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Hello,

I'm working on a project where I want to follow signals being sent to an LCD. measuring across an input pin and a backplane pin, I get a square wave of about -1v~1v when a symbol is off, and a more complicated signal that goes about -1.5v~1.5v when it's on. These go at about 500hz or so.

I want to be able to measure the voltage that the signal gets up to, so I know if a symbol is on or off. I've found some circuits online for AC measurement, but they seem to rely on diodes, and I think with such low voltage, the entire signal would get lost in the diode's forward voltage drop. Any ideas for how to measure this?
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See if you can get hold of an oscilloscope.
www.dpscope.com is a nice one for this kind of stuff,
or try a soundcard scope such as Visual Analyzer, free download from
http://www.sillanumsoft.org/
Make up a simple probe to plug into your sound card.
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I actually probed out the signal on an oscilloscope, which is how I know  the signal's voltage and frequency (it was kind of weird, actually, I had always thought of LCD as being a DC thing)

I want to be able to have the microcontroller follow this signal though, without a human intervening.

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HOWEVER, detecting the signal on an LCD is a really last-ditch method. I would try VERY HARD to find a more suitable method.  Even if you manage to amplify the signal, rectification could very easily mask any slight change in the signal. I would not consider this to be a viable method at all.

I think it's pretty much the best choice in my case. I'm trying to follow the state of an external device, and the controller is pretty much a black box, with no external communication, but all necessary information gets displayed on a purpose-built LCD.
« Last Edit: March 14, 2011, 12:22:52 am by YenTheFirst » Logged

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In that case, buffer it with an op amp, give it some gain, then rectify it and run it thru an envelope follower or peak detecter circuit.
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My favorite device for measuring low level AC (up to 500Mhz!) is the analog devices log amp, AD8307. While not a cheap device, around $15 when I bought one, it has tremondous dynamic range and a easy to interface DC output voltage perfect for an arduino analog input pin.


http://www.analog.com/static/imported-files/data_sheets/AD8307.pdf
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Clearly you need what is referred to as a "precision rectifier" circuit.  In these a simple opamp has a feedback loop which takes into account the normal forward voltage loss across the diodes.

Try googling "precision rectifier" or go directly to   http://sound.westhost.com/appnotes/an001.htm for a write-up on the subject

jack
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