Despite the advances in dextrous hand mechanics over the past decade, fingertip force remains predominantly controlled by flexor cable tension and joint position. A modular fingertip normal force sensor is developed here from basic semiconducting elastomer devices. Construction techniques are defined to shape standard Interlink Force Sensitive Resistors within a conformal mold of the human thumb and provide standard plug-in capability. Current-to-voltage electronic circuits are incorporated to linearize output and minimize the total sensor package. The sensor system is installed on each of three fingers and demonstrated to provide full control in elementary grasping strategies of a robotic hand.