+ 5V - SW1 SW2 A 5V here 0V (ground) 5V, Ground, and a Wire Called "A" close SW1 => A is 5V close SW2 => A is 0V The voltage on A is invisible. Clip a voltmeter across these two terminals and you can read it.
+ 5V - SW1 SW2 A 5V here 0V (ground) fan I Close SW1 and Current Flows SW1 is closed, so A is at 5V and the fan spins Voltage is the push. Current is the flow of charge that push causes. The fan spins for as long as current flows around the loop, and every second it spins the battery gives up a little energy.
+ 12V - SW1 SW2 A 12V here 0V (ground) fan I A Bigger Battery Pushes Harder Same fan, but 12V of push more current => faster fan More voltage across the same fan means more current through it. The fan spins faster, but the battery also drains faster. Nothing about the circuit changed except how hard we push.
+ 5V - SW1 SW2 A 5V here 0V (ground) Back to Digital close SW1 => A = 1 close SW2 => A = 0 A = 1 no current flows here Now A is not a voltage, it is a number: 1 or 0, on or off. 0V always means 0. Anything close to 5V means 1. The exact value does not matter. Nothing is connected to A, so no current flows and the battery never drains. But never close both switches at once, or you will see smoke!
+ 5V - M1 M2 +5V closes it 0V opens it +5V closes it 0V opens it A 5V here 0V (ground) Transistors Instead of Switches M1 closed => A = 1 M2 closed => A = 0 A transistor is just a switch with no handle. You close it with a voltage. The gate is a plate sitting on an insulator, so no current flows into it. That is why a chip can hold millions of these and still drain almost nothing. M1 and M2 must never be closed at the same time.
+ 5V - P1 M2 0V closes it +5V opens it +5V closes it 0V opens it IN A 5V here 0V (ground) CMOS Switches IN = 0 => A = 1 IN = 1 => A = 0 P1 is a PMOS transistor. The bubble means it works backwards: 0V on its gate closes it, and 5V opens it. M2 is still an NMOS. So one control wire can now drive both gates at the same time. Exactly one is always closed. You cannot turn both on, or both off.
+ 5V - P1 M2 C A 5V here 0V (ground) These Two Transistors Are a Gate C = 0 => A = 1 C = 1 => A = 0 We can stop thinking about P1 and M2 separately. Together they do one job: whatever C is, A comes out the opposite. A digital building block like this is called a gate. This gate is called an inverter, because it inverts its input.
C A inverter C A 0 1 1 0 Truth Table The Inverter This is the symbol for an inverter. The bubble is what flips the value. The 5V and ground wires are still there, we just stop drawing them. Two transistors, one symbol, one job, and a table that says it all. A is NOT C.
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