Hi guys,

I am having problems figuring this one out. I need to covert a 60Hz sine wave, into a DC level based on its frequency. for example: If the sine wave is at 60Hz, i need a 4Vdc output, if the sine wave is at 30Hz, i need a 2Vdc output.

I have tried using this device: http://www.national.com/mpf/LM/LM2907.html

LM2907 Frequency to Voltage converter - but its threshold is 1Khz, and my requirements are FAR lower than that.

 

You can do this with op amps. Clip the input to a square wave with an op amp set up as a comparitor. ( or use a real comparitor ) Feed this to a capicator and a resistor to ground. The RC constant should be much more than 1/60 so that you have differentiator, it produces a current spike on each pulse. Feed this to a second op amp thru a diode ( so you only get the positive spike ) and integrate this ( cap and resistor in parallel in the feedback network, RC >> 1/60 ). The signal out here will be proportional to the input frequency independent of the amplitude and wave shape of the input.

Google some of the terms above if you are not familiar with them.

 

You can do it with a 555 timer running from a regulated voltage (like +5v). Setup the 555 as a monostable with a fixed "on" time.

Lots of old car tachos just used 2 transistors as a monostable, that works good enough to measure the engine frequency and output to a analog voltmeter.