MOSFET switch

// A MOSFET switch.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit: the 5 V rail pushes current through the 220 ohm resistor and the
// LED, down through the MOSFET's channel, and back to ground.
//
// Click the slide switch while it runs. In one position the gate sits at 5 V,
// the channel turns on and the LED lights; in the other the gate is on ground,
// the channel is off and nothing flows. Watch the readout under the part: it
// is the drain current, and it is the same current the LED is lit by.
//
// Two things worth noticing. There is no gate resistor, because the gate takes
// no current. This is the whole reason a MOSFET is the switch of choice for
// anything a board pin cannot drive. And the switch is wired across both
// rails, not just to one: a gate with nothing on it floats, and a floating
// gate turns a switch into a radio aerial.
//
// Things to try. Turn the gate supply down by swapping the 5 V symbol for a
// lower voltage and find the threshold, about 1.8 V for a 2N7000; below it
// the LED goes out completely. Or put the MOSFET in the properties panel and
// change the model to an IRF540N, whose channel is a hundred times lower and
// whose threshold is a good deal higher.