Scoping Out the Schematic

You have but one breadboard to put together for this project, but we make up for that by making you string 5 LED/resistor arrays, each containing 38 LEDs and 19 resistors.

Take a look at the schematic for the board, as shown in Figure 10-2.

To make your dolphin shapes light up in sequence, you need to make a circuit 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 counter essentially takes a square wave and breaks it up into ten pulses. For those of you who took Latin, you’ll recognize decade as 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:

IC1 is 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 generated 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 capacitor 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).

The circuit won’t mean a thing if you don’t set up the lights for it to control. That’s where the elements of the LED/resistor arrays come in. An array, in this case, equals the lights that define one whole dolphin. These five arrays each include

38 LEDs, which light up when a river of current runs through them

19 resistors, which are resistors that limit the current running through the LEDs in series with each resistor to approximately 10 milliamps Take a look at the schematic for these in Figure 10-4. Notice that we have not assigned a number to each LED and resistor. That is because we have 190

LEDs and 95 resistors among the 5 dolphins — and we don’t have time.

These LEDs and resistors are wired together such that two LEDs and one resistor are in series. When +V is applied, current runs through each component sequentially. Each group of two LEDs and one resistor are connected parallel with the other groups of LEDs and resistor. Because 19 LED/resistor groups are in each array, the total current running through an array is approximately 190 milliamps.