Help with LED project

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ca00bravo
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Help with LED project

Post by ca00bravo » Mon Apr 09, 2007 5:21 pm

Hello everyone,
I am using a LF353 that has to amplify a 1kHz-500mVp-p sinewave up to 3.5Vp-p. The source resistance must be 600 ohms. The circuit must have adjustable gain and the LED should come on when the Vp-p of the output sinewave reaches 2.8Volts. I can only use +12V from a power supply. I'm only using one amp.

Any suggestions..sorry i cannot attach my schematic...

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haklesup
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Post by haklesup » Tue Apr 10, 2007 1:16 pm

Off the top of my head, start with an inverting amp using the op amp.

The source's resistance dosen't really matter since the op amp input is so very high impedance. If it took more current like when using a transistor to bias the base, then the source impedance is important so as to not to reduce its voltage when current is sourced from it.

Choose R1 and R2 for a gain of 7 make one of these resistors a potentiometer so you can adjust.

Now apply the output of this amp to another inverting amp with unity gain (R1=R2) to re invert it to the proper phase (if you need to)

Detecting 2.8V is the harder part. You could rectify the 2.8V p-p to DC and then compare it using a comparitor to a reference voltage. That would give you a digital output to drive the LED. If you need to use the 2.8V AC signal elsewhere, you could isolate the detector stage with a series capacitor between.

You might put the AC into an integrator (like an inverting amp but with a cap in the feedback) which will output a DC value you can compare to. This uses more op amps but since each has a high imput impedance, it leaves the intermediate signals more useful (unchanged) than if they were loaded by a diode. Come to think of it, the integrator might just output the offset of the input signal if it were just a sine.

Find a way to post the schematic. IF you can't scan it, resketch it using paintbrush. This will help a lot. Have you downloaded the free version of MultiSim (AKA Electronics workbench). It has enough nodes to simulate a circuit this small and is a very useful tool for learning. Student versions are also available. It has a nice op amp design wizard built in.

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