Make: Electronics by Charles Platt (read me a book .TXT) 📕

the negative power through pin 7. So now C4 can start charging through R4. While this is happening, the positive output from the timer continues.

As the voltage increases on the capacitor, comparator B monitors it through pin 6, known as the threshold. When the capacitor accumulates 2/3 of the supply voltage, comparator B sends a pulse to the flip-flop, flipping it back into its original state. This discharges the capacitor through pin 7, appropriately known as the discharge pin. Also, the flip-flop ends the positive output through pin 3 and replaces it with a negative voltage. This way, the 555 returns to its original state.

I’ll sum up this sequence of events very simply:

1. Initially, the flip-flop grounds the capacitor and grounds the output (pin 3).

2. A drop in voltage on pin 2 to 1/3 the supply voltage or less makes the output (pin 3) positive and allows capacitor C4 to start charging through R4.

3. When the capacitor reaches 2/3 of supply voltage, the chip discharges the capacitor, and the output at pin 3 goes low again.

In this mode, the 555 timer is “monostable,” meaning that it just gives one pulse, and you have to trigger it again to get another.

You adjust the length of each pulse by changing the values of R4 and C4. How do you know which values to choose? Check the table on page 157, which gives an approximate idea and also includes a formula so that you can calculate values of your own.

I didn’t bother to include pulses shorter than 0.01 second in the table, because a single pulse of this length is usually not very useful. Also I rounded the numbers in the table to 2 significant figures, because capacitor values are seldom more accurate than that.

Background

How the timer was born

Back in 1970, when barely a half-dozen corporate seedlings had taken root in the fertile ground of Silicon Valley, a company named Signetics bought an idea from an engineer named Hans Camenzind. It wasn’t a huge breakthrough concept—just 23 transistors and a bunch of resistors that would function as a programmable timer. The timer would be versatile, stable, and simple, but these virtues paled in comparison to its primary selling point. Using the emerging technology of integrated circuits, Signetics could reproduce the whole thing on a silicon chip.

Figure 4-19. Hans Camenzind, inventor and developer of the 555 timer chip for Signetics.

This entailed some trial and error. Camenzind worked alone, building the whole thing initially on a large scale, using off-the-shelf transistors, resistors, and diodes on a breadboard. It worked, so then he started substituting slightly different values for the various components to see whether the circuit would tolerate variations during production and other factors such as changes in temperature when the chip was in use. He made at least 10 different versions of the circuit. It took months.

Next came the crafts work. Camenzind sat at a drafting table and used a specially mounted X-Acto knife to scribe his circuit into a large sheet of plastic. Signetics then reduced this image photographically by a ratio of about 300:1. They etched it into tiny wafers, and embedded each of them in a half-inch rectangle of black plastic with the product number printed on top. Thus, the 555 timer was born.

It turned out to be the most successful chip in history, both in the number of units sold (tens of billions and counting) and the longevity of its design (unchanged in almost 40 years). The 555 has been used in everything from toys to spacecraft. It can make lights flash, activate alarm systems, put spaces between beeps, and create the beeps themselves.

Today, chips are designed by large teams and tested by simulating their behavior using computer software. Thus, chips inside a computer enable the design of more chips. The heyday of solo designers such as Hans Camenzind is long gone, but his genius lives inside every 555 timer that emerges from a fabrication facility. (If you’d like to know more about chip history, see http://www.semiconductormuseum.com/Museum_Index.htm.)

Fundamentals

Why the 555 is useful

In its monostable mode (which is what you just saw), the 555 will emit a single pulse of fixed (but programmable) length. Can you imagine some applications? Think in terms of the pulse from the 555 controlling some other component. A motion sensor on an outdoor light, perhaps. When an infra-red detector “sees” something moving, the light comes on for a specific period—which can be controlled by a 555.

Another application could be a toaster. When someone lowers a slice of bread, a switch will close that triggers the toasting cycle. To change the length of the cycle, you could use a potentiometer instead of R4 and attach it to the external lever that determines how dark you want your toast. At the end of the toasting cycle, the output from the 555 would pass through a power transistor, to activate a solenoid (which is like a relay, except that it has no switch contacts) to release the toast.

Intermittent windshield wipers could be controlled by a 555 timer—and on earlier models of cars, they actually were. And what about the burglar alarm that was described at the end of Chapter 3? One of the features that I listed, which has not been implemented yet, is that it should shut itself off after a fixed interval. We can use the change of output from a 555 timer to do that.

The experiment that you just performed seemed trivial, but really it implies all kinds of possibilities.

555 timer limits

1. The timer can run from a stable voltage source ranging from 5 to 15 volts.

2. Most manufacturers recommend a range from 1K to 1M for the resistor attached to pin 7.

3. The capacitor value can go as high as you like, if you want to time really long intervals, but the accuracy of the timer will diminish.

4. The output can deliver as much as 100mA at 9 volts. This is sufficient for a small relay or miniature loudspeaker, as you’ll see in the