The cheap ones? Lights. I guess the air flow sensor needs to talk to be read from something. The disposable ones don't have buttons.

I guess technically you probably (I don't know how the sensors work) could do without a mcu but the big issue is the safety function. Without buttons you usually have something like 3 quick drags to lock / unlock them.

The traditional vapes that are more enthusiast grade (like, the fat ones not something like Juul) actually have screens and different modes to set the power.

Like, a lot of the more expensive vapes are actually just battery pack + electronics and then you can screw in a vaporizer with a coil head that you can make yourself. So you end up in a situation where the manufacturer of the electronics has no idea what you put on top.

The early ones only had variable voltage which kinda sucked. Then variable wattage became a thing but now you need to measure the resistance of the wire and automatically adjust the voltage accordingly so you keep the wattage constant.

Then we got temperature control over nickel wire (I think it was nickel) and the last big vape I bought actually had stainless steel profiles. So you'd use stainless steel wires and based on the alloy the electronics would try to hit a certain temperature on the wire. This also finally meant that you are very unlikely to burn the cotton in the coil head because you could set the temperature below the point where cotton burns. double or even triple digit watts through a dry cotton strip doesn't taste good.

There are actually battery packs without any electronics but those are very old school or very enthusiast grade. You kinda have to build the coils yourself. The first vapes people built at home were basically modded flash lights (and I think that's where the term mod for the battery pack + electronics comes from).

But for the disposable ones it is basically only the air flow sensor and especially the safety lock.