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micropython / micropython-lib

Core Python libraries ported to MicroPython
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Problem with dht library #611

Open gandipg opened 1 year ago

gandipg commented 1 year ago

Hello Problem with dht library not working properly in subzero temperatures with dht22 module. reports (-3264.4°C) it is written here about a domestic problem https://forums.raspberrypi.com/viewtopic.php?t=296097

Keelando commented 1 year ago

Indeed! I'm not well-versed enough to suggest a proper solution at this time but I've just been adding the offset value corresponding to -0.1C and then multiplying by -1

dpgeorge commented 1 year ago

I tested the dht driver with a DHT22 and it works OK for me at temperatures less than 0 Celsius.

The dht driver here assumes that the top bit of the returned temperature value is a sign bit. But if you are getting values like -3264.4 then it could be that your DHT22 returns a 2's complement signed value.

After you do a reading can you print out the buffer, like this:

dht.measure()
print(dht.temperature(), dht.buf)

That will help narrow down the problem.

If it is 2's complement then you have a non-standard DHT22 module and you'll need to use conversion like this:

def dht_temp(dht):
    value = dht.buf[2] << 8 | dht.buf[3]
    if value & 0x8000:
        value -= 0x10000
    return value * 0.1