I want to build kind of keyless entry for my car.
My car will have a radar sensor and whenever some body approach it, it will trigger a transmitter (unknown)
Then, this signal will wake up a microcontroller(Arduino) on my keychain and it will trigger a classic 433mhz module to send a specific message to the car that will open the car.
The problem here is nothing BUT the transmitter and the receiver while the transmitter can have at much as 10-20mA, because it will only works whenever the radar detect someone close to the car, the receiver(unknown) on the other side must be extremely low current because it will ALWAYS listen and will be powered by a much much smaller battery (compared to the car)
I'm a beginner, i don't know much about electronics. This means i don't know if somethings like this exists..