Simplest tool for tablet inspecting / repair

This is the simplest tool I use to see if a tablet is fully bricked or still has some activity. It has just a LED and a resistor, connected to the expander box, so is VERY simple to build. When you power-up your tablet, the green led must turn on and flicker. That means the serial port is outputting data. If the led turns off and your tablet doesn’t start with Android, the device is probably hanged in U-Boot, may be after wrong rom image used for upgrade.
If the led doesn’t turn on, the tablet is fully bricked and you have to reflash the SPI to revive it. If the led stays on until the Android starts and then makes a quick lamp every 60 seconds, your tablet is running fine and so, why are you reading this post ?  😉
The led turns on ‘cause is connected to the TXD data of the tablet. When your tablet sends data thru the serial port, the level that’s normally 3.3V goes to GND so the LED is directly polarized and turns ON. When the tablet doesn’t sends data, the TXD is 3.3V so there isn’t any voltage difference at the led terminals and it remains OFF.
PAY ATTENTION: don’t use a resistor with value less than specified, or you can destroy you serial output ! Look at the picture below for details; remarks are in italian, but I think are readable anyway (this tool is also described here in italian language).

Front and back of the led + resistor connected to the expander box
Front and back of the led + resistor connected to the expander box; click on the image for full size view.