Need help on Humidity sensor

capacitive humidity sensor circuit

Hi to all,
I am designing a Humidity sensor circuit using a Humidity sensor from china. Datasheet of that sensor is also in their language:|. But without those knowledge i am designing sensor circuit that I've placed my resistance dependent humidity sensor as variable resistance across 555 timer configuerd as astable multivibrator. Iam able to generate pulses and the pulses vary with my humidity input. I could see that in my CRO. I wanted to convert the pulses into an analog signal. I tried out a Frequency to Voltage converter LM 2917 an 8 pin IC. I gave the output of the 555 to the Frequency to voltage converter. But I could not find any voltage variations from the F to V converter.
When i attempt this with LM 2917 that IC was producing high heat and no voltage variation. But now no heat on converter IC bt same result. How can i proceed.

Thanks for reply

philips humidity sensor

It's not understandable, how the two observations are related. A LM2917 f-V converter can't directly interface the sensor. Thus LM2917 problems are a different topic. You probably have a wrong circuit.

Generally, a variable resistance or capacitance sensor signal can be directly converted to a voltage variation by using two 555, one as an astable and one as a monostable circuit, having the sensor modulating either the astable frequency or the monostable pulsewidth.

In either case, when the monostable stage is triggered by the astable, you get a variable duty cycle output signal, that can be averaged by a low-pass to a proportional voltage and amplified by an OP circuit width zero and gain adjustment.

resistive humidity sensor circuit

You probably have good reasons to do R->F->V conversion. However, if you have a microcontroller (uC) in your system, you could use a simpler analog front end. Make a humidity sensor one of the resistors in the voltage divider. The output of the divider will be nonlinear with respect to humidity. But you can linearise it on the uC (again, if you have one).

resistive humidity sensor

Thanks for reply. I've placed my Humidity sensor using voltage divider. From that i can see the variation of .01 voltage decrement when blow some Air on my sensor. But my Op amp circuit cant boost this. so i cant see the variation on CRO. So i have to work hard with my op amp right? i hope it wont fail. I ill post here if i succeed. otherwise i have to try as FVM posted.

humidity sensor circuit

My suggestion was assuming, that the sensor must to be AC operated, which has been the case with any capacitive or resistive humidity sensor I've yet seen. DC operation may possibly damage the sensor or give unstable results (or not work at all, in case of a capacitive sensor).

A design, as I sketched it, had been proposed in an old Valvo/Philips application (using CMOS gates rather than 555), and I've used it in several designs. Unfortunately, I don't have any schematic at hand now, otherwise I would have posted it. Today, I also would design an AC circuit based on a microprocessor.

how to make a humidity sensor

Thanks for your reply Fvm. My humidity sensor's PDF is in chinese. From that even i dont know chinese that sensor is a Impedance type sensor.

Rated Voltage 1V AC(50Hz ~ 2 K Hz)
Operating humidity range 20%~ 90% RH
Accuracy ±5%
Operating temperature range The Maximum operating temperature 0℃~+85℃ 120℃ *
Standard characteristics 30 (21 ~ 40.5) KΩ(at 60%RH, 25℃)
Response time ≤12 s(20%~ 90%)
Hysteresis ≤ 1.5%RH
Humidity drift(per year) ≤±2% RH

CAUTION:
Do not use DC to the Humidity Sensor , please use the LRC test equipment.
Thats it from that page. here's the link:http://www.cbsen.com/en/ShowProduct.asp?id=270.
whats that LRC method? And can you get any details from this to help me?

make humidity sensor

LRC method simply measn impedance measurement. I think, it's not critical. As a first step, you can connect a series capacitor to protect the sensor from DC voltage, if you don't use a circuit that is known DC-free.

humidity sensor circuits

So can i apply 1 volt AC to my circuit? or can i use DC here? and Give me how to connect my Humidity sensor with this LRC circuit. Is it a separate circuit? Can i have exact 1 volt AC for my circuit?

valvo humidity sensor

The Sensor voltage must be AC only. A CMOS schmitt-trigger gate (e.g. 74HC14) can be used as simple R/f converter, the frequency can be directly read-in by a microprocessor or processed by an addtional f/V converter. I see, that the sensor has a very strong temperature dependency, it's output has to be corrected with a temperature measurement to get a calibrated humidity anyway, this can be achieved by a processor most easily.

frequency to voltage circuit using 555

Really thanks for reply Fvm. I have to check it out as you posted and ill be posting my result here as well as to your inbox.

Hi I need Help on this Humidity Sensor, I need to interface this humidity sensor to micro controller ADC and Measure the Humidity? Can u post circuit u work on?