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Return to Previous Lesson: Breadboard - Schmitt Trigger Oscillator

Lesson Overview:

Now we'll make a schematic for our circuit!

Step 1: Getting Started

In this lesson you will learn how to create a circuit in the 123D Circuits schematic editor. The circuit you will be describing in this schematic is a Schmitt trigger oscillator that drives a blinking LED.

You may already be familiar with the breadboard editor in 123D Circuits. The schematic editor is significantly more versatile in describing complex circuits, but has both advantages and disadvantages to the breadboard editor.

Some of these advantages are that many more components are available, connecting wires is more versatile, and since most circuits are described in terms of their schematics, it is easy to find examples to work from and to convey your designs to others using schematics.

The disadvantages are few, but significant. There is no simulation of the circuit available, and the schematic can look very different from the physical circuit.

  1. Continue to the next step.

Step 2: What Is a Schematic?

What is a schematic? A schematic is a pictorial view of a circuit that shows how different components are connected.

  1. Continue to the next step.

Step 3: Schematic Components

Each component in the circuit is represented by a picture, with a little text to give it a unique name and perhaps tell more about the part. The type of pictures used can vary. They generally show connection points, but can also include an icon of the standard schematic symbol, or the general shape of the physical part.

Here are some examples.

  1. Continue to the next step.

Step 4: Connections Between Components

Solid lines are then used to show connections (wires or board traces) between the parts. Where the lines cross or intersect, a small black dot shows where they are joined.

  1. Continue to the next step.

Step 5: Gathering the Components

First you will gather the components for the power portion of the circuit.

  1. Click “Components”.
  2. Click “All Components”.
  3. Enter “GND” in the components search bar.
  4. Click “Add” on the top search result.
  5. Place the GND (ground) somewhere on the screen.
  6. Enter “capacitor” in the components search bar.
  7. Find the one named “Polarized Capacitor” and click “Add”.
  8. Place the capacitor directly above the GND, with a little space between them.
  9. Enter “VCC” in the components search bar.
  10. Scroll down to find the one that looks like a circle at the top with a vertical like below (ϙ) and click on “Add”.
  11. Place the VCC directly above the capacitor, with a little space between them.
  12. Enter “diode” in the components search bar.
  13. Scroll down to find the one named “Diode”, with description “Ensures electricity only flows in 1 direction. ...”
  14. Click “Add” and place it to the right of the space between VCC and the capacitor.
  15. Click “Components” again to hide the components selector.
  16. Continue to next step.

Step 6: Adding Connections

Time to hook these up together.

  1. Place the mouse pointer over the top of the GND component until you see a small black box with a red circle in it appear.
  2. Click the mouse to start the wire.
  3. Drag the blue line up to the bottom of the capacitor until the black box appears.
  4. Click the mouse to complete the wire.
  5. Do the same to connect a wire between the top of the capacitor and VCC.
  6. Click on the diode to select it.
  7. Press “R” once (or click the rotate icon) to rotate the diode so that the arrow faces left.
  8. Connect a wire between the left of the diode and the top of the capacitor.
  9. Continue to next step.

Step 7: Adding Power

Add a connection for battery box wires to connect.

  1. Click “Components”.
  2. Enter “2 pin header” in the components search bar.
  3. Scroll down to find the one named “Generic 2 pin header”, with description “2.54mm pitch 2pin header”.
  4. Click “Add” and place it to the right of the diode.
  5. Click “Components” again to hide the components selector.
  6. Click on the circle in the component.
  7. Press the down arrow on your keyboard to see that each pin of this component can be moved separately.
  8. Press “R” twice to rotate the pin so that the line is facing to the left.
  9. Select the other pin and rotate its the same.
  10. Move the pin labelled “U1_1” to the right of the diode with a little space between them.
  11. Move the pin labelled “U1_2” directly below that and somewhere between the capacitor and GND.
  12. Add a wire from pin “U1_1” to the right side of the diode.
  13. Add a wire from pin “U1_2” to GND.
  14. Continue to next step.

Step 8: The Power Circuit

The negative terminal of the battery or power supply (ground) will connect to U1_1. The positive terminal (+6 volts) will connect to U1_2. The capacitor is there to help remove small power fluctuations if the power source is not steady. The diode is there to make sure that if the power is accidentally connected backwards, that no current will flow and the rest of the circuit won’t be damaged.

  1. Continue to the next step.

Step 9: Adding the Schmitt Trigger

Now you will add the Schmitt trigger microchip and give it power by connecting it to GND and VCC (the power rails).

  1. Click “Components”.
  2. Using the search word “schmitt”, search for and add a 74HC14 (Hex inverting Schmitt trigger) to the project to left of your existing power circuit.
  3. Click “Components” again to hide the components selector.
  4. Select the existing VCC that you added earlier (the circle at the top of the power circuit).
  5. Copy and paste to create a second one. Move the new one above pin 14 of the microchip you just added.
  6. Connect a wire between it and pin 14.
  7. Copy and paste the GND from the power circuit, place it below pin 7 and connect it with a wire to pin 7.
  8. Continue to next step.

Step 10: Adding the LED

Unlike other components, when you add additional copies of VCC or GND to your circuit, they are automatically connected together, even though no wire is drawn between them.

Next, you will add the LED and its current-limiting limiting resistor.

  1. Click “Components”.
  2. Using the search word “LED”, find the one with the description, “A type of diode that illuminates…” and add one somewhere below pin 3 of the microchip.
  3. Using the search word “resistor”, find the one with the description, “Resist the flow of electrical…” and add one somewhere below pin 5 of the microchip.
  4. Click “Components” again to hide the components selector.
  5. Select the resistor and press “R” once to make it horizontal.
  6. Connect the right side of the resistor the the GND connected to pin 7.
  7. Select the LED and press “R” three times until the arrow points to the right.
  8. Connect the right side of the LED (cathode) to the left side of the resistor with a wire.
  9. Connect the left side of the LED (anode) to pin 2 of the microchip with a wire.
  10. Continue to next step.

Step 11: Adding Feedback

Next, you will add the feedback portion of the circuit.

  1. Copy and paste the existing resistor and place the new one somewhere below pin 1 of the microchip.
  2. Connect the right side of the resistor to pin 2 of the microchip with a wire.
  3. Click “Components”.
  4. Using the search word “potentiometer”, find the component with the description, “A variable resistor. …” and add it somewhere to the left of the resistor you just added.
  5. Click “Components” again to hide the components selector.
  6. Select the potentiometer and press “R” twice to rotate it such that the arrow points to the left.
  7. Connect the arrow with the left side of the resistor that you just added with a wire.
  8. Connect the top of the potentiometer to pin 1 of the microchip with a wire.
  9. Copy and paste the capacitor from the power circuit and place the new one to the left of the potentiometer.
  10. Connect the top of the capacitor (positive end) to pin 1 of the microchip.
  11. Copy and paste one of the existing GND points and place it somewhere below the capacitor that you just added.
  12. Connect the bottom of the capacitor (negative end) to that GND.
  13. Continue to next step.

Step 12: Setting Values

At this point, the schematic is almost complete. All you need to do now is assign values to the capacitors and the resistors.

  1. Select the capacitor in the power portion of the circuit.
  2. In the details box at the top left, change the capacitance to .1 μF, and check the box next to that to display this value on the schematic.
  3. In the same way, change the feedback capacitor capacitance to 100 μF, and make this value visible.
  4. From left to right, change the values of the potentiometer and the other two resistors to 100 kΩ, 27 kΩ, and 470 Ω (not kΩ), respectively, and make these values visible.
  5. Continue to next step.

Step 13: Finishing Up

You may find it necessary to move the components around to get the schematic to look the way you want it to. Feel free to make any changes you’d like.

The schematic, along with the bill of materials (available at the top right of the editor) is enough information to completely describe the circuit to others.

The schematic is complete! Congratulations!

In the next lesson you will learn to make a printed circuit board for our circuit!

Next Lesson:PCB - Schmitt Trigger Oscillator

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