Simple changes improve Schmitt trigger

Ron Patrick, ECM, Los Altos, CA -- EDN, 5/13/1999

The classical Schmitt trigger circuit that you'll find in textbooks provides some noise immunity, such as for the front end of magnetic pickup in position-sensor circuits (Figure 1a). You adjust the two threshold levels using R₁ and R₂ or R₂ and R₃. The circuit works acceptably but has two problems. First, each resistor affects both levels, making the circuit adjustment an iterative process. Second, the op amp's current-sinking ability limits your choices for R₃ and, hence, your ability to adjust the circuit. The improved trigger circuit in Figure 1b has independently adjustable levels; you first adjust R₁ and then R₂. The circuit also eliminates the sensitivity to the op amp's current-sinking capability. (DI #2361).