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　mm3　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.　©　2025　The　Author(s)