I have a Humirel 1101 sensor which measures relative humidity and has a linear humidity to frequency output. I want to change the frequency range (7350Hz @ 0%RH to 6030Hz @100% RH) to a linear voltage (0 - 1volt) to input to a chart recorder. Frequency DECREASES as humidity increases, but I want voltage to INCREASE with humidity, up to 1 volt. I have a 5volt supply, so the LM2917 won't work. Any ideas please? 555 timer as a F/V converter?

 

Frequency to voltage conversion can be easily achieved using a phase locked loop chip. The PLL uses a simple VCO that you tune to oscillate in your frequency range by varying resistor and capacitor values. Then when you phase lock this oscillator to your incoming signal, the VCO control voltage becomes the DC output of your circuit. If frequency goes up, the control voltage changes to track this frequency change, and it does so with a nice linear characteristic. If the control voltage is going up with frequency while you want it to go the other way, you can add an inverting amplifier using a simple op amp which will do this for you.
I prefer this chip, the CD4046 for such an application:
http://www.fairchildsemi.com/ds/CD/CD4046BC.pdf#search='4046'

You can also use a tone decoder chip for this kind of thing, such as the LM567. Here's an example: http://www.national.com/ds/LM/LMC567.pdf

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RadioRon

 

Excellent post. I would agree 100%, any FM discriminator type circuit (PLL etc) is what is needed.

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An XR4151 will do what you want, Very Nicely.

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Thanks for the ideas. I've never used these PLL specialised chips before. Is there a simple schematic anywhere? I do have some CMOS4046 lying around, but no circuit. The XR4151 needs a min 8V supply, I only have 5V. The NE567 I have seen in a SW direct conversion receiver circuit. I need a working circuit and how do I adjust the output so that 7350Hz = 0 volts, and 6033Hz = 1 volt? This is inverse to the normal.

 

I think that there is enough design information on the CD4046BC data sheet from Fairchild to guide you through the process of selecting resistors and capacitors. The fundamentals are that you need to select your VCO minimum and maximum frequencies to be just below 6030 and somewhat above 7350 Hz. Let's say you design for 6000 Hz to 8000 Hz. The data sheet gives an example circuit and says how to choose R and C for these frequencies. It also shows how to set the low pass filter corner frequency. Since humidity changes slowly you don't need to make your loop react quickly so choose a corner frequency of about 1% of the VCO frequency, say around 50 Hz. Use phase comparator II, not phase comparator I because it will allow a loop to find lock regardless of the start frequency of the VCO. Take your output from the hot side of C2.

The output will be close to zero volts at 6030 Hz and close to 3.4 volts at 7350 Hz. Now all you have to do is use an op amp inverting amplifier to flip these over and offset so that you get your 0 volts with an input of 3.4 volts and 1 volt with an input of 0 volts. This sounds fairly easy to do without further guidance, but ask if you can't manage it.

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RadioRon

 

I've looked at the 4046 datasheet, and it seems from the graphs shown that for a centre frequency of 6700Hz, I need R1=100K, and C1=0.01u. I can't see on the schematic where to input the frequency, nor where the corner frequency comes into it. The component values are blurred on the datasheet, even if I zoom in. I'm also a bit unsure of the inversion part - "Now all you have to do is use an op amp inverting amplifier to flip these over and offset so that you get your 0 volts with an input of 3.4 volts and 1 volt with an input of 0 volts". Could you help me out please? I've done a websearch, and the manufacturers data sheet comes up no problem, but not how to adapt it. Thanks.

 

It is indeed hard to read some of the details in the application section. Your values appear to be approximately correct. It looks like R2 should be about 22K ohms in that case.

What you are doing is locking the internal VCO to your incoming signal. The incoming signal and the VCO are compared in the phase comparator which then drives a DC voltage back to the VCO. Things are arranged with one inversion so that you have negative feedback, not positive feedback. So, to answer your question, the incoming frequency goes to pin 14, Signal In. If you use a coupling capacitor in series to pin 14 to make your connection (and you probably should), make it 0.01 uF and make sure your input signal is about 200mV or more in amplitude.

The corner frequency is just another name for the cutoff frequency (the point where the output is a specific amount lower than the input, usually -3dB) point of the low pass filter comprised of R3 and C2. This is your loop filter. You can choose any convenient value of R3 and C2 that satisfies the criterion that R3= 1/(2 x pi x Freq x C) where Freq=50. I usually estimate these values first, then look for a capacitor in my junk box then adjust the resistor value if I must use a different C value.

I gather you are not familiar with op amp circuit design. You can use a simple "inverting" amplifier to flip over the voltage. You might want to look this up in an op-amp book or find something online. If I had a few minutes I could sketch a circuit for you, but can't right now. Maybe tomorrow if you are patient.

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RadioRon

 

i am using the lm2917 converter but i am not sure how to connect the input pin since i am having an input of 1 to 3 volt.

what must i do? connect the pin 11 to my input supply?

 

I don't find the http://www.fairchildsemi.com/ds/CD/C...arch='4046' link because they move or remove it.

I need to make a FM demodulator and i want an schematic reference.
For example: whitin F1=1000 Hz and F2=1030 Hz range out of circuit should be "1" logic and for Fin<F1 and Fin>F2 out of circuit must be "0" logic.

Thank you for answer. **