3. ASSEMBLY, ADJUSTMENT, AND RUNNING
4.12 Integrated Circuits: The 555 Timer Chip
water demonstration in Chap.2. In this case, unlike either the coffee water experi- ment or the capacitor experiment, there are no factors we can control, like R and C, to alter the time scale of the graph, but we can describe it in a readily understandable way by the quantityradioactive half-life.
This half-life is the time in which the activity declines by a half, symbolT1/2, and it repeats throughout the decay process, as shown here (Fig.4.27):
Two kinds (isotopes) of carbon occur in the earth’s atmosphere, in a more or less fixed proportion. The predominant isotope,126C, often called C-12, is not radioac- tive. The radioactive isotope 146
C, commonly called C-14, has a half-life of 5730 years. All growing plants absorb this mixture during their life. At death the absorption of carbon (as carbon dioxide) ceases, and from that point on the 146
C decays, with exponential decrease, while the 126C content remains fixed. In an archaeological specimen such as an ancient timber stake, the ratio of146C to126C will have halved in 5730 years, so by measuring today’s proportion we can calculate the age since the death of the specimen.
There have been significant fluctuations in this latter ratio in historical time, and the current ratio of objects of known age is used to correct for this.
The method can also be used for animal remains because the food chain for all animals will include herbivores at some point.
The old trick question about the frog wanting to get to a pond and being allowed only to hop such that each succeeding hop is again halfway there—and how many hops does it take to get there?—is another, and not trivial, example of exponential decrease. Of course, it never quite gets there!
other processes so that separate areas could be made to be insulators, conductors, semiconductors, or dielectrics. So you could build up a complex of transistors, diodes, connectors, capacitors, and resistors all within the one piece. The first working IC was produced by Fairchild Semiconductor in 1960.
ICs have many advantages over the equivalent discrete circuits, i.e., those assembled from separate components. They are more compact, less expensive, and more reliable and use less energy. Also, because the links between the internal components are short, digital circuits are faster working.
Furthermore, by the time an IC is put on the market, it has been thoroughly debugged, and circuits built with it will require less development and testing.
The technology is too complex to describe further here, but excellent informa- tion is available on the Internet. We can say however that in almost all cases thin slices orwafers are cut from a large single crystal of silicon, just like slicing a carrot, and hundreds of identical ICs are replicated on the wafer byphotolithogra- phy, a kind of printing process. The wafer is then sawn up into individual ICs, which might be around 0.1 inch square, depending on the complexity of that particular circuit. When you buy an IC, almost all the space in the device is taken up by the connections to the outside world, which are much bigger than the chip itself. Most ICs are packaged in a little brown plastic block like a small square of chocolate (Fig.4.28), with two parallel rows of terminals, called legs, which conveniently plug into the “breadboard” used in this book. This style is called dual in line (DIL).
The 555 timer chip was designed in 1971 by the Swiss-born inventor and electronics engineer, the late Hans R. Camenzind (1934–2012), working as a Fig. 4.28 The IC chip has awhite dotattop left. For size comparison there is a UK 10p coin on theleftand a US quarter on theright. A more complex DIP-24 integrated circuit is also shown
consultant to Signetics Corporation. He was the author of many technical papers and books as well as of a general interest book on the history of electronics.
The chip is still currently in large-scale production by prestigious companies like Texas Instruments and countless small factories in Southeast Asia, and nobody knows how many have been made, although Camenzind suggested in an interview in 2003 that production then was about one billion per year!
The 555 is a relatively simple circuit with about 25 transistors and various other components, and there are many updated versions, all using the same external connections. Because of its simplicity it only needs eight connection legs, a form called DIP-8. The reason for its continuing popularity is its versatility; it can be used as a one-shot timer,e.g., to keep an indicator light on for say 30 s; to give a beep after 5 min,e.g., for an egg timer; to flash a light,e.g., ON for 5 s and OFF for 2 s, as in an automobile direction indicator or cyclist’s rear light; to generate a beep;
and so on. And that’s only a few examples.
Here is a “pin-out” diagram to show the connections. The legs or pins are numbered 1–8, and a little dimple is moulded into the case to identify pin 1. In the photo of Fig.4.28this dimple has been highlighted with a white dot.
We have taken the opportunity to include in the figure a much more complex IC, in configuration DIP-24, which has been produced as a demonstration without the plastic casing, so that you can see the chip and its connections to the pins.
The function of each pin in the 555 IC is as follows:
Pin 1. Ground or zero Volts
Pin 2. Trigger: Normally at the supply or the battery voltage VS, but starts the timing process if it is connected to 0 V
Pin 3. Output: When the timer is switched on, this is connected internally to the battery (+) to activate the light, beeper, relay, etc., which is controlled by the device
Pin 4. Reset: Puts the timer back to its starting condition, ready to repeat the timing process when triggered
Pin 5. Control voltage: Alters the whole time scale of the process. In many simple applications this is not needed and the pin may not be connected!
Pin 6. Threshold: The voltage on this pin is normally around 0 V, but if it is raised to a certain level (the threshold level, of course), it causes the output pin to fall to zero Volt. And the threshold level is by design 2/3 VS. Does that ring a bell?
Pin 7. Discharge: This pin is normally connected internally to 0 V, and so it stops the capacitor getting charged but is disconnected internally when the trigger (pin 2) is activated Pin 8. It is connected to the supply or the battery voltage + VSand supplies the energy to work
the IC and also to drive whatever device is activated by the output (pin 3)
The 555 timer is remarkably flexible as to the supply voltage, which is usually in the range 3–15 V. Within this range its output timing does not vary.
555 timer ICs are readily available (see the components Appendix) at a cost of about 40 cents each.
There are also ICs with more pins, 14 or 16, which contain two or four 555s in the one package. (See below.)
4.12 Integrated Circuits: The 555 Timer Chip 151