Designing, building and testing a circuit

Prep for this week’s class


Who has:

  • Made something with electronics?
  • Used an Arduino?

This week:

  • Tour of some basic components
  • Learn how to draw a circuit in Eagle
  • Assignment: draw a circuit board, ready for milling (ending up where we started last week).
  • (Make and test it)
  • Next week, program it



Ribbon cable used for ISP headers - and post-milling board edits – picture
Also, headers (mechanical convenience)


Surface mount versions for circuits – picture
Momentary buttons - “N.O.” make a circuit when pressed (example use: connect a chip’s reset pin to GND)
You might want to use cool buttons for UI elements – picture


  • Value measured in Ohms (1 Ohm, 5 kOhm, 4k7 Ohm, etc.)
  • Limit the flow of current
  • Ohm’s law: I = V/R


Uses often include:

  • Regulate current to LEDs – picture
  • Keep pins high or low (pull-up/pull-down) without current flowing


  • We use 1206 (0.12”× 0.06”)
  • Hand solderable
  • You can get a trace between the two ends

More: Resistors -


Store charge, and release it when the voltage across the cap reaches a specific level
Q (charge stored) = C (capacitance) x V (voltage across plates)
Value of capacitance (amount of charge it can store): Farad (F, nF, pF, uF, etc.)



  • Filtering high-frequency noise from ICs (“decoupling” or “bypass” capacitors)
  • Supplying current to IC pins very quickly (e.g. when you ask it to switch on and off in your program)
  • Create pulses at specific time intervals (with resistors)


  • 1206 SMD
  • Bigger capacitors often polarised


Crystals and resonators

Used to tell time.
Crystals are very accurate, but need 2 capacitors.
Resonators are more convenient, but less accurate (could cause problems for your IC, e.g. for fast serial comms).
IMHO: crystals are easier to solder, even with caps


  • External clock for microcontroller (IC)


Conducts only in one direction (as shown in circuit symbol)
Current flows from anode (+) to cathode (-) (alphabetical)



  • Prevent current going in the wrong direction (e.g. if you connect a battery backwards)
  • Often used in AC/DC convertors


  • Signal diode
  • Specialised: Schottkey diode, Zener diode
  • LED: emits light and happiness

Diodes are polarised: bar on the symbol is the same as on the package (cathode)
Diodes have no internal resistance, so can draw all the current available, and blow up
So use a current-limiting resistor (e.g. 100 Ohm, 1K Ohm) - very common with LEDs


A whole circuit inside one package: a computer, analogy to digital convertors, clock, PWM outputs, memory, switches, etc, etc.

We’ll mostly use ones from Atmel:

  • ATtiny 44/45, 84/85
  • ATmega 328p (Arduino Uno)
  • ATmega 32u4 (Arduino Leonardo, USB)

Datasheets - learn to love them!

And many more …

Further reading

Fab Academy:

Drawing circuits

Overview of Eagle

See the Detailed guide for more Eagle tips.

Libraries of parts



Logical structure of a circuit.


Board layout

Where the components go on the board, and how to route traces (connections between parts) between them.


Export and verify

Eagle can generate a layered vector file of traced, pads, labels, and board outline. Use your preferred tools to convert these into 2 PNG files for milling.


See the Detailed guide for more Eagle tips.


1) Redraw the Echo Hello World Plus board, starting with this finished circuit:


The parts and what they do

IC1 Microcontroller ATtiny44
R1 Resistor Holds PB3/Reset pin high
C1 Capacitor Smooths out noise on VCC/GND line
J1 FTDI header Serial comms with a computer
J2 ICSP header Programming with a FabISP
D1 LED A simple output device you can test with. Brings happiness
R2 Resistor Limits current to LED
Resonator Resonator External clock source for chip
S2 Button A simple input device you can test with
  • Use a library in Eagle to get all these parts
    • Look at the part bins here to see what we have
    • Make sure the parts you select are the right size
  • Draw a schematic and board layout
  • Export a millable png file of the traces and outline
  • How compact can you make it? ( = less waste, less milling time)

2) Make the board

If you have time, mill and stuff the board.
Do this if at all possible!

3) Upload a pre-written program to the board to test that it works

See the board programming guide.

What do I need to do to pass?

Redraw the board in Eagle and generate a PNG file suitable for milling.

Document all your work on your student blog, with photos and videos to show what you did, what went wrong, and how you fixed it. Cite external sources where you have used someone else’s work.

Extra credit

Mill, stuff and test the board by uploading a program. Make the button work reliably (figure out how and why to use a pull-up resistor).

See the board programming guide.

Source material / further reading/watching

Materials we need this week

  • All components for board we make
  • Atmel programmer or working FabISPs
  • Some pre-made boards for stuffing
  • WIP …?

Failsafe for next week

  • Commercial Arduinos we can program
  • We can continue to mill and stuff boards next week