Design Ideas: July 7, 1994

An integrator powers the light source in Fig 1's position-sensing circuit, eliminating errors arising from variations in the source's intensity. This circuit's accuracy is essentially that of the sensor itself, beating designs that use a more complex analog divider.

Amplifiers IC_1A and IC_1B convert the two detector currents from D₁, a photosensitive strip, into voltages in the conventional manner. IC_2B subtracts these two voltages to derive the output signal V_O. V_O is proportional to the position of the spot cast by the light source upon D₁.

IC_2A integrates these two voltages along with a current from a regulated 5V source, driving the light source via Q₁. This closed-loop scheme supplies illumination proportional to the sum of the two input currents, providing a fixed-scale factor for the signals supplied to the difference amplifier, IC_2B. Using a monolithic resistor network for all the 10-kV resistors in the circuit enhances temperature stability. (DI #1452)