google "frequency to voltage IC" and you will get several hits. e.g. - http://www.ti.com/lit/ds/symlink/lm2907-n.pdf -
(never used it myself disclaimer applies
google "frequency to voltage IC" and you will get several hits. e.g. - http://www.ti.com/lit/ds/symlink/lm2907-n.pdf -
(never used it myself disclaimer applies