Electronic Project Plan Step 2: Design the Circuit - dummies

# Electronic Project Plan Step 2: Design the Circuit

Once you have an idea for an electronic 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 can turn to books 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 may find a circuit that’s close, but you may need to make minor modifications to make the circuit fit your project’s needs.

One helpful strategy for designing circuits is to break complex requirements down into simpler parts. Imagine a Halloween jack-in-the-box project. It would be very complex, but can be made simpler by breaking it into its separate elements, like 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

Now imagine a simple electronic project where you design the circuits to toss a coin for you. 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.

This schematic diagram differs from the coin-toss project’s needs in just two ways. It doesn’t have an on/off switch and it uses a pushbutton instead of the user’s fingers to start and stop the LEDs from flashing.

It’s easy to make those modifications: Add a push-button switch that must be pressed to provide the +9 V voltage needed to run the circuit and replace the pushbutton that was in the original schematic with two open terminals. When the user touches these two terminals, the resistance of his or her 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.

For example, imagine 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.

Once 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 KÙ, 1/4 W resistor
R2 10 KÙ, 1/4 W resistor
R3 470 Ù, 1/4 W resistor
R4 470 Ù, 1/4 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 pushbutton