We built a soldered photo-sensor using a photo transistor to measure a monitor’s optical response. By capturing rising and falling waveforms across gray levels, we analyzed step responses and distinguished drive methods—most notably PWM vs PAM

My role: sensor fabrication & wiring, oscilloscope data capture, waveform analysis, documentation.

• Hardware & Tools

  NPN photo transistor (BJT), resistors (6.8k/15k/56k/82kΩ), breadboard/proto board, power supply (5–30V), oscilloscope, shielding (paper cup + tape)

• Method (Brief)

  We built a photo sensor, attached it to the display, and captured waveforms under different gray levels by varying resistance and supply voltage. From these measurements, we extracted rising/falling times and analyzed step responses.

  

• Key Findings

  • Drive modes: The test monitor’s waveform width changed with brightness → PWM; the reference monitor scaled amplitude more linearly → PAM.
  • Panel behavior: TN/VA panels showed step responses due to dynamic capacitance; IPS minimized them, enabling faster target luminance without “stairs.”
  • Resistor choice: For crisp on/off (LED switching), smaller R (≈6.8kΩ) produced steeper edges. For optical luminance tracking vs gamma, 56kΩ best matched the curve with minimal saturation.
  • Supply choice: Around 20V V_CC gave the closest gamma alignment among the tested values (5–30V).
  • Mitigation: Overdrive (briefly overshooting the target voltage) can reduce response time and perceived step artifacts.

• Outcome

  We verified step responses across gray levels, quantified rising/falling times, and explained why PWM vs PAM and TN/VA vs IPS produce distinct waveform signatures. This informs sensor setup choices (R, V_CC) and display tuning (e.g., overdrive LUTs).