Hello guys and gals..

Over the weekend I wanted to build a light bar (or at least a prototype of it) that would respond to sound frequencies. Decided to use AD820 op-amp because I need it to work from single 9v battery and I can control the gain using potentiometer.

Circuit has few more LED's and high-pass filters, I didn't want to over-populate the picture.

The circuit is supposed to respond to certain frequencies and once certain frequency is reached, a LED is supposed to light up.
I calculated the Xc of each capacitor to respond to desired frequency and adjusted potentiometers to calculated Xc:
80Hz - 22uF - Xc = 90.5
120Hz - 22uF - Xc = 60.2
180Hz - 22uF - Xc = 40
At this point, Xc would start becoming very small, so I changed filters to use 1uF capacitors.
270Hz - 1uF - Xc = 589
405Hz - 1uF - Xc = 393
607Hz - 1uF - Xc = 262
911Hz - 1uF - Xc = 174

Now here is where the problem starts. Using signal generator at 1Vpp, op-amp operates fine. After after first filter (80Hz), signal is 1.2Vp and at the last high-pass filter it if 0.6Vp. When I increase the frequency to 1 kH, last filter has (911 Hz) 1,2Vp and first filter (80Hz)drops it to 0.9V, dimming the LED.

On paper, it all appears fine, but it will not work properly in design. What am I missing?

Thanks for all of the answers in advance.

 

Your problem is as the frequency increases all the capacitors will be low in resistance and they are fed directly into the bases of the transistors, which will clamp at 0.7V, i would put the wipers to the bases of the transistors,and feed the presets from the caps.

 

So for example, on first filter, I should put wiperarm from R4 to base of Q2, what about the input to the potentiometer, where does the wiring go? Only from cap?

 

just move the wipers from earth and connect them to the bases only,leave the feeds from the caps to the pots,

 

I did as instructed, but now it is no longer a filter; adjusting potentiometer does not limit the frequency. All base pins on transistors have same voltage, no matter howI adjust potentiometers.

 

Well, I got it to work (somewhat). I eliminated Q1 & R3, so that op-amp is directly feeding the signal to the filters. I put the wipers from potentiometers in the filters back to ground (like my original schematics show, and I separated the filters to display different frequencies. I guess my frequencies were too close to each other and when I tried to play any audio, everything would light-up - that's because sound it more than one frequency unlike signal generator. Now my frequencies are 80Hz, 180 Hz, 405 Hz, 911 Hz, 2040 Hz, 4590 Hz, and 6885 Hz. Higher frequencies dim entire circuit slightly, but it responds as I wanted.

Thank you DodgyDave for your help and a clue that got me to my solution.