Polymerase chain reaction (PCR) is a cornerstone technique in molecular biology and clinical diagnostics. However, conventional PCR systems are often bulky and prohibitively expensive, limiting their use in resource-limited settings. In this work, we present a portable, low-cost PCR instrument designed to overcome these challenges while providing fast and accurate thermal cycling. The system features a compact four-well aluminum heating block integrated with a semiconductor thermoelectric cooler and a heated lid, all controlled via an Arduino UNO platform and a piecewise variable coefficient PID algorithm. The device achieves heating and cooling rates of 1.78°C/s and 1.52°C/s, respectively, and maintains temperature accuracy within ±0.55°C. The power-bank powered prototype measures 210 × 140 × 105 mm and weighs 670 g, making it ideal for resource-constrained applications. Validation experiments, including successful amplification of kelp genes, yielded results comparable to conventional commercial instruments. Open source design files and detailed build instructions are provided under the MIT license, providing a cost-effective and accessible solution for expanding molecular diagnostic capabilities in resource-limited settings..