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.
Pi as many Voltmeters
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Talk about Adafruit Raspberry Pi® accessories! Please do not ask for Linux support, this is for Adafruit products only! For Raspberry Pi help please visit: http://www.raspberrypi.org/phpBB3/
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- adafruit_support_bill
- Posts: 88093
- Joined: Sat Feb 07, 2009 10:11 am
Re: Pi as many Voltmeters
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
http://www.adafruit.com/products/1083
http://www.adafruit.com/products/1085
http://www.adafruit.com/products/856
- deltafee
- Posts: 93
- Joined: Sat Feb 08, 2014 1:23 am
Re: Pi as many Voltmeters
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?
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?
- adafruit_support_bill
- Posts: 88093
- Joined: Sat Feb 07, 2009 10:11 am
Re: Pi as many Voltmeters
No. That is an 8-channel chip, so you would only need 2.So to have the pi be 16 voltmeters I would need 16 of these chips:
http://www.adafruit.com/products/856#Learn
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.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
- deltafee
- Posts: 93
- Joined: Sat Feb 08, 2014 1:23 am
Re: Pi as many Voltmeters
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.
- adafruit_support_bill
- Posts: 88093
- Joined: Sat Feb 07, 2009 10:11 am
Re: Pi as many Voltmeters
It might help if you explained in more detail what you are trying to build.all have to do is connect a photo diode like the analog thermometer into the channels
- deltafee
- Posts: 93
- Joined: Sat Feb 08, 2014 1:23 am
Re: Pi as many Voltmeters
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.
- adafruit_support_bill
- Posts: 88093
- Joined: Sat Feb 07, 2009 10:11 am
Re: Pi as many Voltmeters
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.
- deltafee
- Posts: 93
- Joined: Sat Feb 08, 2014 1:23 am
Re: Pi as many Voltmeters
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 :/
- adafruit_support_bill
- Posts: 88093
- Joined: Sat Feb 07, 2009 10:11 am
Re: Pi as many Voltmeters
That is the operating voltage range for the ADC chip. You measurement range will be 0v-Vref.the maximum voltage would be 5.5V and the minimum voltage is 2.7V.
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.
- deltafee
- Posts: 93
- Joined: Sat Feb 08, 2014 1:23 am
Re: Pi as many Voltmeters
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.
- adafruit_support_bill
- Posts: 88093
- Joined: Sat Feb 07, 2009 10:11 am
Re: Pi as many Voltmeters
Yes. That is correct.I need to figure out the maximum voltage the chip can take and I believe that is 5.0V.
- deltafee
- Posts: 93
- Joined: Sat Feb 08, 2014 1:23 am
Re: Pi as many Voltmeters
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:
So basically what I am asking is how do I connect them together so I can have the pi read them?MCP3008 CLK -> pin 18
MCP3008 DOUT -> pin 23
MCP3008 DIN -> pin 24
MCP3008 CS -> pin 25
- egutting
- Posts: 297
- Joined: Wed Nov 14, 2012 12:57 am
Re: Pi as many Voltmeters
No, you'll need a different CS (chip select) for each chip. You can use one of the other pins (ie: 8 or 7)
- deltafee
- Posts: 93
- Joined: Sat Feb 08, 2014 1:23 am
Re: Pi as many Voltmeters
So your saying something like this?: https://dl.dropboxusercontent.com/u/775 ... ircuit.pdfegutting 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)
Forum rules
Talk about Adafruit Raspberry Pi® accessories! Please do not ask for Linux support, this is for Adafruit products only! For Raspberry Pi help please visit: http://www.raspberrypi.org/phpBB3/
Talk about Adafruit Raspberry Pi® accessories! Please do not ask for Linux support, this is for Adafruit products only! For Raspberry Pi help please visit: http://www.raspberrypi.org/phpBB3/