Presented here is a flexible and low-cost setup for demonstrating X-ray, electron or neutron diffraction methods in the classroom. Using programmable spatial light modulators extracted from a commercial video projector, physical diffraction patterns are generated, in real time, of any two-dimensional structure which can be displayed on a computer screen. This concept enables hands-on experience beyond simplest-case scenarios of scattering phenomena, and for the students, the transfer to the regime of visible light greatly enhances intuitive understanding of diffraction in real and reciprocal space. The idea and working principle of the modified video projector are explained, technical advice is given for its choice and successful modification, and a Python-based open-source program code is provided. Program features include the design and interactive manipulation of two-dimensional crystal structures to allow a straightforward demonstration of concepts such as reciprocal space, structure factors, selection rules, symmetry and symmetry violation, as well as structural disorder. This approach has already proved helpful in teaching crystal diffraction to undergraduate students in materials science.