Voltage to Frequency Converter LM331 Problem

[syee10]

Hi all,

I had construct a voltage to frequency converter using LM331. The circuit can be found in Figure 3 in the LM331 datasheet which you can found it at **broken link removed**

I use Proteus to simulate through the circuit and it works fine with a DC input signal (0-10V). But when i change my input signal to a pulse signal (square wave 0-10V), the output frequency i get is incorrect. Why is this happen? Is this means the VFC LM331 only accept DC input signal but not a square pulse signal? Or is because of my circuit connection got problem?

Also when i connect the circuit and test it in practical, i cant get output frequency from the oscilloscope. I had check through many times the circuit, and i am sure the connection is correct when i connect it in my breadboard. And yet i still dun understand why the VFC LM331 is not functioning in practical. Why is this happen? Please help me

 

[Prototyp_V1.0]

Hi. If you intend to enable/disable frequency output, you might try to use a simple logic and circuit AFTER the output.

I wouldn't count on that it would work under square wave input, where it under normal circumstances would be some voltage.

 

[syee10]

Prototyp_V1.0,

What you mean by 'you might try to use a simple logic and circuit AFTER the output'?

 

[Prototyp_V1.0]

Hi.

I mean just what I said. See drawing then if I didn't make make you understand.

 

[syee10]

I dun know why i need a AND gate logic and a switch there?

 

[Prototyp_V1.0]

I dun know why i need a AND gate logic and a switch there?

Maybe I haven't get what you want to do with the circuit. Why do you feed it's input with square wave?

 

[syee10]

Because the input is coming from my output of AD633 analog multiplier which is a square wave. The output of AD633 analog multiplier is converted into frequency. So the input to the frequency converter is a square wave =)

 

[Prototyp_V1.0]

That's what I'm saying

Because the input is coming from my output of AD633 analog multiplier which is a square wave. The output of AD633 analog multiplier is converted into frequency. So the input to the frequency converter is a square wave =)

Take a look back to this post.

 

[syee10]

I think i got what you mean. The input after passing through the and logic and switch is actually same with a square wave input. But since the square wave input is not working, i dun think connecting a AND gate there will make the VFC working properly?

 

[Prototyp_V1.0]

I think i got what you mean. The input after passing through the and logic and switch is actually same with a square wave input. But since the square wave input is not working, i dun think connecting a AND gate there will make the VFC working properly?

Your observations is correct. It won't work, and is not supposed to work with square wave input.

 

[syee10]

Ok.. the square wave input wont work for LM331. Can i convert the square wave input to a DC level signal? Say if i have a +1V high level and 0V low level square wave, how am i going to convert it to a DC level +1V signal? Any suggestion so that i can feed this input into my LM331?

 

[Prototyp_V1.0]

Ok.. the square wave input wont work for LM331. Can i convert the square wave input to a DC level signal? Say if i have a +1V high level and 0V low level square wave, how am i going to convert it to a DC level +1V signal? Any suggestion so that i can feed this input into my LM331?

Yes you can convert it to dc. That's also very easy to do. You just make a RC low pass filter. Now you should know something about the input frequency to set the RC time constant to a propriate value.

 

[syee10]

Is low pass filter will give me the perfect DC level signal from a square waveform signal?