Hey, cool project. I would definitely think the problem is in the last state, the one-shot 555 circuit. Measure the voltage on pin 2 and pin 6. They should be changing with the bass beat. As well, measure the other pins just to make sure none of the connections have opened/shorted. Make the pin 2, 6 measurements with your scope. If pins 2 doesn't toggle, then we have to start tracing back.
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In your schematic, C8 is 2.2uF, in the original it is 22uF.
Thanks! Ive gotten lots of compliments from people who have seen it in action.
Here's the board itself:
**broken link removed**
And here it is driving an LED bar.
Anywho, back to troubleshooting!
Pin 2 doesnt toggle. I believe I wasnt clear enough in my first post; I said:
"The expected output is a waveform sitting at VCC until there is a bass beat, at which time Id see a negative pulse to trigger the 555. Right now it just sits at VCC."
I should have been more clearer about the fact that this expected waveform is from the output of IC2 in my schematic (sorry about the numbering being off, it came from a last-second package change). I know /TR of the 555 needs to be triggered low to fire my LEDs, and that isnt happening.
As for the value of C8, this is one of a few values I changed when I first prototyped the circuit on a breadboard. That, along with C9 and C12 gave me the specific look I was after.
What voltage level is Vcc? If the voltage at pin 2 of IC2 isn't high enough to exceed the pin 1 voltage, the comparator (IC2) won't change state at its output. The voltage divider (R15, R16 and VR1) supplies the reference voltage for comparator IC2. The output of the voltage divider depends on the value of Vcc.
What kind of signal do you see at pin 2 of IC2? It should be a varying positive DC voltage that follows the level of the bass frequencies. What is the peak value of this envelope? You'll see it much better with an analog voltmeter ahan with a digital meter. Measure the DC voltage at IC2 pin 1. Does the peak voltage at pin 2 exceed the pin 1 voltage? If not, then you're in the right area.
Either the signal voltage from the first two IC's isn't getting amplified enough or the envelope detector isn't working. Are the diodes OK? If you have low signal voltage here, then you need to check the first two ICs.
Cheers,
Dave M
VCC is ~12.2v. By pin 1 of IC2, do you mean pin 3 (the +ve input)? Pin 1 is for offset nulling on the TL051 (unless Im looking at the wrong diagram, in which case ignore me!)
I believe you are correct though, and this area is where I traced the problem to. The voltage on pin 3, the "reference" voltage if you will, varies from 8.4v to 10.1v, but the DC level of the envelope signal
is no higher than 2v. If it matters, the input signal (output from the amplifier pre-out) is about 250mVpp, but it goes up to 500mVpp depending on the volume.
This tells me I dont have enough gain in the early stages (or I need to lower my reference voltage). I would like to do both - make my reference adjustment able to be much wider. But I would also like slightly more gain (or maybe selectable gain?).
Correct me if Im wrong, but the gain of a Sallen-Key is 1, so the only gain in this circuit is coming from R6/R7 (in my schematic). However, how does C4 affect the gain? I would assume it causes gain to be higher at certain frequencies (Xc = 1/[2*pi*f*C]). Whats the best way to fix my reference adjusting problem?