I have this 2 wire Humidity sensor, HR31, aka HR202L. I see now, after purchasing it that is looking for non-polarizing power at 1.5V AC. I have some of these NE555N timer chips that I purchased locally for 6 cents for a different project. I'm wondering if there is a way to use the 555 timer to get a non-polarizing circuit so I can read the Humidity sensor? I read the 555 spec sheet, but I am a microbiologist, and all I really got out of it was "oscillator", which is what I think I need--somehow. This timer takes a 5V minimum VCC, which I have available as I am using a NODEMCU ESP8266 for this project. I realize i can get a DHT11 temp/humidity sensor for around $1. In fact I have a couple now. I am just wondering if there is a relatively easy and inexpensive way to set up the power conditions for this sensor? Here are the sensor specs:
Blockquote Technical parameters
Scope of work
20 … 95%
Supply voltage
1.5V AC (Max sine wave)
Operating frequency
500Hz … 2kHz
Rated Power
0.2mW (Max sine wave)
Center value (at25 °C 1kHz 1V AC 60% RH sine wave)
31.0k?
Impedance range (at25 °C 1kHz 1V AC 60% RH sine wave)
19.8 … 50.2k?
Humidity detection accuracy
± 5% RH
Characteristic parameters (at 1kHz) unit: ohm
Impedance characteristics (at25 °C 1V AC 1kHz)
Note:
(1) To prevent polarization, driven sensor voltage or current used should not contain DC components.
(2) use the LCR AC bridge measurements, do not use the multimeter to measure.
(3) to avoid condensation of moisture.
Just use DHT11. This HR202L sensor is only for humidity and if I interpret the datasheet correctly you also need a temperature sensor to read the value of humidity. It seems easier to use a DHT11 sensor that measures temperature and humidity.
Yes, I have a DS18B20+ online for temperature, and I was looking to add this and calculate relative humidity. I think that would be a good learning experience, if it is not too complicated getting the humidity sensor to be happy. That datasheet is helpful, though, because it has a lookup table. I'm curious if anyone has done this, since the DHT11 sensor probably has a way to supply AC to the sensor without adding 20 pounds of equipment.
It may be good training, but keep in mind that for someone with little or no knowledge of electronics and software, this can be quite a challenge. It doesn't make any sense to me, but if you want to, no one is stopping you. Little information about what is inside the DHT11 sensor.
Thanks, that sure looks and sounds like the same sort of sensor. I'm not sure why it makes no sense. I mean, I could just go out and buy a digital thermometer/ hygrometer and slap it on my desk, if I had no curiosity. I will try to provide a little more info in hopes of getting a more specific answer from someone with an EE background. Looking at the 555 timer datasheet, it seems to me I can get the top part of a sinusoidal curve on the output at 1.1V with 5V DC supply and 3.3 VDC input current with the proper capacitors and resistors. So I am wondering if I could use this, or would this still be considered polarizing current on the hygrometer?NE555N Datasheet
Anyone with the relevant training or understanding would want more information. The data sheet does not even begin to allow anyone to understand the device requirements.
Does the excitation have to be a sine wave? If so, what is the allowed THD?
How are you planning to measure the "performance parameter", the impedance?
Yes, you can generate an AC sine wave using a 555 timer, but you need to know the allowed THD to design to the required band pass filter.
I was planning to measure the impedance across the A0 pin of the NODEMCU. The response of the sensor itself to environmental changes is on the order of 10 seconds, so I assumed I could do some averaging to filter out harmonics. Maybe not? If I understand correctly, are you suggesting a pulse-width modulated input may be better than a sine wave? If so, would that be something that would be fairly easy to accomplish with the 555 timer and a voltage divider? I'm not looking for super accuracy. Temperature is much more important to me for this project, and the sensor I am using is much more accurate for temperature than the DHT11.
For testing, I would just like to setup a 1V AC 1kHz supply, run it through the sensor to pin A0 , and observe. Maybe the sensor has internal filtering? If not, I would likely stop there.
Really? What to watch, maybe smoke? Did you know that the ADC of the microcontroller does not accept negative voltage and by connecting the AC signal in this way you will most likely burn out the microcontroller? As I said before, this way is unnecessarily complicated for me.
Just and idea, I would use your sensor in a voltage divider or Wheatstone bridge driven by a digital signal (square not sinusoidal), and then use a low pass filter to get rid of the ac signal and measure with the analog read pin.
If you are just looking for a fun challenge, check out humidity sensor calibration. It's fascinating and could be the basis of a lot of neat experiments.
A lot more real than shoehorning an obsolete IC into an uncomfortable role.
Thanks, I was wondering about that part. This gives me valuable information on which topics to investigate before trying it out. For now, I think I will try what aarg suggests and look at the humidity data coming in from the DHT11 and try to process that without the library, then maybe look at using the raw sensor. This is not a critical project, I am just goofing around with this stuff. If I get to a point where I need good data, I will probably do what Wawa suggests. On the other hand, if I can get this sensor to work, I might use the signal as a switch to trigger a fan or something.
These bare sensors were commonly used in VCRs, as dew sensor. To prevent the tape turning into a salad if there was condensation on the drum or tape. That happened frequently after a trip to the video store on a cold day.
The sensor is just a resistor that is sensitive to moisture.
You would expect it's resistance to be less when wet, but it's the opposite.
You can use a DC voltage (pull up resistor or internal pull up) for testing,
but if you do that for several months in a damp environment then the sensor will corrode. AC will prevent that.
You can try the tone() library, set to 1kHz, connected to one pin of the sensor, with the other pin connected to a 100k:100k voltage divider and an analogue pin. Then detect deviation difference from 512. That will only get you a moisture reading, not humidity, which also requires temperature.
Leo..
