[Noisebridge-discuss] seeking circuit idea: frequency to voltage

[Michael Shiloh]

in my case, it is a square-ish wave (555 oscillator) and so is 
absolutely not a pure (sine) wave.

Jonathan Foote wrote:
> That's only going to work for a pure tone, and not for speech or music.
> Typical audio is harmonically rich, meaning lots of frequencies mixed together.
> 
> 
> On Fri, Dec 11, 2009 at 12:01 AM, Lamont Lucas <lamont at cluepon.com> wrote:
>> Lamont Lucas wrote:
>>> Use a 555 as a monostable (one shot) vibrator that kicks out a fixed
>>> width pulse every time the input voltage crosses zero (or some higher
>>> point at the Vthreshold, you know, one fixed-width pulse per input
>>> waveform cycle), then a 741 configured as an integrator after that.
>>>
>>> As long as your fixed pulse width is less than 1/20000th of a second,
>>> and you integrate the output, you should get a output voltage that is
>>> linearly related to the input frequency, with higher voltage indicating
>>> higher frequency all the way from 0 Hz to 20kHz.
>>>
>>> that's a 555, 2 caps, and 3 resistors for the monostable vibrator, plus
>>> a 741, a resistor and a cap for the integrator.
>> Oh, and to go from a linear voltage to a bar graph...hm.  Maybe a
>> resistor ladder with LEDs on the legs that will only turn on when the
>> voltage at each node in the ladder raises above the forward voltage drop.
>>
>>
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