# The best way to build a low pass filter for this?

Discussion in 'The Projects Forum' started by gte, Jun 30, 2010.

1. ### gte Thread Starter Senior Member

Sep 18, 2009
353
4
If my calculations are correct, this would be ~58hz signal? I'd like it to filter out anything above 260hz, so would I need to do this with a microcontroller or can I use a resistor/capacitor combo?

fcutoff = 1/(2πRC)

fcutoff = 1/(2πC) * R
fcutoff * (2πC) = 1/R
fcutoff * 2 * 22/7 * 10uf = 1/R
260 * 2 * 22/7 * .00001 = 1/R
0.016342857 = 1/R
61.1888 = R

If I use 61Ω even, it puts me at 260.804hz
If I use 60Ω even, it puts me at 265.151hz

If the voltage were a square wave as pictured below (which I have not scoped yet) how would the voltage look at 260hz?

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2. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
That's the -3db point, or half power point.

I have a feeling you want more like -30dB down.

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3. ### gte Thread Starter Senior Member

Sep 18, 2009
353
4
I have a feeling you are correct sir, what do you suggest?

Micro controller that passes the signal through until it reaches that threshold, and then it just mimics it?

4. ### gte Thread Starter Senior Member

Sep 18, 2009
353
4
So is this grand fathered in?

If so, any ideas?

5. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Low-pass filters are not expressly prohibited.

Have a look at the attached simulation. You really need to get familiar with SPICE simulations, as they can answer a lot of questions before you take them to a breadboard.

Remember though, the simulations are only as good as the model that was built. With low-frequency, low-power stuff, you are generally OK. However, once you start getting into higher frequency circuits, seemingly inexplicable things start happening.

Increasing the value of R and C of the attached schematic will decrease the ripple on the output waveform, but it will take considerably more time to settle.

• ###### Low pass RC.png
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6. ### gte Thread Starter Senior Member

Sep 18, 2009
353
4
Thanks Sgt!

I have shied away from spice because of the lack of a GUI combined with all of the other stuff I've been trying to cram into my brain. I will take your advice though and try to start learning it and simulate it there first.

7. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Google "LTSpice download" and you'll find it - it is freeware, and runs under Win2k, WinXP, Vista, Win7, and in Unix under wine.

It DOES have a GUI interface. For most of the parameters, you just enter a few numbers in dialogs. LTSpice comes with all of their products in the library, as well as an assortment of other manufacturer's products like diodes, resistors, capacitors, MOSFETs, transistors, etc - and a goodly number of example circuits already made for you to experiment with and learn from.