Automatic samplers represent a convenient way to gather rain samples for isotope (δ<sup>18</sup>O and δ<sup>2</sup>H) and water quality analyses. Yet, most commercial collectors are expensive and do not reduce post-sampling evaporation and the associated isotope fractionation sufficiently. Thus, we have developed a microcontroller-based automatic rain sampler for timer-actuated collection of integral rain samples. Sampling periods are freely selectable (minutes to weeks) and the device is low-cost, simple, robust, and customizable. Moreover, a combination of design features reliably minimizes evaporation from the collection bottles. Evaporation losses were assessed by placing the pre-filled sampler in a laboratory oven with which a diurnal temperature regime (21−31°C) was simulated for 26 weeks. At the end of the test, all bottles had lost less than 1‰ of the original water amount and all isotope shifts were within the analytical precision. These results show that even multi-week field deployments of the device would result in rather small evaporative mass losses and isotope shifts. Hence, we deem our sampler a useful addition to devices that are currently commercially available and/or described in the scientific literature. To enable reproduction, all relevant details on hard- and software are openly accessible.