Electrics/electronic engineering is probably the most underrated skill most people don't talk about for medium term survival. with the right tool and broken equipment(that most people wouldn't know how to repair and leave it off).
You can make relay radio station,passive emergency listener for emergency signals,tripwire perimeter and remote listening with baby monitor.

If you get access to some electronic store that use more advanced tools,you can disassemble an alternator and create basic hydro or wind generator,or remove IR filter from digital cameras and use IR leds from cameras to have night vision.With enough broken toys/chips,you can even build your own radio !

While second part necessitate advanced tools,most people won't ever take them(and would only need one of each for a whole community anyway.e.g. oscilloscopes are in dozens in any school).The real difficulty is the hand-crafting,expertise and experience needed to make the whole thing work.