op-amp PID controller Here's an old-school PID controller based on simple op-amp circuits that I put together a week or so ago. Schematic. Note that the DG418 switches are connected wrong – they were not used. Milled PCB coated with PRF202 thru-solder clearcoat. Schroff front-panel to fit 19″ 3U rack enclosure. Measure twice – drill once. The top switch resets the integrator, while the bottom switch enables the output.