This paper describes a symmetrical, simple pair circuit that can harness thermal noise energy to operate correctly on a 'single-dopant device' and its application to a full adder circuit. The single-dopant device is an atomic scale solid-state device and can be used, for example, as an actual platform for a single-electron circuit. We here aim to fabricate actual single-dopant circuits with a deterministic doping method. As a first step toward this, we design a possible circuit on the device and test its operation by Monte Carlo computer simulations. The simulation results confirmed correct circuit operation and showed that the device exhibited thermal-noise-harnessing abilities. Moreover, we also found that the simple circuit can operate as a full adder circuit. We believe that we will successfully fabricate a simple, actual single-dopant full-adder circuit in the near future.