# Frequency voltage converter?

1. Feb 10, 2004

### IKu

does anyone know where i can find a frequency to voltage converter or know how to make one? preferably using a low voltage supply... thanks!

2. Feb 10, 2004

### Staff: Mentor

Frequency to voltage? I'm not sure I understand what you are trying to do - they are unrelated to each other.

3. Feb 10, 2004

### dlgoff

Use the LM324 quad op amp. With two of the amps, some resistors, a npn transistor, and a capacitor, you can make a good Voltage Controlled Osicllator( w/ triangular and square wave outputs). The chip can use a wide supply voltage; 3 to 30 vdc.

If you want, I can make a drawing of the circuit for you.

Added by edit: I wrote too soon. I read you post as voltage to frequency. Sorry about that. Still you should be able to design what you need with an op amp, resistor and cap.

Last edited: Feb 10, 2004
4. Feb 10, 2004

### IKu

well i'm pretty new with this stuff so i could use a diagram... i went on a google spree and found this:
http://www.national.com/pf/LM/LM2917.html
they have example diagrams using this piece:

would this work?

i guess it would help to tell you guys what i'm doing... i want to convert a tach signal from a car into an analog voltage signal so i can log it... but i'm pretty new to this sort of thing. thanks!

5. Feb 10, 2004

### dlgoff

Since the output is 66rpm per volt, at full scale (12 volt output), your input would be 792rpm. I bet you engine will turn more than this? The way I see the circuit, if you change the 100kohm resistor to 10kohm, the full scale output would be about 10 time grater (7920rpm).

Inorder to display the output voltage you will need to have some sort of voltmeter. You might want to mess around with this above mentioned resistor value such that you get something even (like 1000rpm per volt). It would be easier to scale your display.

Hope this helps.

6. Feb 10, 2004

### IKu

i think the output is 66Hz/volt, so multiplying that by 60 would give me rev/min, right? so even at 2 volt output, i would be at like 8000 rpm i think, unless my understanding is screwed up. anyway, i'm very new to this stuff... my understanding is Vcc is voltage supply so the 12 volt Vcc would be perfect with the car battery, Fin is the frequency signal, so i just connect th tachometer signal to this? and i would also need 2 capacitors and 3 resistors for this to work? also, i'm supposed to take the voltage output from right before the 10k (10,000?) ohm resistor which then goes to ground? sorry if these questions are stupid, just want to make sure i get this right so i don't break anything. oh, and what is that curvy line in a circle by Fin, is that a fuse? how do i connect the tach signal to that?

i'm planning on buying a datalogging tool:
http://www.dataq.com/products/startkit/di194rs.htm
i believe this logs analog voltage signals, such as the output from this circuit. i hope this all works out.... thanks!

7. Feb 10, 2004

### dlgoff

No, I'm the one screwed up. You're right. 66Hz=3960cycles(rpm)/min so at 2volt you would have 7920rpm.
Vcc would be attached to your 12 volt battery source. Fin would attach to some sort of sensor that pulses each time the engine goes through one revolution(sense the No. 1 spark plug firing). Make sure that the pulse in not grater than Vcc so that you don't damage the input circuit of the chip.
You will also need a way to mount the components. Check out prototype circuit boards.
Yes. Attach your voltmeter across this resistor.
No. That is the symbol for AC (Alternating Current). In you case, the AC will actually be more of a square-wave instead of sinusoidal.

8. Feb 10, 2004

### IKu

I think i understand everything now, this is making much more sense, thanks a lot!

9. Aug 18, 2004

### kuhl0040

Did you guys ever get this working?

Hi,
I am looking to do the same thing, your link no longer works to the diagram. Just wondering if you ever got the converter to work and if so how. Thanks.
Tyler Kuhlmann

10. Sep 14, 2004

### spacecowboy

I have met this problem too.. I have another question, for example, if I have a speed and voltage equation, which is 1 volt = 1 meter per second. what does the specification of this frequency voltage converter looks like? Or I cannot use this equation to find specification,for exaple like the resistence and capacitence of this equipment?

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