Block the return wave of ultrasonic sensor

[MarkT]

@javierch23

The idea is that the ultrasonic sensor will works normally with other object like a bottle, folder... but when a person or any part of her body stay in front of the sensor it will not receive a reflected echo signal, so it will detect that there is a person there. A vision system would be really expensive

So you want to distinguish between a limb and any other object? IR detector will do this through body-warmth.