Display off-frequency with just LEDs ?

Thread Starter

Externet

Joined Nov 29, 2005
1,412
Hi all.
How would you make the simplest possible analog sensing circuit that will turn a LED on if an inputted frequency is above 1KHz and another LED if the frequency is below 1KHz ?
Does not matter if at 1 KHz both leds are on or off.
 

#12

Joined Nov 30, 2010
18,159
After about 2 minutes of thinking, a pair of filters starts with the question, "How sharp do you want it?"
If you do a F to V converter and add a comparator at the DC level that represents 1KHz, that eliminates most of the filtering.

Use a window comparator and, "How sharp do you want it?" becomes, "dial a Q".
 
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Thread Starter

Externet

Joined Nov 29, 2005
1,412
Thanks, #12.
The 1KHz frequency can be off about +/- 20 Hz before indicators take action.

Do you know of any PLL IC that some pin will go negative or positive if deviated from the frequency ?
 

#12

Joined Nov 30, 2010
18,159
Ummm...no. Did you know there are F to V chips, and they are different from PLL's?
I never built one of these, I'm just thinking of how a guitar tuner works.

Yes, PLL chips go positive or negative when the input is not the center frequency. I just don't know the part number.
 
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GopherT

Joined Nov 23, 2012
8,012
Use a comparitor on a Microcontroller, any signal that is more than 1000 microsecons between rising edges is less than 1000 Hz, anything faster is more. Adjust output to pin with appropriate LED.
 
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