The Scottish Higher Grade Physics syllabus has recently been altered to include some detailed work on photodiodes. This article is from the Scottish Schools Equipment Research Centre (SSERC) to assist teachers in Scotland in introducing the topic. We include this article as we feel that it has a wider use for older students. Many of the techniques described would make good (and cheap!) practical investigations for ‘A’ level physics and electronics students. Much of the data described will be invaluable to those doing project work in electronics.