[solved] From 6 DHT22, only 4 work

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Hello,

i have a DHT22 problem and do not know where the problem is.

The situation:
I want to connect 6xDHT22 to my Raspberry Pi 2, four of them work perfectly, 2 not.
The sensors are powered by a separate 5V power supply. As cable I use Cat5E S / FTP cable.

The Sensors:

WZ - on BCM GPIO 17 - cable length: 2m - OK
WC - on BCM GPIO 20 - cable length: 7,5m - do not work
BA - on BCM GPIO 13 - cable length: 9m - OK
SZ - on BCM GPIO 27 - cable length: 4m - OK
KU - on BCM GPIO 4 - cable length: 4m - OK
AZ - on BCM GPIO 21 - cable length: 6m - do not work

I have tested:
When i connect the DHT22 from WC to the WZ cable, the DHT22 works - so the DHT22 is not defective
There is a stable voltage of 5V.
Cable was replaced without success. Cable has also been tested separately without any other cables
PI & SD card where replaced
The longest cable with a DHT22 works - Cable length can not the failure
When i connect for example the DHT22 from WZ to the WC BCM GPIO Pin, I get values. When I Connect the DHT22 from WC to the WZ BCM GPIO Pin, I do not get any values.

I have the following stand:
Pin´s are not defective
No misconfiguration in PiMatic
cable is ok
voltage is ok
DHT22 is ok

Who has a hot tip for me? I am just helpless

@Mr-X said in From 6 DHT22, only 4 work:

Who has a hot tip for me? I am just helpless

Just for clarification:

1. Did you also test wiring the WC and AZ cables to alternative GPIO ports?
2. Are the cables already mounted in your flat, or are they just rolled up located next to you Raspi?
3. Are you using pull-up resistors? If yes, which value(s) do you use?
4. Do you have a “common ground” between the separate 5V supply and the raspi?

I think 3) and 4) are important to get stable results. You may also need to further denoise your external 5V supply.

1. yes - for example: When i connect the AZ cable to the KU GPIO Pin, the DHT connected in AZ doesn´t work.
   When I connect the KU cable to the AZ GPIO Pin, the DHT connected in KU works.
2. The Cables are mounted in my flat; I have tested it also with a short cable near my Raspi, to avoid the cable as a source of error
3. yes, 10k Ohms
4. yes, all “Grounds” are connected together

I use a power Source from Meanwell RS-25-5: 5V / 5A

Isn’t it so that some gpio have internal pull ups? Maybe this needs to be changed … ?

@Mr-X May be the PIN numbers you’re using are wrong. Can you please post the devices config you have setup?

@mwittig said in From 6 DHT22, only 4 work:

@Mr-X May be the PIN numbers you’re using are wrong. Can you please post the devices config you have setup?

    {
      "type": 22,
      "pinType": "BCM GPIO",
      "pin": 17,
      "interval": 5000,
      "xAttributeOptions": [],
      "id": "wz",
      "name": "wz",
      "class": "DHTSensor"
    },
    {
      "type": 22,
      "pinType": "BCM GPIO",
      "pin": 4,
      "interval": 5000,
      "xAttributeOptions": [],
      "id": "ku",
      "name": "ku",
      "class": "DHTSensor"
    },
    {
      "type": 22,
      "pinType": "BCM GPIO",
      "pin": 20,
      "interval": 5000,
      "xAttributeOptions": [],
      "id": "wc",
      "name": "wc",
      "class": "DHTSensor"
    },
    {
      "type": 22,
      "pinType": "BCM GPIO",
      "pin": 13,
      "interval": 5000,
      "xAttributeOptions": [],
      "id": "ba",
      "name": "ba",
      "class": "DHTSensor"
    },
    {
      "type": 22,
      "pinType": "BCM GPIO",
      "pin": 21,
      "interval": 5000,
      "xAttributeOptions": [],
      "id": "az",
      "name": "az",
      "class": "DHTSensor"
    },
    {
      "type": 22,
      "pinType": "BCM GPIO",
      "pin": 27,
      "xAttributeOptions": [],
      "id": "sz",
      "name": "sz",
      "class": "DHTSensor",
      "interval": 5000
    },

@Mr-X Thanks. Which PI model do you have? Assuming you have a Raspi 2 or 3 the pinout should be as shown below.
Can you confirm WC and AW are connected to physical pins 38 and 40 or the header?

 +-----+-----+---------+------+---+---Pi 3---+---+------+---------+-----+-----+
 | BCM | wPi |   Name  | Mode | V | Physical | V | Mode | Name    | wPi | BCM |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
 |     |     |    3.3v |      |   |  1 || 2  |   |      | 5v      |     |     |
 |   2 |   8 |   SDA.1 | ALT0 | 1 |  3 || 4  |   |      | 5V      |     |     |
 |   3 |   9 |   SCL.1 | ALT0 | 1 |  5 || 6  |   |      | 0v      |     |     |
 |   4 |   7 | GPIO. 7 |   IN | 1 |  7 || 8  | 0 | IN   | TxD     | 15  | 14  |
 |     |     |      0v |      |   |  9 || 10 | 1 | IN   | RxD     | 16  | 15  |
 |  17 |   0 | GPIO. 0 |   IN | 0 | 11 || 12 | 0 | IN   | GPIO. 1 | 1   | 18  |
 |  27 |   2 | GPIO. 2 |   IN | 1 | 13 || 14 |   |      | 0v      |     |     |
 |  22 |   3 | GPIO. 3 |   IN | 0 | 15 || 16 | 0 | IN   | GPIO. 4 | 4   | 23  |
 |     |     |    3.3v |      |   | 17 || 18 | 0 | IN   | GPIO. 5 | 5   | 24  |
 |  10 |  12 |    MOSI |   IN | 0 | 19 || 20 |   |      | 0v      |     |     |
 |   9 |  13 |    MISO |   IN | 0 | 21 || 22 | 0 | IN   | GPIO. 6 | 6   | 25  |
 |  11 |  14 |    SCLK |   IN | 0 | 23 || 24 | 1 | IN   | CE0     | 10  | 8   |
 |     |     |      0v |      |   | 25 || 26 | 1 | IN   | CE1     | 11  | 7   |
 |   0 |  30 |   SDA.0 |   IN | 1 | 27 || 28 | 1 | IN   | SCL.0   | 31  | 1   |
 |   5 |  21 | GPIO.21 |   IN | 1 | 29 || 30 |   |      | 0v      |     |     |
 |   6 |  22 | GPIO.22 |   IN | 1 | 31 || 32 | 0 | IN   | GPIO.26 | 26  | 12  |
 |  13 |  23 | GPIO.23 |   IN | 0 | 33 || 34 |   |      | 0v      |     |     |
 |  19 |  24 | GPIO.24 |   IN | 0 | 35 || 36 | 1 | IN   | GPIO.27 | 27  | 16  |
 |  26 |  25 | GPIO.25 |   IN | 0 | 37 || 38 | 1 | IN   | GPIO.28 | 28  | 20  |
 |     |     |      0v |      |   | 39 || 40 | 1 | IN   | GPIO.29 | 29  | 21  |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
 | BCM | wPi |   Name  | Mode | V | Physical | V | Mode | Name    | wPi | BCM |
 +-----+-----+---------+------+---+---Pi 3---+---+------+---------+-----+-----+

@mwittig: Thank you I am using a Raspi 2 Model B, also tried a Raspi 3 for testing.
I can confirm that “WC” is connected to Pin number 38 and “AZ” is connected to Pin number 40.

Which I can not explain:
For example the Device “KU” works without any Problem. When i connect the “KU” cable to Pin 38 where the device “AZ” is configured, i get values.
When I connect the “AZ” cable to Pin 7 where the device “KU” is configured, I do not get any values.

When I exchange the DHT22 sensors among each other, every sensor connected in KU, WZ, BA or SZ works; so I think the DHT22 sensors are ok and not defective.

For a better understanding how i connected the sensors:

Thanks. My suggestion is to test the setup with short cables just to make sure the underlying library is able to deal with 6 DHTs (not sure whether or not anyone has tried this yet). Generally, it should work, but one may run into timing problems.

If the previous step works, my next suggestion is to try a smaller pull-up 4.7k with the long cables. Also make sure the cable shield is connected to ground at one cable-end.

A, perhaps, dumb question: Why do you use an external power supply? The power draw of one sensor should be below 1.5 mA according to the data sheet. So, for the described application using the Raspi to power the sensors should be fine.

https://www.sparkfun.com/datasheets/Sensors/Temperature/DHT22.pdf

@mwittig: Thanks for your answer
yeah, that is the next step I will do. This evening first I want to build a test situation with short cables and try my luck.
The cable shield is not connected to ground - I will try it, also the 4.7k idea; provided the test setup works.

In the future i want to connect more devices like relay boards, etc. Thats the reason I use a external power device.

As soon as I have the first test results I will report.

Test 1: New installation of Pimatic on Raspi 3; connected 7x DHT22 with pull-up 10k and short cables - it works!

Test 2: Connected the external Power supply: it works!
Test 3: Replaced “old” Pimatic Installation & Raspi 2 with the new one on Raspi 3, connected all network cable grounds to global ground and renewed all GPIO connector cables: it works !

Thanks for support !

I have named the devices like the GPIO connector number they were connected.
something surprised me (see Pimatic screenshot):
The DHT22 on Pin 26 has a very high humanity, also the DHT22 on Pin 16.
When I exchange the DHT22 - for example i connect the DHT22 who is Room with Pin 26 to the Room with Pin 19, then Pin 19 as a high humidity. I think it is the sensor.
Is it possible that this DHT22 is defective or does this sensors have a high tolerance?

edit:
The Pimatic Picture is from the first test, so all DHT22 sensors where next to each other and each sensor have different values…

Great, the basic setup is working fine for you know!

@Mr-X said in From 6 DHT22, only 4 work:

Is it possible that this DHT22 is defective or does this sensors have a high tolerance?

Normally, DHT22 has a high accuracy and it is calibrated as part of the production process. However, if you bought cheap ones. e.g. from China for 2 Euros, it may be different story.

Keep an eye on the measured values. If the values are fluctuating it may be a problem with false readings. The following post is suggesting the problem can be mitigated using a certain read method for the sensors: http://forum.arduino.cc/index.php?topic=184356.msg1753894#msg1753894. I haven’t tried this myself and I didn’t find a follow-up on this confirming the method to work,

I bought them in germany - for test i replaced the 10k with 4.7k; no changes in the values.
I have now ordered other sensors and will test them.
Thanks for your support !