Building a Photo Gate

Links and useful resources

• Arduino Language Reference
• OnShape
• How To Mechatronics
• Thingiverse

Lesson-specific resource links

• https://www.instructables.com/IR-Photogate/

• https://www.instructables.com/DIY-Photogate/

• Project index: photo-gate-design

Concept summary and connections

• phototransistor
• pull-up resistor
• photodiode
• errors and measurements
  • How to design for accuracy

Examples

• Phototransistor tutorial
• build a photogate for rpi

The Design Process

1. Concept
   1. Requirements/MVP
   2. Block-level design (just like writing code)
   3. Iterate
2. Prototype
3. iterate

Media resources

• Youtube search for "phototransistor": https://www.youtube.com/results?search_query=phototransistor
• Physics classroom search for "phototransistor":
• Youtube search for "pull-up resistor": https://www.youtube.com/results?search_query=pull-up resistor
• Physics classroom search for "pull-up resistor":
• Youtube search for "photodiode": https://www.youtube.com/results?search_query=photodiode
• Physics classroom search for "photodiode":
• USB-C breakout board

Guided practice

• What are our requirements?

• What is our concept?

• What's the minimum viable product?

• Can we make a prototype right now?

  • Use the prototype for a while and gather new requirements

• Can we figure out a way to chain several gates together?

  • One data line
  • one device enable line for each gate
  • Use USB-c and breakout boards for the connections
  • Enable them in sequence using the microcontroller's code, in pin order
  • Mark each gate with its sequence position.

• When should we start the timer?

• Should we output absolute clock numbers or time-since-start? (or both?)

• Problem: Draw a circuit with a pull-up resistor to measure state of photodiode.

• #quiz What is the purpose of a pull-up or pull-down resistor? How does it work?

• #quiz Draw a circuit that uses a pull-up resistor to determine whether a switch is closed or open.

• #quiz Draw a circuit that uses a pull-down resistor to determine whether a switch is closed or open.

• #quiz Draw a circuit with a voltage divider.

• #quiz A circuit has +5v input, then a 1k$\mathrm{\Omega }$ resistor, then a $9k\mathrm{\Omega }$ resistor, then ground. What is the voltage between the resistors?

• #quiz What is the formula for finding voltage drops in a two-resistor voltage divider?

• #quiz What does a phototransistor do in a circuit?

• #quiz Design a voltage divider that will give you 3.5v from a 5v source.

• #quiz What is PWM and what are some common uses for it?

• #quiz Why would you choose to use a pull-up resistor configuration over a pull-down configuration in some designs?

• #quiz How does the engineering design process work? What are the main stages?