Sensors for the turtle aquarium

Hello everyone!
I am currently working on a project that will control the lights in my turtle’s aquarium. The principle is that a sensor checks if the turtle is sitting on the heating spot then uses a switch to turn on the lamps. I am looking for suitable sensors, I was thinking of HC-SR501 or HC-SR04 but I do not know if I am thinking right so I am writing this post.

How big is the turtle?

It is 7.5cm wide, 13cm long and 7cm high

An IR sensor or ultrasonic should work

Micro switch under the turtle.

Why would the turtle go to the heating spot , if it’s not hot ?

There's a sign saying "Wait here if you're feeling chilly." If the lights come on reliably, the turtle will be confident that the sign is trustworthy.