Automatic samplers represent a convenient way to gather rain samples for isotope (δ¹⁸O and δ²H) 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. Evaporative 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.