Collision avoidance for DIY drone is basically none existent unless you want to pay top dollar or have some coding experience. but one company is setting out to change all that Reflex copter is said to make a collision avoidance system that connects between the flight controller and receiver with only one wire to connect and control settings are done via an android app.
The system uses a basic sonar sensor like the DJI Phantom 4v1 that you mount on top of the drone to connect the control board and the app via Bluetooth and your all set if this works out ok then way to go Reflex Copter
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reflex alpha collision avoidance system
get more info on the Reflex Alpha collision avoidance system