Getting in the swim: Exploring the dolphin circuit ........................
To make your dolphin shapes light up in sequence, you need to make a cir-
cuit that uses a timer chip and a decade counter chip in combination with
some resistors and a capacitor plus some transistors. Together, these control
how often each of the five dolphins lights up and how long each stays lit.A decade counteressentially takes a square wave and breaks it up into ten
pulses. For those of you who took Latin, you’ll recognize decadeas related to
the magic number ten. Read more about this counter in the following list.Here’s the overview of the schematic elements that you use to control your
terpsichorean dolphins:IC1is a key component of this circuit; it’s an LM555 timer chip that you
use to generate a square wave at its output on Pin 3.IC2 is the other key component of this circuit. This is a 4017 decade
counter that takes a square wave and generates ten sequential pulse
outputs. A 4017 decade counter does this by placing +V on one of its
output pins at a time, one after the other. The 4017 decade counter
switches to the next output pin at the start of each cycle of the square
wave generated by the timer, as shown in Figure 10-3. This allows you to
control the rate at which the 4017 decade counter switches +V to each
output pin; this is done by controlling the frequency of the square wave
generated by the LM555 timer chip. Because we didn’t want ten dolphins,
we connected the sixth output pin (Pin 1) to the reset pin (Pin 15). This
applies +V to the reset pin after five dolphins dance across the wall and
also resets the counter to the first output pin, skipping the last four
output pins altogether.
R1, R2, and C1 are, respectively, two resistors and a capacitor that form
the RC circuit that determines the frequency of the square wave gener-
ated by the LM555 timer chip.Q1, Q2, Q3, Q4, and Q5 are 2N3053 transistors that turn on when the
output pin of the 4017 decade counter they’re connected to is switched
to +V. You use these transistors to supply the necessary current —
about 190 milliamps — to light the 38 LEDs in each group.
The 555 timer IC generates a square wave from its output. The frequency
of the square wave that is generated is determined by how fast the capaci-
tor fills and drains. You calculate how fast the capacitor fills to two-thirds
of its capacity or drains to one-third of its capacity by using the RC time
constant equation.You can read more about this equation in Chapter 9.C2 is a capacitor that reduces the occurrence of noise on Pin 5 of the
LM555, which could cause false triggering of the IC. This noise can occur
if Pin 5 is left unconnected (also called floating).218 Part III: Let There Be Light