Problems reading the DHT11 sensor

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Problems reading the DHT11 sensor

Postby Oderlando » Tue Jan 01, 2019 11:10 am

Hi. I'm having trouble reading a DHT11 sensor. I was able to read the sensor normally in other flowcharts, but in this particular flowchart I did not succeed. The purpose of this flowchart is to make a Loop work until the sensor returns with the humidity value. After this, the humidity value is shown on the display and the required loop number for this is also shown on the display. Can anyone tell me what could be wrong here? Thanks for any help. I use flowcode 8. I am sending the attached flowchart.
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Re: Problems reading the DHT11 sensor

Postby mnf » Tue Jan 01, 2019 11:49 am

There is no way out of this loop:

Capture.PNG
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Humidity is always >0 (except maybe in the Atacama?) Do you mean

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Until Humidity > 0
or use a flag and set to true if sensor read ok?

Martin

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Re: Problems reading the DHT11 sensor

Postby chipfryer27 » Tue Jan 01, 2019 5:01 pm

Hi

I agree with Martin (and I only know of that desert thanks to a recent rerun of a Top Gear Special :D ).

If you set your "loop while" value to something more reasonable instead of "0", like perhaps your actual target value then it will escape.

Also, you are running at 20Mhz. I haven't used the DHT11 but I don't think it will take very long to sample. Therefore it will loop very quickly indeed (guessing many, many thousands of times per second) and you will soon exceed the Uint range of your variable "x". It will start counting all over again from 0 every time it reaches it's limit so this means that your count "if" it escapes your loop will be incorrect.

I don't know your intended application but do you really need to sample so quickly? Perhaps consider including a delay in your loop?

Hope this is of some help.

Regards

EDIT..>
I just read this very informative post in the forum which explains the operation of this sensor in detail even giving operational limits. I suspect your "loop while" value may perhaps be a typo? Also whilst you won't be looping thousands of times each second you will exceed your "x" range in minutes.
https://www.matrixtsl.com/mmforums/viewtopic.php?f=7&t=11555
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Re: Problems reading the DHT11 sensor

Postby LeighM » Tue Jan 01, 2019 5:51 pm

Perhaps you intended while Sensor > 0 ?

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Re: Problems reading the DHT11 sensor

Postby Oderlando » Wed Jan 02, 2019 5:19 pm

Happy New Year to all !. I'm putting together a controller for incubators. Several functions are performed at the same time, including a stopwatch for a clock. This way, I can not enter loop. I am trying to get the sensor to read and can determine how much time has been spent for this by correcting the timer's time measurement. I am trying to make the loop terminate when the sensor returns with some nonzero value.

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Re: Problems reading the DHT11 sensor

Postby chipfryer27 » Wed Jan 02, 2019 6:01 pm

Hi

From the post I referenced earlier, the sensor can only read humidity between 20% and 95%. Not having played with this I don't know what that will correspond to as a returned value. Maybe someone that has used the sensor can provide details? However from the component macro, it states that if the value is 15.8% then it will return 15%.

As you are unlikely to get 0% humidity even in your incubator we can assume that all readings will be above zero as the sensor will always return with a non zero value (from what I understand, but I stress I haven't used it). Therefore as your "loop while" is set to >0 it will be in a continuous loop as every reading is greater than zero. As Martin pointed out, no escape.

You said:

"I am trying to make the loop terminate when the sensor returns with some nonzero value."

As you will always have a non zero value you don't need this loop at all, simply read the sensor. If however you are using this value to measure a change in humidity, escaping when a predetermined value has been reached then you will need your loop. Using your flowchart, if your predetermined target was 30% humidity and you set your "loop while" to >30, then it will loop until humidity drops to 30 or below.

Hope this helps a little.

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Re: Problems reading the DHT11 sensor

Postby Oderlando » Wed Jan 02, 2019 7:43 pm

I have corrected my flowchart. In the flow chart, I used humidity equal to 0. The loop operates until the sensor shows some humidity value above 20%, ie the loop works until the sensor can be read and "x" shows how many loops were needed to read the sensor. In actual hardware testing the lcd shows that it took around 4000 loop to read the sensor. I was very hungry. Does the sensor take so long to read? I am sending the new flowchart..
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Re: Problems reading the DHT11 sensor

Postby chipfryer27 » Thu Jan 03, 2019 12:13 am

Hi

Full disclosure.... I have just came back from the pub, so any attempt at assistance may not necessarily be correct, especially as the keys on my keyboard aren't where I left them :)

From what I see in your chart you will loop until the sensor reports that humidity is greater than 20%, and from your post it took 4000 iterations to come back with that value.

You don't capture what the initial humidity value was before entering your loop, and whilst in the loop we don't know if you do anything that would physically alter the humidity, so it is difficult to know what is happening. Can you provide any further info regarding such?

It may be helpful to break the program down and check what is happening and when. If you capture and display the humidity before you enter the loop for a reference, that would be helpful. Also, you don't know if it took 4000 iterations because variable "sensor" =1 for part of the time or if variable "humidity" was less than 20%.

Assuming you personally are not doing anything to change the humidity it leaves two possibilities. First is that sensor = 1 meaning it failed to read the sensor, or that your humidity was below 20% and rising. The tolerance is +/-5% so you could have been reading slightly lower than actual. Maybe try averaging your readings?

Perhaps add in some delays to allow you to read / display values whilst in the loop? You certainly don't need speed whilst testing and being able to read / display can be very beneficial in debugging.

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Re: Problems reading the DHT11 sensor

Postby mnf » Thu Jan 03, 2019 10:01 am

It's been mentioned before, the DHT11 is quite a slow sensor- so add a 2s delay to the read loop...

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Re: Problems reading the DHT11 sensor

Postby Oderlando » Wed Jan 16, 2019 8:26 pm

I have a problem that I still can not solve. I'm trying in every possible way. I need to read the sensor at regular intervals, and at the same time make the timing of a clock. I am using a 16F877A and a crystal of 20MHZ (with very low frequencies it was not possible to read the sensor). I want a precise time measurement so I'm using 1: 1 interrupts. Soon, 1 second will have 19531 interruptions. The problem is that when I choose the ratio of 1: 1 on interrupts, there are errors in reading the sensor. Only by using 1: 256 interrupts can I read the sensor, but doing so there is loss of timing accuracy on account of the decimal number of frequencies. What can I do to get a good timing and at the same time I can read the sensor?

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Re: Problems reading the DHT11 sensor

Postby Benj » Thu Jan 17, 2019 10:41 am

Hello,

We would need to see your program to be able to advise really. The fast interrupt is likely effecting the timing of the DHT11 comms. So maybe check that the code in the interrupt macro is as tight as possible, ideally you should be incrementing a counter and that's it.

Can you disable the interrupt before sampling the DHT11 and then re-enable afterwards or would this mess things up?

It's possible to use the timer interrupt on a higher prescaler and then use a C icon to collect the timer count value and use this to increase your timing accuracy.