2023 at 8:56
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    \$\begingroup\$ "sometimes use more volts" Because: Chinaware. Don't buy cheap crap from hobbyist traders or other questionable sources. Adafruit has no concerns about product quality what-so-ever, they'll sell any piece of junk that appears to be electronics. Also breadboards in general are very unreliable and not recommended for any purpose. \$\endgroup\$
    – Lundin
    Commented May 3, 2023 at 10:31
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    \$\begingroup\$ Either way, even with a quality product you wouldn't design something to use the exact specified minimum supply voltage - that's very unprofessional. Components, supply voltages, temperatures, everything comes with inaccuracies and slight drifts, why engineers always design with margins. \$\endgroup\$
    – Lundin
    Commented May 3, 2023 at 10:33

2 Answers 2

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Some sources that sell DHT22 mention in specifications that minimum supply voltage is 3.5V.

Depending on what voltages you have available in your system, you can easily level-shift the bidirectional data wire between a 3.3V MCU and e.g. 5V DHT22.

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  • \$\begingroup\$ You're right - it's listed as both 3.3V and 3.5V. I've upped the local voltage to a (bad) 3.5V, and I'm still not getting a meaningful response. But I suspect that this isn't a true 3.3V component. \$\endgroup\$
    – user2702772
    Commented May 3, 2023 at 12:32
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The DHT22 datasheet says that 3.3V is the lowest it'll run at. Meaning at 3.2999V it won't work.

What are the tollerances on the 3V3 rail? How stable is it? A very accurate 3V3 rail will be in the range 3.2V to 3.4V, so only 50% of the supplies will work, and that assumes zero ripple. You claim the supply is 3.3V and stable as far as you can measure it, what's the tollerances of your volt meter? When was it last calibrated? If it's reading 3.3V, they I would not expect it to work. If it is reading 3.2999V, I would not expect it to work, if it is reading 3.310V (note the three decimal places) I could believe it would work, maybe.

Then there's voltage at the device, you haven't told use the distance between the 3V3 source and the device. What voltage at the DHT22 is seeing on its pins? What's the ripple like?

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  • \$\begingroup\$ So for the purposes of the question, your recommendation is "Validate your 3.3V"? That's not a bad answer, although a little hard for me to do. My multimeter is a touch old, isn't (I don't think) easily calibratable and probably should be replaced. I did specify that the source and device were within 1 inch, or 2.5cm of each other, and the multimeter reading is within 0.01 (3.31 vs 3.30). But I accept that this is of limited value without calibration. \$\endgroup\$
    – user2702772
    Commented May 3, 2023 at 12:36

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