Does anyone have any ideas for a simple circuit to do the following?

I want a circuit that will go from high to low if the input signal drops below 30 Hz. Then go back to high when the signal increases above 30 Hz. The input is a 50% square wave.

Cheap is good. I don't want to use a PIC or get really involved. I do have a supply of 555 timers.

 

No 555s here, but this is pretty cheap. Do a Google search for "retriggerable one-shot" or "retriggerable monostable" for alternatives to the one-shot in the schematic. I believe a 555 can be made to be retriggerable by adding a few components, but it might not be worth it.

 

an Idea is to use a
Frequncy to voltage converter.
Then use a voltage comparator.

 

It's a standard 555 circuit, usually called a 'missing pulse detector', a quick google found this http://www.geocities.com/iecmaster/c...ir_msr002.html.

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If you want to use the 555 circuit that Nigel referred to, you'll still need a "D" flip-flop. I made a slight modification to the original circuit, and added the "D" ff. The output is high when the input is above the preset frequency (nominally 30Hz), and low when the input frequency is below the preset. This circuit does require the input to have a 50% duty cycle (which you said you have) with the values shown. The input voltage must swing from 0 to 9 volts.

Attached Images

30hz_freq_discrim1.gif (6.0 KB, 223 views)