Ah, yes, Janis Joplin rockin’ the Fillmore, the Grateful Dead packing em into the Cow Palace, and Grace Slick and the Jefferson Airplane giving out with …When logic and proportion…have fallen sloppy dead …. We cant bring back the wonder that was the 1960s (and if you can remember the 60s, you certainly didn’t experience them), but we can draw a line from the lyrics of the Airplanes White Rabbit to avionics. Proportion may be dead, but logic is still with us, and we can make the case that logic can be your friend when doing simple tricks with your airplane systems.
Suppose, just suppose, that you want to sound a horn when (a) the airspeed drops below 60 knots or (b) the throttle is brought back below 20% power and the gear is still up and locked. Most aircraft engineers will go through a long song and dance with microswitches, cams, limit switches and other mechanical means to make this happen, but you and I know that electronics are more reliable, cheaper and weigh a lot less.
Normally, I would give you a design that is complete in and of itself. For example, when I did the nav light series, I didn’t just give you the specs on the LEDs and tell you the rest is up to you. No, I did the LED, plus the power supply, plus the optics, plus the flasher circuit. Complete.
However, in the next few months, I will be giving you a toolbox of circuits that you can interconnect in limitless combination to make your airplane systems do what you want them to do… logically.
A Logical Explanation
A brief history of digital logic is in order. The first electronic logic circuits were built out of vacuum tubes. As a matter of fact, the first modern digital computer (ENIAC) wouldn’t fit in the average high school classroom, weighed 30 tons, used 18,000 vacuum tubes and consumed 150 kilowatts of power1. As things progressed and the transistor became a reality, and then later the integrated circuit, things shrank until todays home computer is a billion times more powerful than an ENIAC, and it uses a thousand times less power to do the computing. There were several iterations of digital logic, but the most successful family wound up being a power-sipping extremely powerful series called CMOS (complimentary metal oxide semiconductor). There are several versions of CMOS, but my personal favorite is the 4000 series, which is the logic family we are going to use in this series of articles.
The 4000 logic isn’t the fastest, nor the cheapest, nor does it have the most variety of functions. Those belong to, respectively, ECL, RTL and TTL. However, it does represent the sweet spot of all three parameters.
So let’s talk about digital logic for a bit. For the last 23 years (April 1985 was my first KITPLANES article), Ive been expounding on linear circuits: headphone amplifiers, audio switching panels, lighting, engine instruments and the like. This is my first foray into the digital world with you all.
Analog circuits have an infinite number of input and output levels. Headphone amplifiers can have microvolt inputs to millivolt inputs and put out millivolts to volts. But digital circuits have only two inputs and two outputs. They can have either high or low states. That is, they can be near the power supply voltage or they can be near ground. There is no in-between. That is why they are called digital. High or low, zero or one (0 or 1), up or down, black or white, north or south, but nothing in-between. There is no east or west in a north-south digital system. And there is no north or south in an east-west digital system. They are in either one state or the other, but nothing else is allowed.
Dissecting a Circuit
Just for explanations sake, and before we get to understanding the circuits we are going to be using, let me take the simplest 4000 series digital logic circuit and give you some idea of what we will be talking about.
Take the 4050 buffer. A logical one (high) on the input produces a logical one (high) at the output. That is singularly uninteresting unless you are beginning your study of digital logic. You put in something near the power supply, and you get out something near the power supply. Whats the big deal?
Heres the big deal: You’ve transcended analog to digital. You’ve taken an analog level and translated it to digital values. If the input is greater than 70% of the power supply voltage, then the output is guaranteed to be within 90% of the power supply voltage. If the input is less than 30% of the power supply voltage, the output is guaranteed to be less than 10% of the power supply voltage. Digital switching!
Whew. Some numbers, please? If the power supply voltage is 5 volts2, then an input to our buffer of greater than 4.5 volts (70% of the supply volts) is going to appear at the output of this buffer as a logical 1. If the input to our buffer is less than 1.5 volts, then the output of the buffer is going to be a logical 0.
CMOS says this: A logical 1 at the input has to be more than 70% of the power supply; a logical 0 has to be less than 30% of the power supply. A CMOS logical 1 output will be more than 90% of the power supply, and a CMOS output logical 0 will be less than 10% of the power supply.
Whoa. What happens to those input analog signals between 30% and 70% of digital limits? What happens to the vast universe of analog signals that fall between these limits? For right now, we’ll call them digital no-no levels and not allow them. The very first part of next months installment will take care of that little problem.
Stay tuned. We have lots of digital stuff to talk about. But for right now, I’ll pass on a little digital joke: There are only 10 kinds of people in the world, those who understand binary arithmetic and those who don’t.
1. There is an argument that the first digital computers are the abacus and the slide rule, followed by the Antikythera mechanism from 100 BC, then the Babbage mechanical computer (1837), the Hollerith census machine built in 1899 by the Computing Tabulating Recording Company (later to change its name to International Business Machines, or IBM), then the German Zuse Z3 in 1941, and then along came ENIAC, which is considered to be the first electronic general-purpose machine.
The first bug found in a digital computer was a moth trapped between the contacts of a relay in ENIAC. Ever since then, computer programming errors are said to be bugs.
2. Five volts is the standard voltage supply for a lot of digital integrated circuits, but CMOS is happy to run anywhere from 3- to 18-volt supplies. I use 5 volts simply for compatibility with other logic families.