Here is a picture of two USB heating pads:
As far as I can understand, 5 V comes from the USB and goes into the curly wires, which produce heat since they are like a long resistor that dissipates power.
Will it produce more heat the shorter the curly wires (up to a certain limit where it would melt)?
These pads come with a button that allows you to switch between low/medium/high heat. How does this work? Does that big on/off button hide 3 big resistors that each limit the current to low/medium/high?
I would like to make something similar. Would it be better, in order to control the heat more efficiently, to have some kind of pulse width modulation?
I think I can do it with a tiny Arduino, but is it possible to achieve this with more basic components? Like a low and a high MOSFET and something to controle the on/off rythm aka duty cycle, but what?