555 timer as a voltage to frequency converter.

[rob_croxford]

Hi All,

I am currently working on a project that requires a 0-10V DC supply to be converted into a linear fequency range - square wave. for instance 1V to be a low frequency square wave with the frequency increasing exponetially as the input voltage is increased.

So far i have been playing arround with a simple 555 timer which appears to work however it works the wrong way. (low voltage is high frequency and high voltage is low frequency).

Is there any way this can be reversed?

Does anyone have any other ideas of how this can be achieved with minimum cost issues??

( I am hoping to redesign an excisting circuit which creates the Squarewave output using a MUX and conparator therefore would hope to keep components and costing to a minimum )

Thanks all,

Rob

 

[NickS]

inverting analog buffer.

 

[rob_croxford]

This will not work as the Input is a Variable DC source. Inverting the output will mean that my zero crossing will be well out as the high pulse width will be extended with the low pulse width being small.

Would it be possible to use a VCO or would the linearity be inaccurate??

Thanks for your help

 

[NickS]

Perhaps we do not understand each other. You said "... (low voltage is high frequency and high voltage is low frequency).

Is there any way this can be reversed? "

I said invert, meaning invert the input. If what you said is true then I see no reason why this would not work. Here is a simple block diagram

your strategy(low volt in high freq out)
[low voltage DC]--->[555]--->[High frequency out]

your strategy with my input inverter(low volt in low freq out)
[low voltage DC]--->[invert to high voltage DC]--->[555]--->[Low frequency out]

So now that we have that cleared up(I hope) here is a dirt cheap implementation. note that it is not exactly linear but hey you get what you pay for. Blue is the input and Red is the output

Good luck
-Nick-

 

[rob_croxford]

ohhhhhhh i completely understand now! Wow that had me stumped for a while!! thanks alot works fine now.

Cheers,

Rob

 

[NickS]

That was quick, did you find a different solution?

 

[rob_croxford]

I will use a few comparators and an LM331 (v/f converter IC) however this method is perfect for testing. the finnished output must be linear but using a TIP31c transistor the output is as linear as possible so it works quite well.

Thanks again for the advice

 

[NickS]

Your welcome.