I know there is Google, but why not one place to get everything you need
Sometimes you need a solution for a headache question:
keyboard decoder with ADC:
keyboard created with resister divider connection to one ADC converter,
VCC-Rf-Re-Rd-Rc-Rb-Ra-ADC-Rs-GND, buttons on Ra..f to GND.
It is possible for example 10 buttons on one microcontroller pin, otherwise
you should use a matrix which use more mcu pins.
i2c optical isolation
Think, realise and see the troubles.
i2c slave addon for debugging:
i2c slave device board connected to application to sent debug messages,
often there is no serial port available in small microcontrollers but an i2c port exists.
I use this trick for debugging small cypress usb controllers.
In future these examples must be separated, and if possible:
Smart schematic snippets
--- n keys on one ADC microcontroller pin
--- I2C optical isolation solutions
--- debugging with I2C
I think it is very usable for developers/designers.