Designing Your Circuit
After you have an idea for a project, the next step is to design a circuit that meets the project’s needs. At first, you’ll find it very difficult to design your own circuits, so you’ll turn to books like this one or to the Internet to find other people’s circuit designs. With a bit of Google searching, you can probably find a schematic diagram that’s very close to what your project needs.
In many cases, you won’t be able to find exactly the circuit you’re looking for. You may find a circuit that’s close, but you may need to make minor modifications to make the circuit fit your project’s needs. At first, making modifications to a circuit may seem beyond your abilities. But as you gain experience, you’ll find yourself tweaking circuits all the time to fit specific applications.
One helpful strategy for designing circuits is to break complex requirements down into simpler parts. For example, consider the pop-up jack-in-the-box Halloween prop I mention earlier. The complete circuit for this project required several different elements, including these:
A circuit to detect when someone has entered the room to trigger the prop’s action
A circuit to open and close the jack-in-the-box
A circuit to time how long the jack-in-the-box should stay open
A circuit that plays a screaming sound
A circuit that provides a 30-second delay before the prop is activated again
The coin-toss project is much simpler than the jack-in-the-box project. In fact, a quick Google search will turn up several possible circuits that do almost exactly what the coin-toss project requires. For example, Figure 6-3 shows the schematic diagram for a typical coin-toss circuit you might find on the Internet. This circuit diagram uses a 555 Timer integrated circuit, four resistors, two LEDs, one capacitor, a switch, and a 9 V power supply (most likely a 9 V battery).
The schematic diagram shown in Figure 6-3 differs from our project’s needs in just two ways. First, it doesn’t have an on/off switch. And second, it uses a push button instead of the user’s fingers to start and stop the LEDs from flashing.
Figure 6-4 shows the schematic after I made those modifications. As you can see, I added a push-button switch that must be pressed to provide the +9 V voltage needed to run the circuit, and I replaced the push button that was in the original schematic with two open terminals. When the user touches these two terminals, the resistance of their finger completes the circuit.
One final step you might want to consider when designing a circuit is to create a final version of the schematic diagram that indicates what components will be mounted on your final circuit board and what components won’t be on the circuit board. This diagram will come in handy later when you’re ready to create the circuit board that will become the permanent home of your circuit.
FIGURE 6-3: A schematic diagram for a 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.