which sensor I should use in my project? :(:( urgent!!

Hello, I am a beginner of Arduino.
I would like to do a project that talking about the distance among 2 people (person1,person2) in a dark room.

If person 1 far from person 2, there will be a A sound.
If person 1 reach the ideal distance from person 2, there will be a B sound.
If person 1 is too close from person 2, there will be a C sound.

I would like to ask what sensor should I use? as the IR sensor/ ultrasonic sensor will detect the wall, but I just want to detect another person only. Can PIR sensor can reflect the distance in serial read?

I just want to detect another person only

Very difficult. You will need a sensor to discriminate a person from other objects and something else to measure distances.

A thermal imager can detect warm objects.

If your class project is due soon, pick a different one.

jremington:
Very difficult. You will need a sensor to discriminate a person from other objects and something else to measure distances.

A thermal imager can detect warm objects.

If your class project is due soon, pick a different one.

can RF transmitter and receiver do sth similar?

sodacher:
can RF transmitter and receiver do sth similar?

No, RF cannot detect warm objects

AWOL:
No, RF cannot detect warm objects

if there are two Arduino Boards and one connect to transmitter and the other connect to receiver and different people carry each of the board, can they measure the distance between transmitter and receiver?

measure the distance between transmitter and receiver?

That question has been asked on this forum every few days for years now, and so far, the answer seems to be no (at least not for an Arduino beginner).

You may try laser ranging sensor like VL53L1X

The most common way it seems to measure distance between two boards is to use an ultrasound signal and a radio signal. One board transmits the radio signal and the audio signal at the same time; the other board measures the difference between time of arrival. Radio is as good as instant; audio moves at a mere 330 m/s giving an easy to measure time difference.

Major complication: ultrasound transducers tend to be pretty directional, so if pointed in the wrong direction the signal is not received, or an echo is received and the distance recorded is not the actual distance. Aiming it at the other person while in a dark room may be a challenge.

The VL53L1X (and similar sensors) are also highly directional, and require you to point at the object you want to know the distance of, regardless of what this is (could be the wall, for example). Again the overall darkness may be an issue.

Probably your best bet for this project is to use a set of Pozyx sensors, then based on the locations of the two sensors calculate the distance between them.