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I saw online some cheap hardware ~10$ that can measure volt, amps, watt and kWh and display the results on an LCD screen.

enter image description here

I could buy and plug these on my 12 V solar panel system to monitor it. (5 of them) (Circuit is between 6A and 20A)

But thinking about it, I wish to use my Raspberry Pi to monitor these numbers. - Trigger some events on specific values, log these values over the year to compare efficiency between seasons, etc., and display all of it in a web interface or whatever. (That won't be a problem.)

My question is : What kind of hardware can I use to simply measure the volt, amps, watt and kWh and send the value to my RPi ? (in 5 different places in the circuit.) - Of course i can do the math later for watt and kWh. :)


Answering my own question, so you can tell me if it's a good idea.

I already made a working system with an ADC (Analog to Digital Converter) that can convert the voltage to an LED with a potentiometer with some C code. - PCF8591

enter image description here

  • Can i simply buy 4 ACS712 and send 4 different analog signals to the PCF8591 ? (It does have 4 analog inputs)

enter image description here

And then I should be able to use I2C to send to my Raspberry Pi.

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  • \$\begingroup\$ WHy not use that hardware? Of course the current range and differential voltage may not meet your unstated requirements. \$\endgroup\$ Commented Jun 27, 2019 at 12:48
  • \$\begingroup\$ I could in theory use that hardware but I should open it, remove the lcd and reroute the informations to my RPI instead. And I'm not sure how to do that. - And even if I can do that. (I'm a programming guy, that's my first electronic project.) \$\endgroup\$
    – bob dylan
    Commented Jun 27, 2019 at 12:51
  • \$\begingroup\$ I'm just wandering if there is an hardware like that, but that is built as a "sensor" instead of an all-in-one "lcd display". \$\endgroup\$
    – bob dylan
    Commented Jun 27, 2019 at 12:54
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    \$\begingroup\$ I agree that it is an expensive product. Only if you can find a project which shows a working example that you can rebuild yourself would this be suitable as a first project. There will be quite some issues and small details you need to get right to make this work so as a first project I would not recommend this, it will get too complex especially due to monitoring of multiple places and then avoiding grounding issues. \$\endgroup\$ Commented Jun 27, 2019 at 13:09
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    \$\begingroup\$ "Of course i can do the math later for watt and kWh" - unlikely because power is the average value of the instantaneous multiplication of current and voltage so, unless you store waveforms then you cannot recover a decent value for power. \$\endgroup\$
    – Andy aka
    Commented Jun 27, 2019 at 13:54

2 Answers 2

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Fundamentally, the "kind of hardware [used to] measure the volt, amps, watt and kWh and send the value to my RPi" is an ADC. To measure volts and amps you measure voltage across a parallel and series resistor respectively. Watts and kWh are calculated from that.

But this a big undertaking from where you are. Better to get something that does most of this for you. In this case, look for a "DC power meter IC". Internally they're an ADC, but they take care of a lot of the details for you.

Consider, for example, the Microchip PAC1720 or the Maxim MAX44299 or the Texas Instruments INA219. These are not product recommendations, but advice on what to look for. The typical application circuits in the INA219 datasheet in particular should be enlightening.

From there you can go significantly easier again by finding a breakout or demonstration board for these types of ICs. Adafruit's INA219 High Side DC Current Sensor Breakout (product ID 904) is a great example.

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  • \$\begingroup\$ I saw already the "Adafruit's INA219"but I was concerned about the technical details : "Up to ±3.2A current measurement, with ±0.8mA resolution". My solar system is 6A for now and will probably go up to about 20A. About the others (Microchip, Maxim. Texas Inst) I'm currently reading about them, thx for the names. \$\endgroup\$
    – bob dylan
    Commented Jun 27, 2019 at 14:04
  • \$\begingroup\$ what do you think about the combo PCF8591 (ADC) + ACS712 (sensor) ? \$\endgroup\$
    – bob dylan
    Commented Jun 28, 2019 at 11:35
  • \$\begingroup\$ Sorry, comments are no place for product recommendations. \$\endgroup\$ Commented Jun 29, 2019 at 22:59
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No one can possibly design anything "well" unless you have good interface specs and can define exactly the environment and functions: range and tolerances required. (unless you have done this for decades, when you should have been sleeping)

Learn by reading good specs and Theory of Operation to see how one affects the other.

Start small as I did. The same above goes for IC's in datasheets.

Before I graduated, I read every EDN magazine published since the '60's for the Design Tips page submitted by readers. These EDN's ( Electronic Design News magazines were bound in annual books in our EE library. Now they may be found on www.archive.org

example of today's https://www.edn.com/design/led/4363702/LED-lighting-teardowns-Five-lighting-designs-that-illuminate-the-future-of-lighting (also self-help books in my profile) Specs must include your own needs like:

  • time, effort, skill, cost, environment, choices, learning curve, end-use (make vs buy)

Experience comes from your failures and your ability to detect them before it's complete and correct them. Evidence of experience is the ability to predict worst-case failures and causes then define these limits in your specs, so your design can avoid them and then you can test to verify against these specs.

I learned in 1st job Aerospace R&D as an Electronic Instrumentation Designer to start every design with a functional spec with no physical part numbers. That is what we call "implementation specific" which may be a poor or sub-optimal choice and fails to define the objectives and all acceptance criteria.

If none of this is helpful (?!) start by learning the Taxonomy of application solutions PIC any ONE
( pun intended )

https://www.microchip.com/wwwcategory/taxonomysearch/

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    \$\begingroup\$ Ah...browsing EDN...good times, good times. This is great advice. \$\endgroup\$ Commented Jun 27, 2019 at 13:53
  • \$\begingroup\$ Ah the trolls are back -1 \$\endgroup\$ Commented Jun 28, 2019 at 15:27

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