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

[Lamont Lucas]

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.

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.