AC Electrics -Logic Gates — Page 268, Lesson 280

I want to walk you through the logic gates section of your AC Electrics material. We're starting with an important concept that underpins how digital systems think — and yes, aircraft use these logic gates in everything from flight control computers to warning systems. First, let's talk about negative logic. The normal way we think about binary is that a higher voltage — say +5 volts — is a binary '1', and a lower voltage — near 0 volts — is a binary '0'. That's called positive logic. But negative logic flips that: binary '1' is defined as the lower voltage value, and binary '0' is the higher, more positive voltage value. This is less popular, but some aircraft systems use negative logic to meet certain design parameters. So if you see a system where a low voltage means 'true' or 'on', you're looking at negative logic. Now, let's look at the first gate: the 'AND' gate. The AND gate represents a situation where all inputs must be '1' — or 'on' — to produce a '1' output. Think about the name: input No. 1 and input No. 2 and input No. 3, and so on, must all be '1' for the output to be '1'. If any input is a '0' — off — the output will be '0'. Take a look at Figure 17.1 on screen. You'll see the symbol for a two-input AND gate — it looks like a flat front with a curved back — and next to it, a truth table. A truth table lists every possible combination of inputs and shows you the output. For a two-input AND gate, the only time the output is '1' is when both input A and input B are '1'. A simple way to visualise this is with two switches in series controlling a light bulb. Both switches have to be closed — both 'on' — for the light to turn on. If either switch is open — 'off' — the light stays off. That's why the AND gate is sometimes called an 'ALL or NOTHING' gate. All inputs must be '1', or you get nothing. Let's move to the 'OR' gate. The OR gate represents a situation where any input being '1' will produce a '1' output. Input No. 1 or input No. 2 or input No. 3 — any one of them being '1' gives you a '1' output. The only way you get a '0' output is if all inputs are '0'. If even one input is '1', regardless of the others, the output is '1'. The symbol for a two-input OR gate looks different — it has a curved front and a pointed back. The truth table shows that the output is '1' for three out of four possible input combinations; only when both inputs are '0' does the output go to '0'. A simple circuit for this is two switches in parallel controlling one light. If either switch is closed — 'on' — the light turns on. The OR gate may be called an 'ANY or ALL' gate — any input being '1' gives you output, and if all are '1', you still get output. Now, the 'INVERT' or 'NOT' gate. This one is different — it has only one input and one output, and its job is to reverse the condition of the input signal. If the input is '1', the output becomes '0'. If the input is '0', the output becomes '1'. It flips the signal. Look at Figure 17.3 on screen. The symbol is a triangle pointing to the right with a small circle — called an inversion bubble — at the output. The truth table is simple: input A, output B — they're always opposite. A practical circuit might use a switch controlling a normally closed relay that turns a light on or off. If the switch is turned '1' — on — the relay opens, and the light goes '0' — off. So the output is the inverse of the input. The INVERT gate is most often used in combination with other gates — you'll see it attached to the inputs or outputs of AND and OR gates to create what are called NAND and NOR gates, which we'll cover later. But for now, understand that the NOT gate simply gives you the opposite of whatever you put in. So to summarise: AND requires all inputs to be '1' for a '1' output — it's an 'all or nothing' gate. OR requires any input to be '1' for a '1' output — it's an 'any or all' gate. And NOT just inverts whatever signal it receives. These are the building blocks of all digital logic in aircraft systems.