Fig 2Pi cluster rackHover to set the fan's PWM dutyPWM 60% · ≈1020 rpm
120 mm fan, simplified: blades turn with the PWM duty (shown slowed)Side panels, rods and four trays, 28 mm apartAn empty four-layer rack for Raspberry Pi boards. Two side panels 133 mm apart overall, 165 mm tall and 152 mm deep carry four steel rods and four clear trays; each tray has four standoffs on the 58 × 49 mm hole pattern. A 120 mm fan covers the front. Fan speed is a straight line from 300 to 1500 rpm for the sketch, not a measured curve.