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BAM Blinky Badge

Lesson Overview#

Lesson 2 fully assembled

In this lesson, you’ll design a custom PCB that blinks an LED on and off. If this sounds a lot like a previous lesson, you're right!

The Lesson 1 and 2 circuits function very similarly Bill of Materials. However, this design boasts the following features not found in Lesson 1.

  • It requires no human interaction to flash the LED whereas before you had to press a button every time you wanted the LED to shine.

  • It swaps out the push-button switch and finger (to press it) for a touchless transistor and capacitor based delay circuit.

Once you have this project built, you won't have to lift a finger to make your LED blink.

Bill of Materials#

Photograph of all the Lesson 2 components shot on a colored background

In this lesson the stars of the circuit are two new types of components: the capacitor and the transistor.

You'll also use a new type of switch called a single pole double throw. This is a fancy way to say that the switch has one input that switches between two outputs.

Component NamePatchr SymbolSchematic SymbolDesc.
LEDLEDLED Schematic Symbol5mm, red
CapacitorElectrolytic CapacitorElectrolytic CapacitorElectrolytic
NPN TransistorNPN TransistorSchematic symbol for an NPN transistor2N3904
PNP TransistorPNP TransistorSchematic symbol for a PNP transistor2N3906
ResistorResistorResistor100K Ω
SPDT SwitchSingle Pole Double ThrowSPDTSingle Pole Double Throw (SPDT)
Batter RetainerBattery Retainer

You'll notice the BOM has a new column with a bit more detail about each component. As project complexity increases, you'll need to know more specifics about each component such as resistance, capacitance, size, color, part number, and type.

For example, if you used a different value resistor than the 100K Ω, the circuit would still work, but the LED would blink faster or slower depending on the resistance used.

How the Circuit Works#

An electrolytic capacitor

A capacitor is a device that stores charge. It's a bit like a bucket that holds water. There's only so much that it can hold and when it reaches capacity it dumps its contents.

Much like filling a bucket with water, charging a capacitor takes time. This circuit takes advantage of that filling time to create a delay in the flow of current.

How a Transistor Works#

Diagram of all the ways you might see a NPN transistor

The bipolar junction transistors (BJT) you'll use in this lesson has three leads called the emitter, base, and collector. When a small current flows to the transistor's leads, it allows a much larger current to flow between the other two.

Transistor & Capacitor Cooperation#

Patchr Fundamentals Lesson 2 Schematic

When a small current is sent from the control to the base of the transistor, current is allowed to flow from the collector through the base, and ultimately through the LED. The effect is that the LED illuminates.

Your Solar Lantern customization begins with the board shape. In this step you can create a custom board shape using the built-in drawing tools found in the left toolbar or you can upload a custom SVG file from your computer using the toolbar on the right of the screen.

Board Shape Design#

Regardless of which customization tool you decide to use, there are a few things to keep in mind while designing.

  • The custom board size may not exceed 100x100mm. The design window shows this with light red shading and OUT OF BOUNDS text. If your shape exceeds the boundary, you will get an error notification and be unable to advance to the next step in the design process.

  • The white area inside the red shading denotes the space you have to work with for your board shape.

  • The dotted red lines that make a small rectangle indicate the minimum size your board can be — if you made your board any smaller than this area, you would not be able to fit all the components on the PCB let alone route traces between them.

Zooming & Moving#

  • Zooming in and out can be done with the icons in the left toolbar or by using your mouse's scroll wheel.

  • To reorient your design workspace, hold the right mouse button down and drag the workspace to your desired location.

  • To move your custom board shape, click on the Move Selected tool in the left hand toolbox. Then click and drag your shape to the location you want.

Built-In Drawing Tools#

The toolbox on the left of the screen features the draw polygon path, draw freehand path, draw rectangle, and draw circle tools. These tools allow for simple shape customization within Patchr.

Once you have a shape drawn, you can use the tools on the right side of the screen to scale up or down your shape and rotate it.

When you are happy with your custom board shape, click on the NEXT > button to advance to the layout step.

Uploading an SVG File#

To really get the most out of board shape customization, you can upload single-path SVG files into the editor.

  1. First ensure your SVG is using a single path.
  2. Click on the Choose File button under the Upload SVG heading in the right hand side toolbar.
  3. Assuming your SVG is compatible, you should see your custom board shape in the editor. You may need to use the scaling and rotation tools — also in the right hand side toolbar — to tweak the placement and size of your SVG.
  4. Once you're happy with the size and shape of your custom board, click the NEXT > button to advance to the layout step.

Note that SVG files are a specialized type of image file which are made up of vector coordinates. If you're new to vector files and vector drawing, check out Inkscape, which is a popular, free, and open source vector drawing program.

Component Layout#

Click and drag all of the components from your component tray onto the PCB. Don't worry too much about the orientation of each component, just get them all on the board.

Once you have all the components on the board, then you can tweak things like the orientation of the switch.


Routing this circuit will require a bit more puzzle solving than the last one. Remember to keep these tips in mind while you're working your way through the routing.

  • Start by routing your power and ground traces.

  • Next tackle components that have multiple pins, since they will require more complex routing.

  • Do not have routes overlapping

  • Avoid using 90° angles in your routes

  • Keep routes as short as possible

  • Routing is as much an art as a science. As you route more PCBs, you'll begin to develop your own habits and patterns.


Last but not least is the silkscreen for your PCB.

Lesson 2 PCB with white silkscreen

A silkscreen layer is typically used for marking where components go and adding company logos and product names to a PCB.

But, silkscreens can also be used to bring your board to life and give it some extra pizzaz. That's what you'll see is going on in the silkscreen shown above. Without the silkscreen, the look of the Patchr Squid just wouldn't be the same.

Submit Your Design#

Once you have your PCB looking the way you want. Click NEXT > and your board will head to your teacher for review before manufacturing.

Congratulations, your first custom PCB is almost ready for solder!