Sensitive AC voltage detector

[skyler]

I am launching a high altitude balloon in a few weeks with the High School club I started, and we are rushing design after design to get things working.

The experiment we are launching on the balloon is a Field Mill. We based the design off this link .

After finally getting the field mill to work, we need to somehow feed the AC voltage into the 0-3.3 volt datalogger. I am fairly inexperienced, but I saw that I might need a preceision rectifier, a peak detector, and maybe an amplifier if the signal is too low.

Could someone help with a circuit, we don't have much time to research what is needed, and what I order today has to work.

[Kleinstein]

The amplifier of choice is usually a FET based OP. At low voltage a CMOS OP like MCP602.
To get good separation of the AC signal from the field mill and noise a kind of Lock-In scheme is helpful instead of simple rectification or peak detection. This can be done with the help of CMOS switches to make an amplifier that is switchable as +-1. However this also needs a reference clock, from an extra detector (e.g. CNY70).

Depending on the datalogger / µC one could also feed the AC voltage to the ADC and do the amplitude determination in software.

[LaserSteve]

Maybe not sensitive enough for a atmospheric mission:

http://freespace.virgin.net/paul.zimmermann/Electronic/ep3.htm#My

One with synchronous (lock-in) detection:

http://www.qsl.net/dh1stf/aufbauae/images/schaltung.gif

Steve

[CaveMannDave]

Hi:

My audio died, but watching the Pretty Moving Pictures, I infer this is some kind of electrostatic Field detector, measuring tiny changes in intensity by chopping techniques.

Excellent Idea, IMHO.

I recently ran across a reference to this method, and searched it out (that is my way of learning, I run-to-ground any New concept, and devour it whole.)  The Mongoose clan, as related by Riki-Tiki-Tavi{sp?}, (their spokes-critter in the R. Kipling Jungle Book stories) had a motto or Mantra, that if you wanted to Know a Thing, go run-and-find-out...            Sorry for that digression...

Anyhoo, it boils down to, you can recover minuscule signals from Overwhelming background interference through using chopping methods and synchronised demodulators: Also known as Lock-in Amplifiers...                BINGO!     Just fits your need.

They can cost tens of thousands for Lab-Grade units, but an Analog Devices, or other analog multiplier IC, a clock recovery PLL, (I'm assuming single channel, Manchester-encoded {Way old School} or otherwise data w/clocking transmission method) to recover synch, some active filter stages, and Very Little Else will deliver a nice, clean DC output of the signal desired.     The S/N can scare you.

Just get a GOOD synch signal from the remote sensor, and use daWebz to find the other trivia, ask for detail help, if needed, and breadboard away!

It is a really simple concept, with Dozens of ways to get there, and YOU get to choose what fits your needs/personal-biases/wallet/whims or whatever.

There are many ways to Skin that Cat!

ETA:  Look into Log Amps, to compress wide data ranges into a low-res DAQ.
Also consider Voltage-to-Frequency conversion for fidelity of transmission and logging.

Cheers,

Dave