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When arduino starts up, RX TX lights doesn't blink. Only the PIN 13 and Green light turns on. Pin 13 light blinks thrice before getting stable. RX TX lights are not blinking at all. I am not able to upload any sketches. The error I get is "Not in sync resp=0x00" or "Programmer not responding" or "Not in sync resp=0x95" or "Not in sync resp=0x69" and so on. A new error everytime. Most probably the error is with serial communication.

A brief history of what I have done

I have a Relay which is operated at 12V. But Arduino works at 5V logic. So I used a comparator (IC741) to improve the logic (By setting Vcc and -Vcc at 12 and 0 and the input pins connected to arduino). I also connected HC-05 bluetooth module to use the relay using my cellphone. You know what I was trying was home automation. Also Arduino was connected to 12V DC supply (I think this was my mistake). Everything was going right.

Suddenly IC741 started burning,maybe I shorted some pins when I was trying to measure the voltages using multimeter. I quickly removed it. Suddenly HC 05 started burning. I immediately removed all the connections. After some time I turned on Arduino again (This time using USB connected to laptop). I connected HC05. The bluetooth module heats up again. I thought the module is now gone. I turned to check Arduino and as I stated I am unable to upload any more sketches. Most probably the bluetooth module cause an error in serial communication. May be not.

So my question is 1. What's the real problem. Is my bluetooth module gone? Or is Arduino gone? If arduino is still fine, how can I get the serial to work back and enable me to upload sketches? 2. If Arduino is gone, most probably the microcontroller Atmega328 is still fine. How can I use it now without Arduino (I don't have any extra Arduino)?

Things I have already tried :

  1. Loopback test : No echo back. Serial data can neither be sent nor received.
  2. When I open the Serial window, Arduino doesn't resets as it it earlier.
  3. RX TX Vin and 5V pin reads 4.99V. I hope they are fine.

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    say goodbye to the arduino. – Marko Buršič May 11 '16 at 18:43
  • @MarkoBuršič Could you please state a possible cause? – Sushant May 11 '16 at 18:51
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    I'm not going to write a speculative answer, but you've probably shorted 12V through to something and fried the comparator, the Bluetooth module, the ATMega328 serial pins if not more of it. If you remove the ATMega328 and connect the TX RX pins together on the Arduino, then send data through the serial monitor, do you see it coming back? If not, you've probably fried the serial pins of the ATMega32U4 as well. – Tom Carpenter May 11 '16 at 19:27
  • Same here. I am pretty sure the damage was done when testing various things, never plugged a motor in AFAIK but it turns out capacitors on the wrong pin (eg D6) can sometimes send a spike back down into the USB chip and fry it. The actual 328P works fine in my other Arduino. Incidentally an out of spec 16MHz crystal can cause strange problems too. – Conundrum Jan 28 '18 at 6:33
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Pin 13 light blinks thrice before getting stable.

It is supposed to do exactly that. See What happens when code is uploaded using the bootloader?

RX TX lights are not blinking at all.

That's not so good. See Have I bricked my Arduino Uno? Problems with uploading to board

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Can you test the bluetooth module separately? i.e. just connect the power pins directly to the correct voltage source, and check if it overheats. In this way you may be able to find the source of the short circuit.

If you shorted one or more of your arduino's I/O pins, you may have damaged your Atmega328 chip, since there's no protection between the chip and IO headers. The other circuitry on the Arduino board is mostly power conditioning/regulation. So I'm assuming that's not the problem. Check if your 328 is getting the correct voltage at the VCC pin, to eliminate the power circuitry as the problem.

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If you are getting three blinks, most likely bootloader is fine but don't take it for granted. Also, post a schematic and picture of your original setup to get better answers in future.

Here is how you should start the debugging process:

1) Disconnect everything from Arduino especially from Tx Rx pins, and try to dump the blinky sketch. If it works, then arduino is good. If it doesn't work, follow this article to burn a fresh bootloader in your atmega. Check again. If not able to burn bootloader or code afterwards, get a new atmega IC for cheap and replace the damaged one.

2) Check the voltage requirements for HC-05 whether it's 3.3V or 5V. Make sure you are not giving it a higher voltage. If it starts to smoke as soon as you connect the GND VCC, then probably it has been damaged. Measure the resistance between GND and VCC of HC-05 before attaching. If it's less than 10-20 ohms, you already know that it's damaged.

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