On Monday the MIWOBA (Monitoring Infant Work Of Breathing Apparatus) capstone te

Question

On Monday the MIWOBA (Monitoring Infant Work Of Breathing Apparatus) capstone team presented their system, which includes a Flex Sensor to measure "retractions" in which the abdomen narrows as the lungs expand. They could count the breaths per minute reliably if they converted the shallow, sinusoidal signal into a square wave with the same timing as the fundamental sine wave, while ignoring some obvious high-frequency noise. Let's assume that the Flex Sensor produces a 0.5-Hz signal with an amplitude of 1 V and an average of 2 V, and has added noise with a frequency range of roughly 5 to 20 Hz with an amplitude of about 0.3 V. Because of the noise, we apply a Schmitt trigger, which is essentially a comparator with two trigger thresholds. To make this trigger circuit we will use a single-supply, rail-to-rail operational amplifier with power supplies of 5 V and 0 V. In the trigger circuit schematic below, note that the R4 rightarrow Vout wire does not connect electrically to the negative supply wire. Initial here (or on your answer sheet) to indicate that you understand they are distinct voltages: _____. Explain briefly how the trigger works, especially why it has two thresholds. Derive the formula(s) necessary to calculate the two thresholds as a function of the power supply voltages and four resistances. Assume that the resistors are numbered as shown below. Given R_1 = 47 k Ohm, R_2 = 33 k Ohm, R_3 = 10 k Ohm, and R_4 = 51 k Ohm, determine the two thresholds for this circuit. Show, by means of a labeled sketch, whether the thresholds are adequate for the Schmitt trigger to perform its desired function.

Explanation / Answer

B ) Schmidt trigger takes an analog input and converts it into digital output.

In the given circuit diagram, a non-inverting schmidt trigger circuit is implemented.

When the input signal voltage, Vin >Vupper-threshold , the circuit will set the Voutput to HIGH.

When the input signal voltage, Vin <Vlower-threshold , the circuit will set the Voutput to LOW.

BUT When Vlower-threshold<Vin <Vupper-threshold , the circuit wont change the Voutput.

So, with the use of the the two thresholds the trigger circuit is able to remove noise from the input signal.

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