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. This paper describes two such sensors that have been developed and fabricated for use with the LEGO platform, a collaboration between the MIT Media Lab and the National Centre for Sensor Research. 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. In addition, simple color sensors have been produced. The aim of developing such sensors is to familiarize students with the concept of colour detection and to introduce them to the basic principles of spectroscopy. The performance of both sensor types and preliminary classroom results are reported.