Make Electronics

Experiment 18: Reaction Timer

172 Chapter 4

Grounding Yourself

To avoid the frustration that occurs when you power up a circuit and nothing happens, be sure to take these precautions when you

use the older generation of CMOS chips (which often have part numbers from 4000 upward, such as 4002, 4020, and so on):

Chips are often shipped with their legs embedded in black foam. This is electrically conductive foam, and you should keep the chips

embedded in it until you are ready to use them.

If the chips are supplied to you in plastic tubes, you can take them out and poke their legs into pieces of conductive foam or, if you

don’t have any, use aluminum foil. The idea is to avoid one pin on a chip acquiring an electric potential that is much higher than

another pin.

While handling CMOS components, grounding yourself is important. I find that in dry weather, I accumulate a static charge merely

by walking across a plastic floor-protecting mat in socks that contain some synthetic fibers. You can buy a wrist strap to keep your-

self grounded, or simply touch a large metal object, such as a file cabinet, before you touch your circuit board. I am in the habit of

working with my socked foot touching a file cabinet, which takes care of the problem.

Never solder a CMOS chip while there is power applied to it.

Grounding the tip of your soldering iron is a good idea.

Better still, don’t solder CMOS chips at all. When you’re ready to immortalize a project by moving it from a breadboard into perfo-

rated board, solder a socket into your perforated board, then push the chip into the socket. If there’s a problem in the future, you

can unplug the chip and plug in another.

Use a grounded, conductive surface on your workbench. The cheapest way to do this is to unroll some aluminum foil and ground

it (with an alligator clip and a length of wire) to a radiator, a water pipe, or a large steel object. I like to use an area of conductive

foam to cover my workbench—the same type of foam that is used for packaging chips. However, this foam is quite expensive.

Figure 4-35. IC3 is a 4026
counter. IC4 is a triple
seven-segment display chip.
The arrows tell you which
pins on the LED display
should be connected to the
pins on the counter.

1 2 3 4 5 6 7 8

16

15

14

13

12

11

10

9 1 2 3 4 5 6 7 8 9

10

11

12

13

14

28

27

26

25

24

23

22

21

20

19

18

17

16

15

IC3

IC4

5

1.

5

5

27

28

4

1

2

24

25

Disable clock

Enable display

Enable out

Carry output

Reset

Not-2 output

To segment c

To segment f

To segment g

Clock input

To segment b

To segment e

To segment a

To segment d

3

4

1

2

16

15

14

13

6

8

7

12

11

9

10

5

4026

Counter

Figure 4-36. The 4026 decade counter is a CMOS chip that accepts

clock pulses on pin 1, maintains a running total from 0 to 9, and out-

puts this total via pins designed to interface with a seven-segment

LED numeric display.