Pi as many Voltmeters

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deltafee
 
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Pi as many Voltmeters

Post by deltafee »

Hey there,
I need to figure out a way to configure the raspberry pi to become at least 16 voltmeters. I know these means that I would have to program it to work, but I am not sure how to create the electronic part of it. I think I would do is create a custom one from a humble pi type of board but I am not sure what connectors to put on it.

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adafruit_support_bill
 
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Re: Pi as many Voltmeters

Post by adafruit_support_bill »

The Pi does not have any built-in analog to digital converters (ADCs). To measure voltage, you would need to add some ADCs. We have a few that are compatible. If you click on the Tutorials tab for each of them, you will find links to tutorials for connecting them to the Pi.

http://www.adafruit.com/products/1083
http://www.adafruit.com/products/1085
http://www.adafruit.com/products/856

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deltafee
 
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Re: Pi as many Voltmeters

Post by deltafee »

So to have the pi be 16 voltmeters I would need 16 of these chips:
http://www.adafruit.com/products/856#Learn

but if I use this with the pit I would only need to have four boards:
http://www.adafruit.com/products/1085#Learn

and to ensure I have connected the voltmeter correctly I would need a
Differential measurements
http://learn.adafruit.com/adafruit-4-ch ... onnections

is that correct?

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adafruit_support_bill
 
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Re: Pi as many Voltmeters

Post by adafruit_support_bill »

So to have the pi be 16 voltmeters I would need 16 of these chips:
http://www.adafruit.com/products/856#Learn
No. That is an 8-channel chip, so you would only need 2.
but if I use this with the pit I would only need to have four boards:
http://www.adafruit.com/products/1085#Learn

and to ensure I have connected the voltmeter correctly I would need a
Differential measurements
http://learn.adafruit.com/adafruit-4-ch ... onnections
Differential connections are not necessary for most applications. And differential connections require 2 channels each. So if you use differential measurements with those boards you would have 2 differential connections per board and require 8 boards. This is not possible because there are only 4 configurable addresses for these boards.

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deltafee
 
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Re: Pi as many Voltmeters

Post by deltafee »

So now that I have it all have to do is connect a photo diode like the analog thermometer into the channels. I just want to make sure.

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adafruit_support_bill
 
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Re: Pi as many Voltmeters

Post by adafruit_support_bill »

all have to do is connect a photo diode like the analog thermometer into the channels
It might help if you explained in more detail what you are trying to build.

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deltafee
 
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Re: Pi as many Voltmeters

Post by deltafee »

What I want to do is measure the voltages from 16 photo diodes. The diodes are going to get the voltage from 16 individual laser diodes shining right into them, the maximum voltage they all will produce is under 3 mV.

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adafruit_support_bill
 
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Re: Pi as many Voltmeters

Post by adafruit_support_bill »

You would connect the anode to GND, the cathode to the ADC input and a pullup resistor from the ADC input to your reference voltage. The value of the pullup will depend on the photodiode. Check the spec sheet for app notes and/or circuit diagrams.

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deltafee
 
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Re: Pi as many Voltmeters

Post by deltafee »

according to this http://www.adafruit.com/datasheets/MCP3008.pdf the data sheet the maximum voltage would be 5.5V and the minimum voltage is 2.7V. It also gives max current to be 0.000549A and it doesn't give a value of its minimum current but I assuming that it is like 0.000001A. So by using I=V/R => R=I/V. Max Resistance would be 0.0001 ohms and min resistance would be 3.7E-7 ohms and by RPullup = sqrt(RMax*Rmin) => Rpullup would be 6.08E-8 ohms. But I am not sure, I am not sure but I did see this RSS= Source Impedance but it didn't say how I would calculate it. So basically I am kind of guessing on my end :/

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adafruit_support_bill
 
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Re: Pi as many Voltmeters

Post by adafruit_support_bill »

the maximum voltage would be 5.5V and the minimum voltage is 2.7V.
That is the operating voltage range for the ADC chip. You measurement range will be 0v-Vref.
The value of your resistor will have more to do with the characteristics of your photodiode than the input impedance of the ADC. 10K is probably a reasonable value to start with. If your lasers max out the readings, use a higher value resistor.

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deltafee
 
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Re: Pi as many Voltmeters

Post by deltafee »

Oh okay, I understand now; I am connecting the photo diode to an opt amp before it connects to the chip. Which would mean that I need to figure out the maximum voltage the chip can take and I believe that is 5.0V.

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adafruit_support_bill
 
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Re: Pi as many Voltmeters

Post by adafruit_support_bill »

I need to figure out the maximum voltage the chip can take and I believe that is 5.0V.
Yes. That is correct.

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deltafee
 
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Re: Pi as many Voltmeters

Post by deltafee »

okay so my final question is how do I connect both chips to the pi? I know there's a tutorial that comes with the chip on adafuit but it doesn't mention how to connect two. I am stating the obvious question but do I just connect both of them to the same pins that take data like:
MCP3008 CLK -> pin 18
MCP3008 DOUT -> pin 23
MCP3008 DIN -> pin 24
MCP3008 CS -> pin 25
So basically what I am asking is how do I connect them together so I can have the pi read them?

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egutting
 
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Re: Pi as many Voltmeters

Post by egutting »

No, you'll need a different CS (chip select) for each chip. You can use one of the other pins (ie: 8 or 7)

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deltafee
 
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Re: Pi as many Voltmeters

Post by deltafee »

egutting wrote:No, you'll need a different CS (chip select) for each chip. You can use one of the other pins (ie: 8 or 7)
So your saying something like this?: https://dl.dropboxusercontent.com/u/775 ... ircuit.pdf

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