Observer do not update itself if you this iteractions:

1. Build this thing like in photo 1 (on picture this is moving, so put slime block closely to the piston) (piston is sticky).
2. Put a lamp on the piston.
3. If observer & slime aren't closely to a piston, put a lamp of signal's exit, else do all from the beginning.
4. Lamp is powered, but observer's enviroment isn't changing, you can break all other blocks, like on anoter pictures.

If also break the lamp and put it again, it will be powered again.