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I used 6 rechargable AA batteries (total 7.2V) to power my Arduino (UNO R3) Car's two DC Motors (using L293D IC as motor driver). I used separate 9V battery to power arduino itself. But I want to share same 6 battery pack to power my arduino via barrel jack. Please suggest me how can I do this? What precautions Should I take? Like decoupling capacitors or any other?

  • If you want to operate the uC reliably, then take a switching step down regulator (I usually use the LM2596 modules from ebay, 1€/each, but a lot of them are fine), then push the 5V voltage in the board directly (through the USB connector if you want some protection - e.g. fuse - or directly in the 5V pin if you don't want it) – frarugi87 Jan 22 '16 at 15:12
  • I have LM317 and LM7805 can I use any of this? @frarugi87 – palash Jan 23 '16 at 9:00
  • The LM7805 has a 2V dropout, so as soon as the voltage drops under 7V you won't have 5V again. The LM317 has a 3V dropout, so it will not work. Moreover they are linear regulators (they will waste much power, and you don't want this to happen with a battery powered system). Just use a switching regulator (the modules you find on ebay are perfect for your needs) – frarugi87 Jan 23 '16 at 13:44
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You may have read that the UNO can accept 7-12 volts as input power. Your AA battery pack (7.2 volts) is very near the low limit of 7 volts. After just a little discharge of the battery (and also voltage drop from motors running), your batteries are likely to deliver less than 7 volts.

As your AA battery goes below 7 volts, then the 5 volts (regulated) will also begin to go lower. A reasonable scenario would be if AA battery was 6.5 volts (1/2 volt low), then the 5 volts regulated would be about 4.5 volts. The UNO is likely to operate ok at this voltage, but as voltage goes lower operation could become a problem depending upon your program used.

Since you are experimenting with a toy car, I would say to go ahead and run both the motors and UNO from the same battery pack. Not a lot to lose on trying.

The UNO already has decoupling capacitors in it's 5 volt regulator circuit.

Just for fun, you could use an Analog input to measure your 5 volts and if the voltage drops to 4.5 volts, you could dis-allow motor operation and flash an LED (or other action). Just a low battery indicator.

If you decide to measure the 5 volts with an analog input, read up on using resistor dividers and the Analog reference (A-ref). Because 5 volts dropping to 4.5 volts could effect your analog reading, depending upon what you use for Analog reference.

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    When the motors are running the battery voltage can drop quite a bit too. If the motors allow it, I'd use 8 batteries (9.6 volt) instead. – Gerben Jan 22 '16 at 15:52
  • I like your analog input read up idea. To do so, should I connect arduino 5V pin to any one analog pin directly? Could you tell me more? I also like frarugi87 comments on main quesion. Could you comment on this that which one is the best approach? @marla – palash Jan 23 '16 at 9:51
  • @palash he is saying that if you want to measure the battery voltage, you have to use a stable analog reference instead of Vcc, because if the 5V voltage drops to 4.5V your analog reading will raise. So if you want to measure the battery voltage, you can use the internal analog reference (1.1V), then use a voltage divider to lower the 7.2V to around 1V (for instance 82k and 13k). I suggest you to use a PMOS to disconnect this from the battery (otherwise it will consume power) – frarugi87 Jan 23 '16 at 13:51

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