Problem 1: We would like to design a transducer for blood pressure measurements using strain-gauges. We mounted four strain-gauges on a membrane in such a way that a pressure change of 1 mmHg exerted on the membrane would introduce a 0.00025% change in the length of each transducer. Two of the strange-gauges are stretched and the other two are compressed by the blood pressure. We connected them in a bridge configuration and used a 5 Volt DC supply as an excitation source. The gauge factor for all the transducers is 2.1 and the resting resistance is 500 Ohms. We assume that the output voltage of the bridge changes linearly with pressure up to 200 mmHg.
What is the bridge voltage output for systolic (120 mmHg) and diastolic (80 mmHg) pressures?
Problem 2: Design a system for measurements of snoring sounds in heavy snorers using a piezoelectric crystal. The minimum expected frequency content in the signal is about 20 Hz. The crystal has a capacitance of 800pF. The combined input capacitance of the amplifier and the cables is 200 pF. What is the minimum value for the input resistance of the amplifier to be able to amplify the lowest frequency components in the snoring sounds?
Problem 3: We would like to design a thermometer to take oral temperatures using a thermistor. First we need to characterize our termistor. Its resistance at room temperature (To=20oC) is Ro=1 k and β = 4000 K. Assume that β does not change with temperature in the range that the thermistor is used. Your assignment is to plot the thermistor resistance for the range of 36 - 42oC at every 0.1 degree Celsius using Excel
Problem 4: A capacitance sensor with 1cm2 surface area will be used to record breathing sounds. The circuit diagram in Figure 2.9 from the textbook will be used. Input impedance of the amplifier is 100 MΩ. What is the x, the palate spacing, required to pass sound frequencies above 50Hz? The dielectric material between the plates is air.
Problem 5: Assume a dissipation coefficient of 4mW/ºC for the termistor in Problem 3. If we use this thermistor in room temperature with 0.6 Volt dropping across its terminals, what would the temperature rise in the sensor due to self-heating effect?
Problem 6: In Figure 2.23, for a glass fiber with n=1.62 and a cladding of n=1.52,
a. what is the critical angle (Qic) below which the light beam cannot stay inside the fiber?
b. If light is entering the fiber from air (n=1), what is the maximum incident beam angle (Q3)?
c. What would be the maximum incident beam angle if the cladding was removed and the fiber was exposed to air on the sides?
d. Can you explain why cladding is needed in optical fibers of communication?
Problem 7: What would be the band gap voltage needed for a material to fabricate an LED that can emit blue light?