Chapter 6 Challenges

Phototransistor Voltage Output Circuit

Phototransistor voltage output circuit with 2 k-ohm resistor in series with the phototransistor


  1. What does a transistor regulate?
  2. Which phototransistor terminals have leads?
  3. How can you use the flat spot on the phototransistor’s plastic case to identify its terminals?
  4. Which color would the phototransistor be more sensitive to: red or green?
  5. How does VA3 in the circuit above respond if the light gets brighter?
  6. What does the phototransistor in the circuit above do that causes VA3 to increase or decrease?
  7. How can the circuit above be modified to make it more sensitive to light?
  8. What happens when the voltage applied to an I/O pin that has been set to input is above or below the threshold voltage?
  9. If the amount of charge a capacitor stores decreases, what happens to the voltage at its terminals?


  1. Solve for VA3 if I = 1 mA in the circuit above.
  2. Calculate the current through the resistor if VA3 in the circuit above is 4.5 V.
  3. Calculate the value of a capacitor that has been stamped 105.
  4. Write an rcTime statement that measures decay time with  pin 7 and stores the result in a variable named tDecay.
  5. Calculate what the ndShade measurement would be if the Arduino measures decay values of 1001 on both sides.
  6. Write a for loop that displays fifty equal sign characters in the Serial Monitor.


  1. In Activity 1, the circuit, along with the HaltUnderBrightLight sketch, made the BOE Shield-Bot stop under a light at the end of the course.  What if you will only have a limited time at a course before a competition, and you don’t know the lighting conditions in advance?  You might need to calibrate your BOE Shield-Bot on site.  A sketch that makes the piezospeaker beep repeatedly when the BOE Shield-Bot detects bright light and stay quiet when it detects ambient light could be useful for this task.  Write and test a sketch to do this with the circuit in Activity 1
  2. Develop an application that makes the BOE Shield-Bot roam and search for darkness instead of light.  This application should utilize the charge transfer circuits from Building the Photosensitive Eyes.
  3. Develop an application that makes the BOE Shield-Bot roam toward a bright incandescent desk lamp in a room where the only other light sources are fluorescent ceiling lights.  The BOE Shield-Bot should be able to roam toward the desk lamp and play a tone when it’s under it. This application should use the charge transfer circuits from Building the Photosensitive Eyes.