Online teaching, which has been the only way to keep teaching during the pandemic, imposes severe restrictions on hand-on robotic courses. Robot simulators can help to reduce the impact, but most of them use high-level commands to control the robots. In this paper, a new holistic simulator, HoRoSim, is presented. The new simulator uses abstractions of electronic circuits, Arduino code and a robot simulator with a physics engine to simulate microcontroller-based robots. To the best of our knowledge, it is the only tool that simulates Arduino code and multi-body physics. In order to illustrate its possibilities, two use cases are analysed: a line-following robot, which was used by our students to finish their mandatory activities, and a PID controller testbed. Preliminary tests with master's students indicate that HoRoSim can help to increase student engagement during online courses.