Which sensor do you recommend for this project?

Hello guys, this is my first time posting.

I have a design that I want to do. The design is all about preventing the situation of forgetting a baby by himself inside of a locked car. For this, I am planning to use some sensors withing the car seat such as (temperature, weight, etc) and then use an arduino microcontroller to control them. One of the features that I wanted to add is to detect the proximity between two points. The first one is the car seat and the other one is the user or designated driver. My question is, what is the simpler approach to have this ? I was thinking to use a bluetooth connection between the microcontroller and the cellphone to detect the proximity but I read that is not really possible to detect range or proximity with a bluetooth signal. another way that I was considering is with a RF transmitter and receiver but I dont really know how this works. By any chance any of you have an idea on how to properly implement this ?
Thanks a lot!

There used to be a product called Smartag.

You put a unit in your brief case and carried a second unit in your pocket.

If there was more than 50 feet between the two units, the one in your pocket would beep.

Why not have birth control for people who are incapable of remembering their children?

It's that damn law that says you shouldn't put your offspring in the front seat where you'd be less likely to forget you made one of those things.
But to be fair, I'm sure sleep deprivation plays a role for some parents with poorly trained meatbags.

As for a sensor system, sounds like a dead man switch of some sort could work. Put transmitter on the meatbag, transmit a signal every minute or so. Receiver on the parent that resets a timer every 45 seconds or so when it receives the signal. If the irresponsible parent gets too far away and consequently doesn't receive the signal, an alarm goes off.

I’m sure sleep deprivation plays a role for some parents with poorly trained meatbags.

Unfortunately, parents use this as an excuse to justify the death of their children when it happens.

Recently, a father left his child in a unlocked car while he went into a bar for a drink :o
I guess his excuse was he was too sleep deprived from being hung over.
Probably would ignore a beeping child alarm.

Guess the beeping has to be a vocal alert saying "You're a bad parent!" to be effective

“Guess the beeping has to be a vocal alert saying "You're a bad parent!" to be effective”

Or maybe: 'This parent has left their child in the car.'

Or the Mr T version:
"I pity the fool who left the baby in the car!"

analysis
People that forget their children, also forget to put the receiver in their trousers, or to replace batteries.

It is solving a problem with a system that has the same “problem” (the adult). Therefor the solution should be looked for in the “supersystem”, that is the system where both the baby and the person are part of. The first that comes in mind is the CAR.

exploring solution space
So I propose to make a solution that is hard wired in the car. Then you do not need to think about putting a receiver in your pants. And a car battery is most of the times suffiecient for a loud alarm.

If your safety belts are not on, and a person is seated on a chair (weight sensor), some cars give an alarm.

If my lights are on and I stop my car/open the door, it goes beep beep beep … <AHA!>

proposal
So the car should detect the baby - weight sensors do exist in many cars - and sound an alarm after the motor stops/door opens → Crying baby noise (no beep beep). The loudness of the alarm should increase with time until baby is taken out - poor baby ears. If you take the baby out the system could give positive feedback → laughing baby sounds.

So a car based system could take two existing signals from the car (weight & door open) and plays a warning.

afterthoughts
If the weight-sensor does not exist, the car could detect the baby seat by a seatbelt sensor?
If that doesn’t exist either, one can add another detction system, e.g. IR beam, …

A baby safety system should also detect poo poo smell. As that is uncomfortable for the baby, it might become a distractor for the driver when baby starts to cry. But that is another project…

Quote:
“The design is all about preventing the situation of forgetting a baby by himself inside of a locked car.”

From this statement ahead I refrained to read.

Arduino and related sensors are not designed for life-threatening situations.

There is a huge variety of incidents that can prevent this project to work - from running out the battery to just to incidental cuts in wiring.

If this project is for school or for learning, fine. Otherwise, better think again. There is a life of a baby we talk about.