What function is the op-amp doing in this configuration.

[Helix Vlad]

The op-amps seems to have a few low pass and high pass filters, how are they filtering my signal, from their arrangement?
I know this is the finest electronics forum around, I am anticipated to hear help from you guys, Thanks!

[DmitryL]

Err.. what is the question exactly ?
Op amps in red circles make Sallen-key filters, see
http://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology
Filter frequencies are written on the schematics along with filter type (low-pass, high-pass etc.)
Whole circuit is supposed to blink LEDs when music plays, I suppose

[Helix Vlad]

Indeed they do, The correct question would be how exactly is it filtering my signal within the quotes nodes.
High five! same name.

[DmitryL]

The given filters filter the input signal exactly the way they are designed and accordingly to the laws of nature.

[T3sl4co1l]

Looks like first order bandpass filters.

[Paul Price]

The op-amps are functioning as signal frequency detectors and activate LED's as indicators when certain input signal frequencies are input that match the band-pass frequency of the op-amp circuits.


So, the op-amps are used to make a frequency detecting circuit, with a different frequency being detected by each op-amp.

A common use of a circuit like this would be to detect certain frequencies to activate some control circuits.

For instance, a dual-tone frequency signal generated by a telephone keypad or some other device output could be sent over a single wire to create a multi-frequency signal used to control one or more remote circuits by detecting pairs of frequencies.

A single frequency input to the op-amps could be used to detect just one frequency of interest, so it would be possible to select the LED of interest to be lit, or music sounds could be input to activate the LEDs as a light show that responds to the different frequencies of various instruments.

[c4757p]

The op-amps are functioning as signal frequency detectors and activate LED's as indicators when certain input signal frequencies are input that match the band-pass frequency of the op-amp circuits.

No, that's what the whole circuit does. He asked about the op amps, and made it quite clear that he knows the op amp is acting as a filter and wants to know how.

[LukeW]

The filters in that circuit, which are bandpass filters, are a topology known as a multiple feedback filter.

http://www.analog.com/static/imported-files/tutorials/MT-218.pdf

There are several different topologies for opamp-based active filters, eg. Sallen-Key, multiple feedback and what not, and of course you can design for different filter types, i.e. HPF, LPF, BPF or notch filtering. But the basic principles are much the same.

Basically an active filter like this feeds back an inverted signal in the unwanted frequency domain, so it cancels out the unwanted part of the signal.

Here are various links and resources on how they work. But be aware that it can get a little bit math-heavy.

http://www.electronics-tutorials.ws/filter/second-order-filters.html
http://www.analog.com/static/imported-files/tutorials/MT-220.pdf
http://www.ecircuitcenter.com/Circuits/MFB_bandpass/MFB_bandpass.htm
http://www.electronics-tutorials.ws/filter/filter_7.html
http://www.sensorsmag.com/sensors/electric-magnetic/an-introduction-analog-filters-1023

[dannyf]

The correct question would be how exactly is it filtering my signal within the quotes nodes.

Think of those capacitors as resistors whose resistance varies with frequency. Then work out the gain / attenuation of the circuit with regard to the "resistance". Then you have the frequency response of the whole circuit.

[Helix Vlad]

Thank you all for the wonderful help. This information will be of great aid to my project. 

[jlmoon]

I'm going to put this one in:  How about variable (neg) feedback (or variable gain) based on the oh so popular formula for Xc... 1/2(pi)(f)(c)