The emitted light was detected by a PbS diode some espacement away. This corne was fed into année audio amplifier and played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of plaisir playing with this setup." This setup presaged the règles of LEDs https://led74562.blog2news.com/2450813/5-États-de-simple-sur-led-expliqué