tach switch

I need a circuit that will output 9-24v dc when an engine is running or cranking. I was thinking about using a frequency to voltage converter from National Semiconductors such as this one.

http://cache.national.com/ds/LM/LM2907.pdf

But I still need help with the actual circuit design. I'm open to suggestions as far as the IC goes if someone knows a better choice. I need it to work off any source such as a coil, fuel injector, tach gauge, crank sensor, etc. I need 0v dc when there is no input (engine not running or cranking) and 9-24v dc when there is any input to the circuit (engine cranking or running).

Any help is appreciated.

I've messed around with the 2907 and never got really good results out of it.  I was probably doing something wrong, but I could never get it to do what I thought it should do based on the provided data sheet.

You can purchase premade frequency switches that are programmable.  Anything above 10 hz and you can assume the car is on, below that have the output turn off. 

Any links for ones you've had good luck with?

Thanks for your replay.

Something like this maybe?  http://www.siliconchip.com.au/cms/A_108920/article.html

It uses the same family of chip you were talking about already.

I, myself, made my own RPM switch using a programmable microcontroller to measure the time between pulses - it's a lot more work but once the software is done the circuit is extremely simple.

A rectifer bridge won't convert a pulsing DC signal to a non pulsing signal.  A rectifier bridge will "convert" a pulsing DC voltage in to a pulsing DC voltage with a slight voltage drop.

A bridge works with AC because AC has three different potentials - +, 0, and -.  A pulsing DC only has the + and 0 as the voltage potential never drops below 0.  Since the voltage never goes negative the bridge won't conduct anything on the second stage of the bridge.

You can typically convert a pulsing DC signal to constant voltage by using an RC circuit - they are slow to respond and wouldn't be very accurate for real time RPM information but it may work for a simple "motor running" signal.  You would definately want some sort of isolation between the raw signal and the RC circuit, like maybe a transistor or a voltage follower.

Alternators don't use just a rectifer bridge to convert AC to DC, they have to smooth out the AC ripple, typically done with a large cap or a car battery.  The key here, though, is the fact that the alternator generates AC voltage, thus allowing the bridge to rectify.