# PCB - Schmitt Trigger Oscillator

1,562

37

## Intro: PCB - Schmitt Trigger Oscillator

The following information is a single lesson in a larger project. Find more great projects here.

Lesson Overview:

Now we'll make a PCB for our design!

## Step 1: Getting Started

In this lesson you will learn how to layout a simple circuit in the PCB (printed circuit board) view of the 123D Circuits editor. This will teach you how to position components, route the copper traces, and resize the board.

Once all of these steps are complete, it is possible to have physical boards made by either ordering them directly in 123D Circuits, or downloading the Gerber file and ordering through the PCB manufacturer of your choice.

1. Continue to the next step.

## Step 2: PCB View

In the editor, you can see a Schmitt trigger oscillator circuit that has already been created. To work on the PCB layout, you will need to switch to the PCB view.

1. Select PCB view in the upper right of the editor to switch to the printed circuit board (PCB) layout view.
2. Continue to next step.

## Step 3: PCB View - Outline and Footprints

The large brown square with the orange border is the board outline. The gray grid in the background is used for aligning things and has a spacing of 0.100 inch (2.54 mm).

All of the white and red objects are the footprints of the different components from the schematic. You may notice that they look somewhat different than they do in the schematic view. This is because in the PCB view, you are concerned with the physical dimensions of the actual components, and how they connect to the board.

1. Continue to the next step.

## Step 4: PCB View - Connections

The red part of the footprints represent the exposed copper where the conductive elements of the component will make contact with the board. The white portions are referred to as the silk screen. These will be printed on the circuit board to help identify different components and their orientations. They do not conduct electricity.

The green lines show the connections between the different components. These are equivalent to the wires in the breadboard and schematic views.

Any yellow highlighted sections show where problems with the placement of the components are occurring. These are known as design rule checking (DRC) errors. One of your jobs when creating a PCB layout is to eliminate these errors.

1. Continue to the next step.

## Step 5: Initial Placement

Most visible DRC errors can all be eliminated by moving the components completely within the PCB area (brown rectangle) and not overlapping. Try to arrange them somewhat similar to the picture. with R1 above R2 and C1 above C2.

1. Move the components into the PCB area to be similar to the picture.
2. Continue to next step.
3. Stuck? HINT: If you are having trouble seeing the picture, right click on it and select "Open Image in New Tab" to get a better view.

## Step 6: Selecting Footprints

Many components can come in different physical shapes and sizes. The footprints will need to match the components that will ultimately be soldered to the PCB. It’s time to make sure the footprints are correct for your parts. Some of them here are already what you will want. The others will need to be changed.

1. Select the diode D1.
2. In the information box that opens up, select “Change Footprint”
3. Search for “D041-7.6” and select it to change the footprint.
4. Change both of the resistors R1 and R1 to use the “0204/7” footprint.
5. Change the LED D2 to use the “LED5MM” footprint.
6. Change both capacitors C1 and C2 to use the “E2,5-6” footprint without the capacitor symbol in the middle.
7. Continue to next step.

## Step 7: Component Orientation

Now you will change the orientation of the different parts to further shorten the connections and reduce their crossings.

1. Select resistor R1 and press “R” four times to flip it around.
2. Select resistor R2 and press “R” four times to flip it around.
3. Select the microchip U2 and press “R” six times to rotate it until it is horizontal with white circle in the lower left corner.
4. Select the LED D2 and press “R” six times to rotate it until the flat white part is at the top.
5. Select capacitor C1 and press “R” two times to rotate it until the holes are in a vertical line with the “+” at the top.
6. Select capacitor C2 and press “R” six times to rotate it until the holes are in a vertical line with the “+” at the bottom.
7. Continue to next step.

## Step 8: Placing Components Close Together

It is a good idea to place the components close together. This means shorter traces between the components, and smaller boards cost less money. Now that the basic orientation is worked out, it’s time to move all of the components closer together. It is possible to get all of them to fit in a 1.00 inch (25.4 mm) space without making them so close that assembling the board becomes difficult.

1. Select the left hole of the diode D2 and move it so that it is one grid space (.100 inches) to the right and down from the top-left corner.
2. Select the left hole of resistor R1 and move it one grid space directly below the left hole of the diode.
3. Select the left hole of resistor R2 and move it one grid space below R1.
4. Continue moving the rest of the components so that they match the picture.
5. Continue to next step.

## Step 9: Board Size

Now that all of the components are close together, you can reduce the board outline to match. Change the board size to 1 inch x 1 inch (25.4 mm x 25.4mm)

1. Click the “Zoom To Fit” button on the toolbar to make sure that the entire board is visible.
2. Click anywhere on the orange board outline to make the green board vertex grip points visible.
3. Click on the upper right corner grip and drag it straight left and snap to the major grid line (lighter gray grid line).
4. Move the lower two grip points until you have a 1 inch square board matching the picture.
5. Click the “Zoom To Fit” button.
6. Continue to next step.

## Step 10: Copper Trace - Power

Now it is time to replace all of those green lines with actual board traces.

This is the part of PCB layout where you will usually spend most of your time.

You will start with the power traces. Power traces need to be wide enough to support current for the whole circuit to pass through them. For these you will use .020 inch (= 20 mil = 0.51 mm) wide traces.

1. Select the “Copper Trace” tool to make it active.
2. In the toolbar area, select “0.51 (20mil) Power” for the trace width.
3. Click on the red circle (through-hole) of the power connector U1.
4. Click the upper left corner of the white rectangle around U1 (this will make a short diagonal line).
5. Click the top of the circle around capacitor C1.
6. Click the red through-hole circle on the right side of the diode D1 to complete the trace.
7. Continue to next step.

## Step 11: Copper Trace - Power (3)

Place the next trace.

1. Click the red through-hole on the left side of the diode D1.
2. Click straight to the right, somewhere in the middle of the silkscreen for diode D1.
3. Move the mouse over the left-most edge of the silkscreen circle around capacitor C1.
4. Adjust the vertical position until the red trace is mid-way between the right through-holes of diode D1 and resistor R1. Click to set this position.
5. Click on the top through-hole of capacitor C1 to complete the trace.
6. Continue to next step.

## Step 12: Copper Trace - Power (3)

Place the third trace.

1. Click the red through-hole on the left side of the diode D1 again.
2. Click down and left to start a diagonal trace.
3. Move the mouse down to the left of the top-left pin of the microchip U2.
4. Adjust the horizontal position until the red trace is mid-way between the left edge of the board and the left through-hole of resistor R1. Click to set this position.
5. Click the top left pin of the microchip (VCC) to complete this trace.
6. Continue to next step.

## Step 13: Copper Trace - Power (4)

Complete the power signals.

1. Click the right through-hole of resistor R1.
2. Click the bottom through-hole of capacitor C1 to complete this trace.
3. Click on the the bottom through-hole of capacitor C1 again.
4. Click on the bottom (square) through-hole of U1 to complete this trace.
5. Click on the bottom through-hole of U1 again.
6. Click on the top through-hole of capacitor C2 to complete this trace.
7. Click on the top through-hole of capacitor C2 again.
8. Click down and to the right to start a diagonal trace.
9. Click the mouse near the top-right corner of the microchip silkscreen rectangle (this sets the horizontal position of the trace).
10. Click on the bottom right through-hole of the microchip (GND) to complete the trace.
11. Continue to next step.

## Step 14: Copper Trace - Signals

That completes the power traces. Next you will work on the signal traces. For these, you can use narrower trace widths.

1. In the toolbar area, select “0.30 (12mil) Signal” for the trace width.
2. Click on the left through-hole of resistor R1.
3. Click somewhere in the center of the silkscreen for resistor R1 to start a horizontal trace.
4. Click mid-way between the right through-holes of resistor R1 and resistor R2.
5. Click on the top through-hole of LED D2 to complete the trace.
6. Click on the bottom through-hole of LED D2.
7. Click on the right through-hole of resistor R2 to complete the trace.
8. Click on the center through-hole of potentiometer R3.
9. Click on the left through-hole of resistor R2 to complete the trace.
10. Click on the right through-hole of potentiometer R3.
11. Click mid point between the nearest top pins of the microchip U2 to set the horizontal position of the trace.
12. Click on the bottom-left through-hole of the microchip U2 to complete the trace.
13. Click on the second through-hole from the left at the bottom of the microchip U2.
14. Click up and to the right of the hole to start a diagonal trace.
15. Route this trace between the through holes at the top of the microchip U2 by clicking mid point between them.
16. Click on the lower through-hole of LED D2 to complete the trace.
17. Continue to next step.

## Step 15: Copper Trace - Signals (2)

At this point, you are left with one green line. It crosses directly over one of the traces. It needs to be routed so that it doesn’t come into contact with any other trace. There are several ways to handle this situation. Since this is a two sided board, you could put this trace on the back-side of the board.

You would do this by clicking the “Layers” button, selecting “Bottom” and then drawing the trace.

1. Continue to the next step.

## Step 16: Copper Trace - Signals (3)

What if you want to keep all of the traces on a single side? Often there are ways to make this happen. The remaining green line connects the right through-hole of the potentiometer R3 to the bottom through-hole of capacitor C2. Notice that the potentiometer is already connected by a trace down to the lower left through-hole of the microchip U2. You can make this last connection there, instead of at the potentiometer, since these two places are already electrically connected.

1. Continue to the next step.

## Step 17: Copper Trace - Signals (4)

The way to solve this is to draw this last trace down between the microchip’s through-holes to near the bottom edge of the board and across to the lower-left through-hole.

1. Click on the lower through-hole of capacitor C2.
2. Click diagonally down to the left to start the trace.
3. Click mid-way between the lower right through-holes of the microchip U2 to set the horizontal position of this trace.
4. Click diagonally down to the left to position the trace mid way between the bottom through-holes of the microchip U2 and the lower edge of the board.
5. Click on the lower left through-hole of microchip U2 to complete the trace.
6. Continue to next step.

## Step 18: Finishing Up

The layout is complete. This board could now be built and components soldered to it.

Remember, there isn't just one right way to lay out a circuit board. Feel free to restart this lesson and try laying out the board in other ways.

In case you should want to try building this board, here's a list of the components and where to get them.

www.digikey.com:

C1 - 493-­12624­-1-­ND

C2 - 399-­6103­-ND

D1 - 1N456A­-ND

D2 - 754-­1731-­ND

R1 - CF14JT470RCT­-ND

R2 - CF14JT27K0CT­-ND

R3 - 987­-1501-­ND

U2 - 296­-1577-­5­-ND

If you want to power this circuit from common AAA batteries, you will need a 4 x AAA battery holder.

www.digikey.com: BH24AAAW­-ND

Congratulations, you have completed this project!

Check out other great projects here.

## Recommendations

• ### Large Motors Class

10,080 Enrolled