I have a arduino uno r3, by mistake i connected the 5v pin to 12v supply, now pin 13 led is always on, and can't upload any sketch, i have tried following methods to test the board,
1.replaced the atmega 328p with a new atmega 328p consisting of blink sketch, in this case pin 13 light is
blinking as it should, but can't upload any sketch,
2.inserted the atmega328p of damaged arduino in another working arduino board, in this case also i can't
upload any sketch.
Please tell me whether it can be repaired or i have to buy a new board.
Likely destroyed all the chips on the board. Time to buy a new one.
The 12V likely took all the devices connected to 5V supply pin - the '328P, the 16U2 USB/Serial chip, maybe the 3.3V regulator, maybe the 5V regulator, probably not the LM358 Opamp. Nothing you can fix yourself without a hot air rework station except the '328P. The dead 16U2 will keep you from uploading sketches via serial port. If Blink works, then perhaps the regulators survived - measure the 5V and 3.3V pins, see what levels they are at.
You can likely use an external FTDI Basic, pick one up from www.tinyosshop.com, connect to 5V, Rx, Tx, Gnd, and connect DTR thru a 0.1uF ceramic cap to Reset for software autoreset; or press reset on the Arduino when the IDE shows "compiled xxx of 32xxx bytes". Can be easier to get the timing right for a reset press if you open File:Preferences and turn on Verbose outputs.
Or pick up a new board, be more careful with the next one. Check your wiring before turning supplies on. Don't move wires around when power is live.
Thanks to all , i think i should buy a new board. Next time i will be more carefull, thanks for suggestion.