simple coin-toss circuit.
For example, Figure 6-5 shows a version of the coin-toss circuit that uses a dashed line to delineate the items that won’t be mounted on the circuit board: the battery power supply (that is, the +9 V voltage source and the ground), the push-button power switch, the two metal finger contacts, and the two LEDs. Instead, they’ll be mounted separately within the project box. Thus, the circuit board will need to hold only six components: the 555 timer integrated circuit, the four resistors, and the capacitor.
FIGURE 6-4: The schematic diagram for the coin-toss circuit after it has been modified a bit for our project.
After you’ve completed your circuit design, you’ll want to compile a list of all the parts you’ll need to build the circuit. Then, you can rummage through your parts bin to figure out what parts you already have at your disposal and what parts you’ll need to purchase. Here’s a list of the components you’ll need to build the coin-toss circuit:
| Part ID | Description |
|---|---|
| R1 | 1 kΩ, ¼ W resistor |
| R2 | 10 kΩ, ¼ W resistor |
| R3 | 470 Ω, ¼ W resistor |
| R4 | 470 Ω, ¼ W resistor |
| C1 | 0.1 μF capacitor |
| LED1 | 5 mm red LED |
| LED2 | 5 mm green LED |
| IC1 | 555 timer IC |
| SW1 | Momentary-contact, normally open push button |
FIGURE 6-5: A schematic diagram that indicates which components are on the main circuit board and which aren’t.
Prototyping Your Circuit on a Solderless Breadboard
Before you commit your circuit to a permanent circuit board, you want to make sure it works. The easiest way to do that is to build the circuit on a solderless breadboard. The solderless breadboard lets you quickly assemble the components of your circuit without soldering anything. Instead, you just push the bare wire leads of the various components you need into the holes on the breadboard and then use jumper wires to connect the components together.
The beauty of working with a solderless breadboard is that if the circuit doesn’t work the way you expect it to, you can make changes to the circuit simply by pulling components or jumper wires out and inserting new ones in their place. If you discover that your schematic diagram is missing an important connection, you can add another jumper wire to create the missing connection or, if you want to see how the circuit might work with a different resistor or capacitor, you can pull out the original resistor or capacitor and insert a different one in its place. Figure 6-6 shows a typical solderless breadboard.
FIGURE 6-6: A typical solderless breadboard.
Understanding how solderless breadboards work
Although many different manufacturers make solderless breadboards, they all work pretty much the same way. The board consists of several hundred little holes called contact holes that are spaced
Beneath the plastic surface of the solderless breadboard, the contact holes are connected to one another inside the breadboard. These connections are made according to a specific pattern that’s designed to make it easy to construct even complicated circuits. Figure 6-7 shows how this pattern works.
FIGURE 6-7: The contact holes in typical solderless breadboards are internally connected following this pattern.
The holes in the middle portion of a solderless breadboard are connected in groups of five that are called terminal strips. These terminal strips are arranged in two groups, with a long open slot between the two groups, like a little ditch. It is in these holes that you will connect components such as resistors, capacitors, diodes, and integrated circuits.
It’s important to note that the rows of holes are not connected across the ditch. Thus, each row comprises two electrically separate terminal strips: one that connects the holes labeled A through E, the other connecting the holes labeled F through J.
The holes on the outside edges of the breadboard are called bus strips. There are two bus strips on either side of the breadboard. For most circuits, you will use the bus strips on one side of the breadboard for the voltage source and use the bus strips on the other side of the board for the ground circuit.
Most solderless breadboards use numbers and letters to designate the individual connection holes in the terminal strips. In Figure 6-7, the rows are labeled with numbers from 1 through 30, and the columns are identified with the letters A through J. Thus, the connection hole in the top-left corner of the terminal strip area is A1, and the hole in the bottom-right corner is J30. The holes in the bus strips are not typically numbered.
Solderless breadboards come in several different sizes. Small breadboards usually have about 30 rows of terminal strips and about 400 holes altogether. But you can get larger breadboards, with 60 or more rows with 800 or more holes.
Laying out your circuit
The most difficult challenge of creating a circuit on a solderless breadboard is the task of translating a schematic diagram into a layout that can be assembled on the breadboard. Only in rare cases will a circuit assembled on a breadboard look like the circuit’s schematic diagram. In most cases, the components are arranged differently and jumper wires are required to connect the components together.
