Simple and cheap 2 way collision detection with a single potentiometer.
How i did:
Warped a zip tie around the potentiometer axis and fixed the other end from the zip tie around the corner outside from the robot.
While startup reading the analog input to find the neutral position from the potentiometer.
A collision from front increases, a collision from side decreases the analog read value. Simple as it is!
On the pic you see a old autonomous robot i build around 2003 with a Conrad D-Control uC.