The PCB handwriting board sensor is introduced in this paper. With the model of PCB handwriting board, the numerical simulations of the electric fields are conducted and the driving modes fitting for weak signal detection are optimized using the results of the simulations. And then, the driving source and weak signal detection system based on the principle of phase-locked amplifier are designed and realized, which focus on the design principles and methods of AC current source, narrow band pass filter, high gain low noise amplifier, dual phase lock-in amplifier, etc. The method of equipotential line intersection with linear interpolation is presented and verified experimentally. The simulation and experimental results show that the designed system can realize the location of the contact point with the PCB printed board.