Hi All,

I have a motor with a tacho output. The frequency changes depending on the rpm between 0hz to 360hz.
What would be the best why to detect when the frequency is lower then some setpoint, lets say 200hz?
Accuracy only needs to be about +\- 20hz.

I could easily do this in a PIC, but all I really want here is to have a 5v output when the frequency falls below the setpoint.

What would be my options?

Thanks

 

This is how to get a voltage that changes with frequency. More frequency = more voltage. Follow this with a comparator to decide what voltage will cause switching and design for a high output when the frequency is too low.

 

Or you might look at LM2917 designed for the purpose.

 

Or, a LMC555 as a missing pulse detector.

Ken

 

You need a frequency to voltage converter circuit and a comparator, like Lm2907, 555 timer,..

Or a tone detection circuit like lm567 chip, the set frequency will make pin8 go low.

http://www.scary-terry.com/more_stuff/tonedet/tonedet.htm

 

Thanks guys.

I think I will go with #12's solution. Nice and easy. Will keep the LM2917 in the bag, might come in handy..

Thanks

 

Also LM2907 /LM2917 can be used for frequency discrimination.