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I want to measure 12v. Car battery with arduino. Three questions:

  1. Is the R1 and R2 the best values? The car battery can go from +10v. To +15v.
  2. How and what can I use to save power between the measure reads? Like a transistor to switch on and off or something like.
  3. Must I add something to protect arduino pin from car battery overvoltage?

I have this:

———————————————— +10v. To 15v. Car battery
         |
         |
    Something here to switch off and save power? 
         |
         |
         R1 = 150kOmh
         |
         |——————————- to analog arduino pin to read voltage 
         |
         |
         R2 = 470kOmh
         |
         |
——————————————————- GND negative battery 

Solved!

———————————————— +10v. To 18v. Car battery
         |
         |
         R1 = 36kOmh
         |
         |———————————————- to analog arduino pin to read voltage 
         |            |
         |            |
         R2 = 10kOmh  5.1v Zener diode to protect arduino pin
         |            |
         |            |
—————————————————————————— GND negative battery and arduino 
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  • \$\begingroup\$ The divider value is too high, which will affect the accuracy. A few kOhms is ok. \$\endgroup\$ – Long Pham Apr 9 '18 at 16:41
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    \$\begingroup\$ if you want protection, maybe you should add a 5V zener diode \$\endgroup\$ – Long Pham Apr 9 '18 at 16:42
  • \$\begingroup\$ A few KOhms of resistance won't drain much your battery \$\endgroup\$ – Long Pham Apr 9 '18 at 16:43
  • \$\begingroup\$ How is the Arduino itself powered? \$\endgroup\$ – Claudio Avi Chami Apr 9 '18 at 16:45
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    \$\begingroup\$ you can connect R2 to a GPIO instead of GND. To save power, you inputMode(x, INPUT_PULLUP); digitalWrite(x, HIGH) and to read you use inputMode(x, OUTPUT); digitalWrite(x, LOW), which connects the divider to the GND with a low-impedance path, whereas the INPUT_PULLUP mode throws it behind dozens of more kohms... \$\endgroup\$ – dandavis Apr 9 '18 at 20:46
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to Q2. To save power, before you switch off something using transistors. Do these two first 1. write proper code to save power eg use very low cpu clock, make mcu sleep etc 2. choose low quiescent current LDO because a cheap L7805 has 2-3mA quiescent current.

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You can use the divider resistor values as shown in your first diagram but you will need to put a 0.1uF capacitor across R2. The input to the A-D shows up largely as a capacitive load which can introduce inaccuracies if you have large resistors and no charge storage at the input.

This will slow down your reading response to fast changes but in this application that will probably not matter.

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    \$\begingroup\$ +1, use C0G or film capacitor as X7R is piezoelectric and will pick up vibrations. \$\endgroup\$ – peufeu Dec 29 '19 at 12:47

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