Undergraduate Measurements of Planck’s Constant using Light-Emitting Diodes (LEDs) are widely employed in current-physics education; however, many classroom variations still exhibit demonstration characteristics or insufficient consistency among different users or students. Developed a reductio ad absurdum lab routine to fix the apparatus shape, changed nominal LED wavelength to be measured by spectrometer peaks; And defined the cutoff voltage by linear extrapolation on the rise section.-voltage curve. Five visible LEDs with nominal wavelengths of 405, 470, 525, 590, and 635 nm were characterized spectrally and measured electrically under controlled conditions. The protocol was executed by six student groups, each completing five independent trials for every LED, yielding 150 threshold records in total. Under the standardized protocol, the pooled threshold-voltage-frequency relation remained strongly linear, with a fitted slope of 4.14 × 10-15V·s, an intercept of -0.064 V, and R2= 0.999. The corresponding estimate of Planck’s constant was 6.63 × 10-34J·s. Group-wise estimates ranged from 6.60 × 10-34to 6.67 × 10-34J·s, with a mean ± SD of 6.63 ± 0.03 × 10-34J·s. Robustness analysis further showed that the relative error remained below 1.2% when ambient illuminance was kept below 5 lx and the voltage step size was maintained within 10-15 mV, but rose to about 5.8% at 50 lx and 30 mV. Variance ranking identified wavelength assignment (44%) and threshold determination (38%) as the two dominant sources of remaining uncertainty. These results show that the classic LED experiment can be rewritten as a quantitatively stable undergraduate laboratory protocol. Its value lies in making spectral calibration, threshold definition, repeatability, and uncertainty control explicit parts of the teaching task rather than hidden assumptions of a convenient classroom demonstration.
