By Jitlada Sunnu, Siriya Satsanapitak, Kheamrutai Thamaphat, Chutima Oopathump, Piyarat Bharmanee, Pichet Limsuwan

Year 2014


The key parameter for determination of Planck’s constant using LED is the voltage required to turn on an LED. Many experiments have been used a voltmeter to measure the voltage. The accuracy of voltage obtained depends on experience of experimenter in deciding when the LED stared to emit. Therefore, in this work, we have proposed a computer based approach that utilizes curve fitting to determine the voltage required to switch on an LED. The experimental setup consists of a circuit with a 6 V dc source, a 100 Ω resistor, a variable resistor (0-100 Ω), a capacitor (0.14 F), and five LEDs. A voltage probe connected to MultiLogPro Data Logger and a computer are used for monitoring the voltage across the capacitor during the discharge of capacitor through each LED. Then, the threshold voltage which is the minimum voltage to turn on the LED obtains from curve fitting. By plotting the graph of photon energy versus frequency for the five LEDs, the Planck’s constant is achieved from its slope. The Planck’s constant from our experiment was found to be (6.625 ± 0.003) × 10 -34 J•s. It can be concluded that this method is reliable and accurate with a percentage error of 0.015.

Download : Experimental set for measuring the planck’s constant using LED