[Microbit-Python] Microbit DHt11 temp and humidity sensor

[Nicholas H.Tollervey]

This is epic. :-)

On Sat, 2017-07-08 at 21:51 +1000, Jim Mussared via Microbit wrote:
> OK you piqued my interest and I remembered I had a humidity sensor in
> my drawer which fortunately is a (very similar) DHT22.
> 
> The way the sensor works, like a 1-wire or 12C device, is both the
> micro:bit and the sensor share the same line which is pulled-up to
> 3.3V. Either device can pull it low to transmit data.
> 
> To initiate a read, the micro:bit needs to pull it low momentarily,
> then the sensor will do it 40 times. A short pulse (!30us) is a '0'
> and a long pulse (70us) is a '1'.
> 
> There's no way you can measure those pulses using read_digital etc on
> a micro:bit, but there is a very easy way to sample a pin a high
> speed
> using SPI.
> 
> So we wire the temperature sensor's data line to two pins - one in
> SPI
> mode for reading, the other in digital mode to do the initial pulse.
> The complete wiring is GND=GND, Vcc=3V, Data=Pin0,Pin1, and a 10k
> resistor between data and 3V. You could use any pair of pins though
> instead of Pin0, Pin1.
> 
> Basically the way this code works is it toggles pin1 low, then
> switches it back to input mode (high impedance). Then it reads at
> 250000 bits/s on the SPI line, which means we get approximately 4-5
> bits for a '0' and 11-12 bits for a '1'. The rest of the code just
> unpacks this into an array of 5 bytes that contain two bytes of
> humidity data, two bytes of temperature, and one byte checksum.
> 
> On your DHT11 it only has one byte each for the humidity and
> temperature, I think the changes you'll need are:
> temp = result[2]
> humidity = result[0]
> 
> I used https://github.com/adafruit/DHT-sensor-library/blob/master/DHT
> .cpp
> as a reference.
> 
> Let me know if that works for you!
> 
> Jim
> 
> On 8 July 2017 at 21:01, Jim Mussared <jim at groklearning.com> wrote:
> > I just realized you had the sensor details in the subject line -
> > DHT11.
> > 
> > It looks a lot like a 1-wire device -- datasheet at
> > https://cdn-shop.adafruit.com/datasheets/DHT11-chinese.pdf (and
> > similar to https://cdn-shop.adafruit.com/datasheets/DHT22.pdf). I
> > think you're going to have a hard time doing this from MicroPython
> > on
> > the micro:bit because you don't have microsecond-level timing.
> > 
> > I recommend trying to find an I2C sensor instead - that'll be much
> > easier to use. Or if you want to make it really easy, some sort of
> > analog interface sensor (i.e. where you read a voltage from it).
> > 
> > 
> > On 8 July 2017 at 17:10, Jim Mussared <jim at groklearning.com> wrote:
> > > Hi,
> > > 
> > > The micro:bit doesn't have a humidity sensor built-in, so you'll
> > > need
> > > to use some sort of external device. I've seen a few I2C sensors,
> > > but
> > > it's also pretty common to see Dallas 1-wire used in these sorts
> > > of
> > > sensors. You'll find I2C a lot easier on the micro:bit.
> > > 
> > > So if you could let us know which sensor you're using then we can
> > > give
> > > you some advice.
> > > 
> > > Jim
> > > 
> > > 
> > > On 8 July 2017 at 12:55, sasithaweerasekera via Microbit
> > > <microbit at python.org> wrote:
> > > > Hi I want to measure and display the humidity by the microbit
> > > > using microbit
> > > > and the leds of the microbit, do you know the coding, because I
> > > > can't find
> > > > any except for bit banging which is a concept I am not familiar
> > > > with, if
> > > > there is a library it would help a lot
> > > > Thanking you in advance for ur time;
> > > > Sasitha
> > > > 
> > > > 
> > > > 
> > > > Sent from my Samsung Galaxy smartphone.
> > > > 
> > > > _______________________________________________
> > > > Microbit mailing list
> > > > Microbit at python.org
> > > > https://mail.python.org/mailman/listinfo/microbit
> > > > 
> 
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