Hi guys, I wanted to design a humidifier, and this humidifier will turn on when it senses a high mental stress level in humans ( I was using a PPG heartbeat sensor to sense human stress level). However, the problem is, the humidifier does not have a signal pin, which means it only consists of VCC and GND that can be connected to the Arduino Uno microcontroller. So my question is, how could I control my humidifier to turn on if I don’t have a signal pin?
You need to switch the power for the humidifier on and off. A relay, FET, solid state switch are some options. Each of those has their own needs (the relay probably needs a driver). Start by figuring out how much power the humidifier needs.
The first atomizer has no control signal, and has a push button that needs to be pushed (which you would need to modify to be shorted by something like a FET) in order to make it mist. The Seeed one has an enable pin which appears to do what you want out of the box (the necessary FET is on the board and will start misting when the enable signal is present from the schematic.)
Peter
Hi microMerlin, thank you for the information 
Hi Peter , thank you for the information