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Help me Rectify Sine Wave

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I'm trying to pulse some LED's to the bass. I have got it working, but the LED's are only on for half the wave.

I am using this in a car so I only have one supply voltage. The sine wave coming out of the low-pass filter is riding on 4V with the negative alternation peaking at 0V and the positive alternation peaking at 8V.

I would like to rectify the negative alternation. I was looking online, and saw people suggesting using a precision rectifier. However, none of the ones I found have worked, and I'm not sure how to "set" the ground to 4V. Any help would be greatly appreciated.
 
You would use a combination of a voltage regulator (eg. a simple voltage divider) and a voltage follower to make 4V steady.
 
Without knowing your circuit it is hard to suggest something, but here is a schematic that may help you. Note the diodes and capacitor on the output of the op amp.

**broken link removed**
 
I will contribute with a much less complex circuit.
**broken link removed** have a fairly good description on a full wave rectifier. You will need to get a negative voltage supply (to power the opamps) in order to get it to work. Reference voltage should be zeero voltage in your case.
 
I will contribute with a much less complex circuit.
**broken link removed** have a fairly good description on a full wave rectifier. You will need to get a negative voltage supply (to power the opamps) in order to get it to work. Reference voltage should be zeero voltage in your case.
The article plainly shows the circuit with a single positive supply voltage. Why do you say it needs a negative supply? :confused:
 
The article plainly shows the circuit with a single positive supply voltage. Why do you say it needs a negative supply? :confused:
Because it's supposed to rectify the negative parts of the "waves" and because sound (waves) is expected to swing around zeero volts.

But - After I posted this, I admit that makes the circuit more complex than neccesary. Instead, use a voltage clamp (excuse me if messing with phrases) to rise DC level to same level as the virtual ground (4 volts as mentioned earlier).
 
Because it's supposed to rectify the negative parts of the "waves" and because sound (waves) is expected to swing around zeero volts.....
The referenced circuit is designed to operate with a signal that goes below ground. It inverts the negative signal to generate one of the positive halves of the full-wave rectified output. The positive half is, of course, not inverted.

Edit: To input his signal with the 4V offset you just use a series capacitor at the circuit input to block the DC.
 
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