Touch Switch Operation Using Schmitt Trigger
The touch switch operates based on a Schmitt trigger with a hold contact, constructed from IC1a, IC1b, and R3. IC1b’s output is looped back to IC1a through R3, ensuring both gates maintain their stable states. When the circuit is at rest, meaning the anode of D1 is low and the cathode of D2 is high, the gates maintain their status.
Understanding Schmitt Trigger Circuit
A Schmitt trigger is a comparator circuit with hysteresis, achieved by applying positive feedback to a comparator or differential amplifier’s noninverting input. This active circuit converts a digital input signal to an analog input signal. The term “trigger” is used because the output maintains its value until the input changes significantly enough to cause a switch. In the non-inverting configuration, when the input surpasses a predetermined threshold, the output goes high. When the input drops below a different (lower) threshold, the output goes low. If the input falls between the two levels, the output retains its current state.
Touch Switch: A Simple Tactile Sensor
Introduction to Touch Switch
A touch switch is a special type of switch that activates when touched by an object. These switches are commonly found in metal-clad lamps, wall switches, and public computer terminals. In essence, a touchscreen comprises an array of touch switches, making it a fundamental type of tactile sensor.
Switching Process Explained
Imagine Output A is initially low. When the touch contact is bridged by a finger, the voltage across R8 increases. This voltage rise generates a brief pulse due to the R1-C1 configuration. This pulse results in a high input for IC1a via D1. Consequently, both the output of IC1b and output A go high, maintaining this state as explained earlier.
State Change and Reset
Simultaneously, the voltage across C2 gradually rises. Once C2 is sufficiently charged, the circuit becomes primed to change state during the next pulse across R1. This pulse causes the output of IC1a to go low, leading IC1b’s output to follow suit. As a result, the voltage across C2 decreases, returning Output A to its low state and effectively resetting the circuit back to its initial state.