Circuit Design Tutorial

Session 1

Sajid Sadi

Logistics

• Time: 5 – 7 pm (earlier if possible)
• Location:
  • E15-235 Monday, Wednesday, Thursday
  • Weisner Room Tuesday
• Mailing List: circuits @ media
• Website: http://ambient.media.mit.edu/~circuits/

What will we cover today?

• Basics of circuits
• What various parts do
• Where to get parts
• How to read datasheets (part 1)
• Chip-to-chip chatter
• Application circuits
• Intro to Eagle (LIVE!)

The basics

Voltage and Current

• Voltage (V) is like volume
• Current (I) is like flow
• Measuring V and I
  

Resistors

• resistor color chart
• SMD: formats are 1002, 102, 10K5, 33R3, etc.
• Important formula #1 : V = I × R
• Important formula #2: Vout = Vin × (R2 / (R1 + R2))
  

Capacitors

• simple rules:
  • if capacitor is in the signal path, high frequencies pass
  • if capacitor is parallel to signal path, low frequencies pass
• Physical and SMD: no rules, most SMD lack markings or any sort
  • The SMD are beige, if that helps

Transistors and MOSFETs

• Transistor
  • BJT (the “transistor”) : current-controlled current amplifier
  • MOSFET : voltage controlled current amplifier
    

Operational Amplifiers

• Go read about it here
• Comes in multiple packs
• Simpler version: comparator
• More complex/precise version: instrumentation amplifier

Logic Bits

• Gates
• Multiplexers/Decoders
• Registers
• And plenty of other random bits…

Sourcing Parts

The usual suspects

• FindChips
• Digikey
• Mouser
• Jameco
• Small Parts

Sourcing random stuff

• MPJA
  • Good stuff, mediocre prices, well organized
• All Electronics
  • Good for random things like solenoids, price not always lowest
• American Science and Surplus
  • Science fair goodies for all ages

… and even more random stuff

• BGMicro
  • Cheap, random, more or less organized
• Electronic Goldmine
  • Just like BGMicro, but do comparison-shop prices
• FuturLec
  • Cool stuff for cheap
  • … but sometimes the package comes from Australia after a few months

Reading Datasheets

The purpose of datasheets

• For most of the world
  • Specify every single bit of data known about a part
  • … but that doesn’t imply “in a readable way”
• For certain Asian companies
  • Specify the footprint so that you can randomly stick the part to your board and brush up on those crystal ball skills

Anatomy of a typical datasheet » 1

• Features (just the highlights)
• Description
• Absolute maximum ratings
• Operating conditions (static)
• Operating conditions (dynamic)

Anatomy of a typical datasheet » 2

• Pinout and device block diagram
• Dynamic characteristics graphs
• Testing patterns
• Principles of operation
• Applications
• Packaging information, layout diagrams, etc

Chip-to-Chip communication

• SPI
• I2C
• Serial (RX/TX)
• Parallel
• Common wires

SPI

• Clock polarity
• Sampling edge
  

I2C

• More of a complex protocol, with better multi-device support
• More latency and overhead
• read more about it here
  

Serial

• 2 wire, can be synchronous, has speed limits
• Additional lines often used (RTS, CTS, DSR, DTR)
• Can interface easily to PC
• Most common protocol
  

Parallel

• Send the data just as a byte
• Sometimes, you see half-byte transfers
• Usually need extra lines (RD/WR/etc)

Common additional wires

• Usually inverted
• Output enable (EN/OE)
• Read (RD)
• Write (WR, W)
• Clock (CLK, LATCH)

Reading application circuit diagrams

Application 1

Application 2

Application 3

Other figures to pay attention to

On to Eagle…