A Schmitt trigger is a type of electronic circuit that converts an input signal into a binary digital output based on hysteresis. It has two trigger points: an upper trigger point (VUTP) and a lower trigger point (VLTP). The purpose of these trigger points is to provide noise immunity and prevent rapid switching between the two output states.
The upper and lower trigger points in a Schmitt trigger circuit are determined by the resistor values (R1 and R2) and the supply voltage (Vcc). The trigger points can be calculated using the following formulas:
Upper Trigger Point (VUTP):
VUTP = Vcc * (1 - R2 / (R1 + R2))
Lower Trigger Point (VLTP):
VLTP = Vcc * (R2 / (R1 + R2))
Where:
VUTP is the upper trigger point voltage.
VLTP is the lower trigger point voltage.
Vcc is the supply voltage.
R1 is the resistance connected between the input and the non-inverting terminal of the op-amp.
R2 is the resistance connected between the non-inverting terminal and the output of the op-amp.
To understand how these formulas work, let's consider the operation of a basic Schmitt trigger circuit:
When the input voltage is below VLTP, the output of the op-amp is saturated to the positive supply voltage (Vcc).
As the input voltage rises above VLTP, the output remains at Vcc until the input voltage crosses the upper trigger point (VUTP).
Once the input voltage crosses VUTP, the output of the op-amp switches to its negative saturation voltage (usually ground).
The output remains at ground until the input voltage falls below VUTP.
When the input voltage falls below VLTP, the output switches back to Vcc.
The resistor values R1 and R2 determine the threshold levels at which these transitions occur. By adjusting the ratio of R2 to (R1 + R2), you can control the hysteresis and the width of the input voltage range over which the Schmitt trigger switches states. This hysteresis helps in noise rejection and stable operation.
Remember that component tolerances and other factors can affect the actual trigger points in a real circuit. It's a good practice to simulate the circuit or perform experimental measurements to verify and fine-tune the trigger points according to your specific requirements.