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Good troubleshooting skills are vital to the electronics hobbyist. More often than not, a circuit is not going to work perfectly on the first try, especially as a beginner. It can be very intimidating when a component on a board gets hot, exhibits unexpected behavior, does nothing at all, or even explodes. Where does one start troubleshooting? The answer is different for every situation and even with practice one will come across errors that can't be immediately explained. Jedi-like troubleshooting skills come only with experience, but there are some common problems that can be diagnosed quickly, and understanding these common problems and how to check for them is a starting place common to beginners and masters.
Let's say you plug in your circuit, and something is wrong. It could be not working at all, or behaving in a way that you don't expect. There are issues you should ALWAYS check for before you even get out the multimeter. These may seem obvious, but it's better to start simply than troubleshoot more complex issues for hours just to find a silly mistake.
So -- you've preformed all of the tests above and something is still not working. This indicates a potentially more complex problem, but still there are some general tests one should perform before hooking up the logic analyzer.
The above tests can all be performed in about two minutes, and even though they're simple (and it hurts a designer's ego to find out she/he made such a simple mistake) they rule out the biggest common issues with improperly functioning circuits. If a circuit passes all these tests, then one can start digging into manuals, debugging code, and checking voltages with a multimeter with the knowledge that the error is not due to a simple common problem.
You have a circuit board with the circuit shown below. You power the device, and the LED lights up, but when you attempt to program the ATTiny, the programming fails. The ATTiny chip is soldered in the correct orientation. Which of the following should you do next?
A) Check the VCC and GND traces on the board for a short
B) Check that the 330 Ohm resistor is the correct value
C) Inspect the soldering of the chip for cold joints and jumpers
The LED is functioning properly, so you know that there is no short from VCC to GND on the board. The LED is lighting up, so the resistor is probably the correct value. The ATTiny is in the correct orientation, so if it's not programming, it's likely not soldered properly to the board. A single cold solder joint will cause bad connectivity and not allow programming. Check the board for "gray" (as in not shiny) solder joints, as this is usually indicative of an incorrectly soldered pin.