Make Electronics

Chips, Ahoy! 201

Experiment 20: A Powerful Combination

To help it make sense, I’ve shown the logic gates that exist inside the chips. I’ve
also colored the power supply wires, as before, to reduce the risk of confusion.
The positive side of the supply goes only to the common terminal on your
keypad, and you have to press the asterisk key to send the power back down
the ribbon cable, to supply the chips.

Note that the “wrong” numbers on the keypad are all shorted together. This
will create some inconvenience if you want to change the combination in the
future. I’ll suggest a different option in the “enhancements” section that fol-
lows. For now, ideally, you should run a wire from every contact on your key-
pad, down to your circuit on its breadboard, and short the “wrong” keypad
numbers together with jumper wires on the breadboard.

Also note that if you use a meter to test the inputs to the AND gates, and you
touch your finger against the meter probe while doing so, this can be suffi-
cient to trigger the sensitive CMOS inputs and give a false positive.

Computer

power-up

switch

0

7

1

4

5VDC Regulated

(from 9V battery)

2 3 5 6 8 9 #

74HC08

74HC04

555

330

330

330

10K

10K

10K

10uF

0.1uF

100K

Figure 4-84. The combination lock schematic redrawn to show how the components can
be laid out on a breadboard.

If you build the circuit and you can’t

understand why everything’s dead,

it’s most likely because you forgot to

hold down the asterisk button.