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?
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.
Please draw a circuit to show us how it was wired. Hand drawn and photographed will do.
If the loopback test fails (and you were able to upload before just fine) and the pin13 led blinks when you reset I would say the ATmega328 is fine but the USB serial converter is dead. If it's a original Arduino it's a ATmega16u2, if it's a clone some other chip like a CH340. If it has a ATmega16u2 you can try to reburn it (Google it ;) ), if it has a CH340 then uhm, yeah, just buy a new one. Or you can use a external USB serial converter like it's a Pro Mini. (But buying a new clone is almost as cheap.)
Btw, I have no idea what you try to do with the comparator. Driving a relay with that isn't going to work, just use a transistor (and don't forget a flyback diode).