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Feather M0 Adalogger with DHT22s on cables
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Feather M0 Adalogger with DHT22s on cables

by CanofBeans on Sat May 08, 2021 2:54 am

Hi,
I am trying to log data to Feather M0 Adalogger microSD from multiple DHT22 sensors attached to 4 m Cat5e cable, The sensor strings work fine on Arduino Uno, but give NaN when attached to Feather M0 Adalogger. The Feather circuit works fine for 1 or 2 DHT22s mounted on the board, but if I attach a single DHT22 on cable the board DHT22s give NaN, but come to life again if the cable is removed. I am using the USB for PWR and measure 5V between PWR and GND. I've got pull-up resistors between data and PWR for all sensors and currently powering over microUSB to a PC, but eventually want to put this on Lipoly batteries.

Is there a fundamental limitation of the Feather M0 Adalogger that prevents it connecting such remote sensors? The plan is to have 12 sensors on 12, 11, 10, 9, 6, 5, A0, A1, A2, A3, A4 & A5 and log to microSD at about minute intervals. I saw the DHT22 is limited to about 20 m cable, and a comment that Feather M0 Adalogger cabling should be kept to a minimum, but don't understand the fundamentals that might be limiting.

I'd appreciate any advice.
Cheers
Ross

CanofBeans
 
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Re: Feather M0 Adalogger with DHT22s on cables

by mikeysklar on Sun May 09, 2021 4:15 pm

If you are using a Feather M0 you will need to be connecting the DHT22 to a 3v line not a 5v or communication will fail. I don't think you have hit any sensor issue other than using a voltage that it is too high for the Feather M0 to handle.

mikeysklar
 
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Re: Feather M0 Adalogger with DHT22s on cables

by CanofBeans on Sun May 16, 2021 11:18 am

thanks for the reply, it may be then that the DHT22 needs more than 3,3 V when at the end of such long cables, so Catch22.
I also seem to have encountered the same problem with the MKR GSM 1400, which I thought I could use as a clip on to a Arduino Uno to transmit the data. When I realised the MKR was a microprocessor itself I thought I could log directly to the MKR GSM without the Uno, but then again run into the 3,3V problem with remote DHT22 sensors. Same problem with operating 5VDC relays.

I end up with the impression that these are components for education on the individual functions, but not intended for real world applications connecting sensors-logging and sending?

CanofBeans
 
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Re: Feather M0 Adalogger with DHT22s on cables

by mikeysklar on Mon May 17, 2021 6:28 pm

Ross,

These components are not meant for medical purposes or mission critical products, but they are good for many real world applications including sensor logging and sending. You will of course need to work within their spec constraints.

Twenty meters is a long run so there is a lot shielding and line cleaning tricks you need to do to keep the 1 wire bus protocol happy. Have you tried using 5k pullup resistors at each ends and using shielded cat-5 for the connections?

Some mention of long runs here:

The data sheet indicates it can cope with a cable length of more than 20m. For less than 20m a 5k pull-up is recommended (most breakout boards have this pull up resistor built-in). If you use more than a 20m cable run then the resistor is not defined and you have to find it experimentally - for a longer cable, resistance increases, therefore you need more current to pull up the voltage successfully, so you you will need a smaller pull-up resistor .


https://www.best-microcontroller-projec ... dht22.html

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Please be positive and constructive with your questions and comments.