How Do You Design an Op-Amp Bandpass Audio Filter with LED Indicators?

[hutch3ka]

I am attempting to build an Op-Amp circuit in which a sinusoidal input is used. Below is the description:

An active, op-amp bandpass filter circuit for audio frequency inputs. The
implemented filter circuit must provide a maximum gain of 14 dB inside the pass-band and less than 9 dB of gain below and above the critical frequencies of 1000 Hz and 5000 Hz.
Design and implement an LED indicator circuit that uses the output of the bandpass filter circuit as the input. Light a green LED when the frequency of a 1 V peak-to-peak amplitude sinusoidal signal is between 1000 Hz and 5000 Hz. Light a red LED when the frequency of the input signal is outside of this range.

Any help greatly appreciated!

 

[yungman]

Get an Active Filter Cook Book and look up Sallen Key:

http://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology

What your requirement should be quite easy. As for the gain, put an op-amp gain stage before the filter to get the total gain you want.

Don't worry about all the math, all you have to do is follow the formulas to find the R and C to get the effect. I have the old Active-Filter Cookbook by SAMS, it is easy and it is good.

As for LED, did not give enough info. Is the input of the filter constant amplitude, only the frequency change? If so just use a peak detector and use a comparator to get the LED driving signal. If the input amplitude varies, then it would be difficult.

 

[dlgoff]

Single supply LM124/LM224/LM324/LM2902 Low Power Quad Operational Amplifiers are good for audio.

https://docs.google.com/viewer?pid=bl&srcid=ADGEESiPas-kKLjSJuAr8mm5bkY-KwaKtWhO_-fAl9GjwlnDzRtulYCbI6q9Gu0K-OwzqKfMkd93M2s4TqlpxybWUwKd0ejeka-aWhZIyLBmREhaUnN6y9NZHlp9wXqjy4Kaq6apDWGm&q=cache%3AdH6RhBxV2NMJ%3Awww.ti.com%2Flit%2Fds%2Fsymlink%2Flm124-n.pdf%20&docid=adbf9b408a0b278dff75a06f531d2e60&a=bi&pagenumber=11&w=800

https://docs.google.com/viewer?pid=bl&srcid=ADGEESiPas-kKLjSJuAr8mm5bkY-KwaKtWhO_-fAl9GjwlnDzRtulYCbI6q9Gu0K-OwzqKfMkd93M2s4TqlpxybWUwKd0ejeka-aWhZIyLBmREhaUnN6y9NZHlp9wXqjy4Kaq6apDWGm&q=cache%3AdH6RhBxV2NMJ%3Awww.ti.com%2Flit%2Fds%2Fsymlink%2Flm124-n.pdf%20&docid=adbf9b408a0b278dff75a06f531d2e60&a=bi&pagenumber=14&w=800

https://docs.google.com/viewer?pid=bl&srcid=ADGEESiPas-kKLjSJuAr8mm5bkY-KwaKtWhO_-fAl9GjwlnDzRtulYCbI6q9Gu0K-OwzqKfMkd93M2s4TqlpxybWUwKd0ejeka-aWhZIyLBmREhaUnN6y9NZHlp9wXqjy4Kaq6apDWGm&q=cache%3AdH6RhBxV2NMJ%3Awww.ti.com%2Flit%2Fds%2Fsymlink%2Flm124-n.pdf%20&docid=adbf9b408a0b278dff75a06f531d2e60&a=bi&pagenumber=18&w=800

And other tips can be found here: "www.ti.com/lit/ds/symlink/lm124-n.pdf"

 

[berkeman]

I am attempting to build an Op-Amp circuit in which a sinusoidal input is used. Below is the description:

An active, op-amp bandpass filter circuit for audio frequency inputs. The
implemented filter circuit must provide a maximum gain of 14 dB inside the pass-band and less than 9 dB of gain below and above the critical frequencies of 1000 Hz and 5000 Hz.
Design and implement an LED indicator circuit that uses the output of the bandpass filter circuit as the input. Light a green LED when the frequency of a 1 V peak-to-peak amplitude sinusoidal signal is between 1000 Hz and 5000 Hz. Light a red LED when the frequency of the input signal is outside of this range.

Any help greatly appreciated!

Welcome to the PF.

In the future, please be sure to show your attempt at solving your schoolwork question/project, or you post will likely be deleted. Thanks.

 

[rezeew]

I would suggest starting by researching and understanding the principles of op-amps and bandpass filters. This will help in designing an effective circuit that meets the given requirements.

To achieve a maximum gain of 14 dB, the circuit can be designed using a non-inverting configuration with a gain of 7 (as 20*log(7) = 14 dB). The cut-off frequencies of the filter can be set using appropriate resistors and capacitors in the circuit. For example, a combination of a high-pass and low-pass filter can be used to create a bandpass filter with a passband between 1000 Hz and 5000 Hz.

To implement the LED indicator circuit, the output of the bandpass filter can be connected to a comparator. The reference voltage for the comparator can be set to 1 V, and the output of the comparator can be connected to the green LED. When the input signal is within the desired frequency range, the output of the comparator will be high, turning on the green LED. When the input signal falls outside of this range, the output of the comparator will be low, turning off the green LED and turning on the red LED.

It is also important to consider the power supply and component tolerances in the design to ensure accurate and stable operation of the circuit. I would recommend simulating the circuit using a software like LTSpice or Proteus before implementing it in hardware.

Overall, building an op-amp bandpass audio filter can be a challenging but rewarding project. By carefully selecting components and designing the circuit based on sound principles, the desired results can be achieved.