[...] due to sonar signals cone shape.
Iwas wondering if this is possible with IR obstecle detectors (TSOP based simple circuit) ,how to cover front side of small robot with one IR detector.
It sort of depends on both the width
of you 'bot and how far in front of it
you want it to be how wide
You wouldn't need to use the full range of the TSOPxxxx (from 10m to 30m = 30' to 90' with direct light), so you could take advantage of the wider "beam" at a lower range (120° with some of them).
You can test this with your TV or HiFi - when you're far away, you have to be at a direct line of sight in front of it, but if you're close to it, you can usually zap from ackward angles.
Many remotes will work if you stand say 2m in front of it and zap towards your own body, simply by you reflecting enough IR to operate the receiver.
Further, you will get some spread from the diffusion of the IR on the object.