Finger Position Sensing with a Photodiode Array

This device finds the X and Y position of a finger in a 2"x2" square area. The resolution is about 256 counts per axis and the update rate is 2ms. The board output is a 0-5V analog signal for each X and Y position.

For each axis, there is a 768-pixel photodiode array (TSL1406R) illuminated by laser light. The laser light is produced by a "line generator" laser, which is nothing more than a laser pointer capped with a cylindrical lens. The cylindrical lens creates a triangular area (not a cone) of divergent light. To redirect the divergent light into parallel light in the sensing area, a 2" cylindrical Fresnel lens is placed between the laser and the photodiode array. This parallel light creates a distinct shadow on the photodiode array and allows for a a full 2"x2" working area.

The center of the shadow that the finger casts on the photodiode array is found and output over I2C to MAX517 D2A chips, which output the X and Y analog values.