In the last three years, a number of Irish primary schools have been using LEGO Mindstorms technology in order to investigate the use of project-based learning as an alternative teaching tool. This has involved the use of LEGO bricks combined with standard electronic motors and some commercial sensors (e.g. temperature). In order to develop this project into the area of science education, we have developed a range of miniaturized optical sensors, which are compatible with the LEGO platform. In particular a working oxygen sensor has been designed and fabricated. The principal design features were compatibility with the programmable LEGO platforms and robustness for classroom use. This sensor uses the method of intensity quenching to determine oxygen concentration. This sensor has been demonstrated in a learning environment. Similarly a carbon dioxide sensor has been developed. In addition, simple colour sensors have been produced. The aim of developing such sensors is to familiarise students with the concept of colour detection and to introduce them to the basic principles o f spectroscopy. A specific use for the colour sensor has been identified, in the role of a pH sensor. A simple method of detecting nickel sulphate using colorimetric diffuse measurements has been demonstrated. The performance of both sensor types has been evaluated.