After the development and use of several scanning tunneling microscopes for different environments (air, UHV and low temperatures) we designed and built a simple STM. Only standard and affordable components are used, to make this microscope equally applicable to a student laboratory. The controller generates a very smooth scan with a resolution much greater than the digital-to-analogue converters in the control computer. In the last 2 years it has proved to be a reliable system with good resolution. It has been used, for example, to study dye molecules on silver-bromide surfaces.