Need help with Schmitt trigger application!

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.

I have to ask, what are you REALLY trying to achieve?

1 Do you want a notch filter?
or
2 Do you want an LED to illuminate when a certain frequency is present?

If the answer is 1, then where is the notched output taken from?

If the answer is 2, I think you would be better off using something like an NE567, a dedicated tone decoder.

JimB

That is not any notch filter I ever saw. National Semiconductor website has circuits in application note AN31 that you may be interested in.

Russ

I always like to see the positive supply at the top and negative supply at the bottom. The LM348 is a quad op amp, you can't have one section operating on +-15 V and another operating on +5 and gnd. U2 output will be at +vcc-2v which puts the input at (vcc-2)/2 + whatever voltage is on C3. Making R10 smaller will get to a lower voltage.

Russlk said:

That is not any notch filter I ever saw. National Semiconductor website has circuits in application note AN31 that you may be interested in.

Russ

Click to expand...

Same here, Russ. I sim'ed it, and it is indeed a 2kHz notch filter.

What simulation software do you use?

polus said:

What simulation software do you use?

Click to expand...

Linear Technology's SwitcherCAD III. Search this forum for "switchercad" or "LTspice" for more info.

Got a bit of a learning curve for people that've never used it before, but it is phenomenal software, there are better and more advanced simulators out there, but none free. It beats a lot of paid simulators as well.

The only thing I would like them to do is open source it (or at least make it shared source), so it can be compiled for different platforms and other people can contribute to it.