This blog post comes from Dr Gavin Williams, Senior University Teacher with both MEE and EEE departments at the University of Sheffield - firstname.lastname@example.org
The Electronic and Electrical Engineering (EEE) department conducts extensive optoelectronics research . This research focus influences our undergraduate and post-graduate teaching, where we make extensive use of The Diamond’s teaching cleanroom.
This facility was forced off-limits to students during the Covid 19 pandemic. Instead, we have been running live online laboratory (lab) sessions. This article describes one such lab, in which we investigate the electrical and optical properties of light emitting diodes (LEDs). Even though the laboratory has now fully reopened in-person, a similar activity has been used for Recruitment/Outreach and has been delivered to over 100 sixth-form students during lunchtime ‘Taster’ sessions. The remote access lab technology enables participation by more students than ever before, increasing our geographical reach in encouraging access to higher education.
The system (Fig.1) has two light sources: a red/green/blue (RGB) LED and a white LED. The RGB LED is powered via an Arduino Uno running code that allows the pseudo-brightness of the three channels to be independent controlled by adjustment of the pulse width modulation (PWM) signals . The signal to the red channel is displayed on an oscilloscope. The white LED is powered via a source-measure unit (SMU). A fibre-coupled CCD optical spectrometer is positioned to capture the emission from both LEDs. A webcam is positioned to show the two LEDs, the fibre and, in the background, the oscilloscope display. The instruments are all under software control [3,4,5] running on a local PC. The PC can be accessed remotely, enabling the student to configure and control the various components.
There are a number of experiments that the student can perform with the system (Fig. 2). The typical current-voltage (IV) characteristics of an LED can be measured by configuring the SMU to perform a voltage sweep (Fig.2f). PWM brightness control can be investigated using the red channel of the RGB LED. The emission spectra of the RGB and white LEDs can be recorded (Fig.2e). With the prior collection of a ‘null’ background spectrum, it is possible to display the position of the lights in CIE colour space . This enable the student to explore the generation of white light by two methods: by combining RGB or by realising that the ‘white’ LED is, in fact a blue LED plus a yellow phosphor . One deficiency of the system is the fact that the camera is easily saturated, hence the colour is often bleached. However, once this fact is explained to the students, it provides an opportunity to discuss the operation of charge-coupled devices (CCD).
a) Camera view, showing LEDs, optical fibre and oscilloscope, b) Circuit simulator (Tinkercad), c) Arduino IDE code, d) Arduino serial monitor, e) Optical spectrometer interface, f) SMU control
The lab enhances some of the formal aspects of our courses. It help to demystify the physics of p-n junctions (diodes) by enabling them to equate photon energy to the band gap of the semiconductor. At a system level, it provides a simple example of PWM control. The students collect their own unique data sets, which they analyse and plot within a lab report – this is a much better than giving all students the same data set to analyse! Even more broadly, the lab gives the students an appreciation of the complexity of colour vision and of the world of detail behind components that can be purchased for just a few pence.