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

[Christoph Maier]

On Thu, 2009-12-10 at 20:55 -0800, Michael Shiloh wrote:
> a student wants to build a bar graph type display that will show 
> frequency, not voltage. the frequency is in the audio range.
> 
> i know there are chips to do this, but i'd like to find a circuit using 
> more basic and common components, like a 555, 741, 2n2222, etc.
> 
> precision and linearity is not an issue. it's merely a visual element.
> 
> any ideas?

Is independence from the _shape_ of the input signal an issue 
(i.e., that a sinewave [=flute] should give the same effect 
as a rectangle [=highly overdriven <whatever>])?

What do you want to have shown if you feed in a _mix_ of frequencies,
i.e., a chord??

Jonathan's comments are the correct answer of someone highly Skilled In
The Art, 
under the implicit assumptions 
that you actually want to measure frequency with as little
cross-sensitivity as possible, 
and that you want to measure the amplitude of each frequency
separately. 

The other suggestions so far trade off simplicity against
cross-sensitivity. 

Christoph