[time-nuts] cheap USB voltage sensor

[Chris Albertson]

If you have a PC and an AC coupled audio interface then send a low
frequency audio saw tooth wave to the audio out.  Connect that and the
device to be measured to an LM311 comparator.  The  comparator will
flip when your output voltage passes the DUT's voltage.   One could
get fancy and use multiple comparators Then connect the comparator(s)
to a parallel port.   You get 8 channels of low bandwidth analog input
for about 25 cents per channel, if you already have the parallel port.

I think that is the cheapest possible way to get voltages into a computer.

Also this kind of ADC can be very accurate.  You can tie one or more
of the LM311s to a voltage reference and then your instrument is
continuoly calibrated.

This works because in our case the signal, has low bandwidth so we can
take out time and collect 1000 samples

On Fri, Jan 13, 2012 at 12:02 PM, beale <beale at bealecorner.com> wrote:
>> Bringing up a question: Does anyone know of a cheap (<$20ish) USB voltage
>> sensor (16 bits or better, ideally)..  I can see one of those Atmel USB
>> capable micros (like the one on the Arduino Uno) hooked to a dual slope or
>> successive approximation ADC.
>
> Doesn't quite meet your price, but there's a 3.3V version of an Arduino called a "JeeNode" designed for sensor work, and there are a number of I2C based sensor plugins for it. For example the "analog plug" based on Microchip MCP3424 with 4 channels of differential inputs at 18 bits.  Jeenode (kit) is $23 and Analog plug (assembled) is $12. It's the standard Arduino architecture, so it is simple to use and (re-)program from your PC via USB, no extra programmer needed.
>
> http://shop.moderndevice.com/products/jeenode-kit
> http://shop.moderndevice.com/products/jeelabs-analog-plug
>
> I have some and they work well. Here's a plot of voltage vs time on an AA battery, showing the 18 bit performance (1 LSB = 15 uV). Noise is generally +/-1 LSB.  https://picasaweb.google.com/lh/photo/q7Lq4oAX_0347S8BO0eHSA
>
> You can plug up to four "analog plugs" directly into the JeeNode (software I2C) and these can actually be daisy-chained as well, with 6 different I2C addresses per I2C chain, for up to 24 total plugs per JeeNode which would be 96 ADC channels. If you are in Europe you can buy hardware direct from the designer at http://jeelabs.com/
>
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-- 

Chris Albertson
Redondo Beach, California