Most likely you will mount your LEDs on a separate
display panel, in a configuration such as the one
in Figure 5. The little dice icons in the LED circuit
indicate inputs that correspond with outputs from
the circuit in Figure 4. Just connect a wire between
each matching pair of icons.
When you have finished building your circuit, you
should short the unused pins on the 74LS27 chip
together and ground them, to reduce the chance
of errors caused by electronic noise.
S1 at the top of the circuit is a momentary push-button, supplying power to a 555 timer, which
sends about 500 pulses per second to the counter.
The LEDs become a blur, and no one can tell which
number is being generated at any moment.
When the button is released, this is equivalent
to throwing the die. Power to the 555 is cut off, but
capacitor C2 has accumulated a potential and now
slowly discharges itself. As its voltage diminishes,
the 555 runs more slowly, until the LEDs finally stop
flickering and show one number — much like a die as
it rolls across a table and stops with one face up.
If you want two dice instead of one, you will need
to duplicate the entire circuit (except for the 74LS27
chip, which still has two spare NOR gates on it). To
ensure randomicity, the second die must run at a
different speed from the first, requiring slightly
different values for C3, R2, and R3. You can double
the value of C2 so that the second die takes longer
to stop flashing, like the second reel on a Las Vegas
slot machine. The two dice displays should still start
simultaneously, which will entail using a double-pole pushbutton for S1, providing power separately
to each circuit.
If you need to troubleshoot the circuit, try adding a 10µF capacitor in parallel with C3 to make the
555 timer run very slowly. Disconnect IC3 and use
a meter to check voltage on the outputs from IC2.
If they are counting properly, reconnect IC3 and
check its outputs one at a time. Be sure to use a
power supply of only 5V; any more will burn out
the counter chip.
Of course I could have simulated dice more easily
by writing a few lines of software to generate random
numbers on a screen, but even a fancy screen
image cannot have the same appeal as a well-made
piece of hardware. Also, I derived satisfaction from
using simple, dedicated chips that demonstrate
the binary arithmetic and Boolean logic that are
fundamental in every computer. Best of all, I ended
70 Make: Volume 10
PARTS LIST
S1: SPST momentary pushbutton switch
R1: 100 resistor
R2: 100K for time increment
R3: 100K for time increment
R4: 100
R5, R6, R7, R8: 10K for IC protection
R9, R10, R11, R12: 500 for LED protection (adjust
the values to suit your LEDs)
C1: 100 F capacitor for decoupling
C2: 22 F for timer slowdown
C3: 0.01 F for time increment
C4: 0.01 F for decoupling
D1, D2, D3, D4: 1N4148 (or similar) signal diodes
Q1, Q2, Q3, Q4: BC550 signal transistors
IC1: NE555N (or similar) timer
IC2: NTE 74LS92N (or similar) counter
IC3: 74LS27 triple 3-input NOR gates
Also:
7 LEDs (your choice)
LM7805CT (or similar) 5V voltage regulator
Source for chips: ebay.com or mouser.com
7400s on the Moon:
If you noticed the similarity between part num-
bers for the 74LS92 counter and the 74LS27
NOR gate, there is a reason for this. They both
belong to the pioneering 7400 family of inte-
grated circuits developed by Texas Instruments
back in the day. The 7400s travelled on NASA’s
manned moon missions and have become leg-
endary in electronics, even meriting a Wikipedia
entry ( wikipedia.org/wiki/7400). 7400s are still
used for prototyping and for teaching computer
science. It doesn’t take much Boolean logic
to combine AND, OR, NOT, NAND, NOR, XOR,
and XNOR gates in ways that will take you far
beyond electronic dice.
up with an object representing my particular tastes
and idiosyncracies; and to me, that’s what making
things is all about.
Charles Platt, a frequent contributor to MAKE, has been a
senior writer for Wired, and has written science fiction novels,
including The Silicon Man.
