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Old 10-20-2010, 02:21 PM
bonanzaman bonanzaman is offline
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Join Date: Oct 2010
Posts: 6

power is three 9v batteries for 27 volts.

Opamp 1 appears to work ad expected. I get a large signal at the output (10V+). I put the cap at the output to remove the dc and the diode to rectify the signal for the charging of the RC circuit.

I can get the circuit by itself to charge up to 1-200 mv or more but when I hook up the second stage it seems to have a great effect on the charging.
I've played with both the cap and resister.

I've played around with the resistance values on #2 as to not load the rc circuit.

The pulses from the first stage around 300 us and the amplitude is to the top rail or 12 volts peak with lesser peaks after initial stimulus.
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Old 10-20-2010, 02:36 PM
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Kermit2 Kermit2 is offline
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try a different arrangement of your 1st opamp output to the second opamp input. Pull out the series cap. or try the diode as a parallel to ground component instead of series. Something is preventing transfer of charge and the second op amp is supplying the problem current by keeping its inputs balanced.

Also the wave shape may need to be cleaned up as suggested earlier. A schmitt trigger buffer might be called for to clean up the 'pulse' you are looking to amplify.
I'm too old.
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Old 10-20-2010, 05:06 PM
Audioguru Audioguru is offline
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Location: Ontario, Canada
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I removed many useless resistors and removed the coupling capacitor that feeds (and is charged by) the rectifier.

I don't know why the second opamp had a 20k attenuator at its input that cut the signal in half then had two resistors as feedback to amplify the reduced signal back up to the the original signal level.
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Last edited by Audioguru; 10-20-2010 at 05:14 PM. Reason: Removed two more useless resistors.
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Old 10-24-2010, 02:35 AM
gootee gootee is offline
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Join Date: Apr 2007
Location: Indiana, USA
Posts: 443

Using Vcc to derive the reference, to "subtract out" the 12 volts so you can amplify only the tiny pulse will probably cause difficulty.

It's a lot more robust if you create a low-pass filtered version of the sensor output and take the difference between that and the original sensor output. That way the absolute DC levels won't matter. Otherwise, tiny differences in the DC levels of the inputs will swamp the output with DC.

My circuit is probably a lot more complicated than it needs to be. But the most important parts are the difference amplifier (U1 and R2 through R4) and the low-pass filter for the sensor (R5, C7, and the U5 buffer). Note that the V3 voltage source is your accelerometer output voltage.

D2 and C3 plus R5, R12, and R9 make up the detector. And I added an adjustable output voltage clamp (U2, D1, and R7, R8) and an output buffer, U3.

You might want to vary some of the component values, to better-suit your signal levels and desired LED on-time. Almost any opamp that can handle the supply voltage should work.

The LTspice circuit simulation file is at . After downloading it, remove the ".txt" from the file name.

Edit: My ISP must be having some sort of problems. My link and picture aren't working.

Last edited by gootee; 10-24-2010 at 02:51 AM.
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