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I'm making an active Low/High Pass filter based on the equal component sallen key design I found in the Active Filter Cookbook.

equal_component_sallen_key

In the book I read that by exchanging the position of the Frequency determining Resistors and Capacitors it is possible to switch between a high pass and a low pass response, using a 4PDT switch.

I'm very confused about how to connect the switch to the components/board, I've been drawing out the connections for a good few hours! It's simple enough I'm sure, but I can't seem to get something which makes sense, or would work!

My 4PDT is connected like this:

4PDT

Could someone please help me with a sketch of the connections or any tips on how to hook up the switch? :) Thank you very much,

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seriously, OPamps and even precision caps are far, far cheaper than 4PDT switches these days (it's been a couple of years since the active filter cookbook was written). Just have the two filters in parallel and switch the output with a simple switch.

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  • \$\begingroup\$ You know that is a very nice solution! I already have the switch though, so I'll try and hook it up. Will remember this for sure, thanks very much. Maybe I'll do this and save the switch after I figure out the connections ; ) \$\endgroup\$ – SOOTY Feb 28 '17 at 9:47
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Look at the non inverting input and imagine an SPDT switch connected to it (using the switch's common terminal). Now use that switch to choose which network of components feeds the non inverting terminal. That should be all you need to do.

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  • \$\begingroup\$ Thanks very much Andy, I will look at it from this perspective! I've really needed a fresh angle to look at this from, cheers! \$\endgroup\$ – SOOTY Feb 28 '17 at 9:48
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You should keep your expensive 4PDT switch for an application that needs it, and follow the advice already offered. If you MUST use the SAME components for the HPF and LPF functions, then in the simplest case you would need to switch BOTH terminals of all 4 passives, using an 8 pole switch. It is possible to use fewer than 8 poles with various messy kludges, but I doubt it can be done with 4 poles.

Or (without knowing what you're trying to achieve) you could use a state variable filter instead, which is more controllable and has simultaneous LPF, BPF and HPF outputs.

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