■ FIGURE 12
■ FIGURE 13
going to the off LED; it will become lit and the other one
will go off. It will stay that way until you press the other
pushbutton. The circuit is often called an SR Latch where
S and R stand for SET and RESET. It’s also a flip-flop: one
bit of memory.
Build an LED Blinker
Just for fun, let's build a circuit that will blink the two
LEDs in an alternate pattern; when one is on, the other
will be off. We will use a CMOS digital IC: the CD4093.
The IC contains four two input NAND gates as shown in
The signals in digital ICs are either high (+V) or low
(0V). The output of a gate will switch between high and
low, depending on whether its inputs are high or low. The
input-output behavior of a gate is defined by its truth
table. Table 1 is the truth table for a two-input NAND
gate, where A and B are the inputs and X is the output.
A 0 represents low (0V) while a 1 represents
The CD4093 has what's called Schmitt trigger
inputs. That means the value of the input voltage
required for a 1 depends on whether the input is
switching from low to high, or from high to low.
It's meant to guarantee a “clean” transition on the
output but, as we will see, it also allows us to
build a simple square wave oscillator.
The circuit you will build is shown in Figure
13. Unlike Figure 11, this circuit is shown as a
schematic. Part of your job will be to translate
Figure 13 into something like Figure 11 before
you start to build the circuit on your solderless
breadboard. Q1 and Q2 are MOSFET transistors.
Use a device like the 2N7000 or the BS170. Use
a 1 µF capacitor for C1; either monolithic or
tantalum. For R1, start out with a 1 megohm
resistor which will give a slow blink. Try using
smaller resistors to make it blink faster. When
the LEDs blink fast enough, they will appear to be
on all the time.
Now that you've got your Proto Buddy and an SBB,
you can breadboard all sorts of circuits. You can find
many circuits in issues of Nuts & Volts, and on the
Internet. It's even more fun to dream up your own circuits
and try them out. Instead of a battery, you can get a wall
wart power module from many vendors; the surplus units
are usually inexpensive. So, go have some fun with your
new Buddy. NV
A complete kit for this project can be purchased
from the Nuts & Volts Webstore
orcallour order desk, 800 783-4624.
ITEM QTY DESCRIPTION
■ S1, S2 1 DIP Switch, Two SPST Switches
■ S3, S4 2 N.O. Momentary Push Button Switch
■ TB 1 Three Position Terminal Block
■ L1, L2 2 LED, T1 Bright
■ R1, R2 2 Resistor, 1K , 1/4W, 5%
■ R1 (EXP) 1 Resistor, 1M , 1/4W, 5%
■ C1 (EXP) 1 Capacitor, 1 µF
■ IC1 (EXP) 1 CD4093B
■ Q1, Q2 (EXP) 2 Transistor 2N7000
■ N/A 15 Pin Jacks
■ N/A 10 Header Pins (Five Pair)
■ N/A 2 Jumper Block
■ N/A 2 HEX Standoff, M/F, 4-40, 1/4 in, 3/8 in long
■ N/A 2 4-40 Nut
■ N/A 2 #4 Lock Washer
■ N/A 1 Printed Circuit Board
It might be useful to include some 22 guage, solid, insulated
hook-up wire; maybe five feet of red, five feet of black, and five
feet of yellow.
January 2009 47