Hi. I'm currently in the process of designing a tunable notch filter for frequencies between around 2kHz to 15Khz. The notch filter is tuned by adjusting resistors R2 and R3. The device will need to 'switch on' an LED when the notch frequency is being presented at the input. I chose to implement the LED activation mechanism by using a schmitt trigger that would 'compare' the output voltage of the filter to the output voltage of the input signal, upon passing them both through a basic rectifier/smoothing section.
I'm using an Op-Amp for the schmitt trigger. What i'm having trouble with is configuring the schmitt trigger's ON and OFF hysteresis limits. Essentially, when the voltage difference at the inputs equals roughly 2.6V, i want the schmitt trigger to go 'negative rail'. When the difference at the inputs equals 3.8V i want it to go 'positive rail'. How can i go about doing that? Please take a look at the circuit diagram for opinions.

Thanks alot.

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circuitpic.png (30.0 KB, 21 views)

 

Please do not make duplicate posts or threads, they do not increase your chance of a helpful response but only reduce it, as people get confused and can't always see what others have suggested in the other thread(s).

If you have made this post or thread in error then please do not be offended. We get lots of people making duplicate threads under the misapprehension that it will increase their chance of a helpful response when in practice the reverse is true.

Have a look at the tutorial; it will help you to calculate the hysteresis values.
http://home.cogeco.ca/~rpaisley4/Comparators.html

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The LM348 has four very old 741 opamps in it. They have poor response above 9khz and lots of phase shift.
I recommend that you use a TL074 quad opamp that works well up to 100kHz.

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