A microcomputer will be used to provide automatic temperature regulation for a shower faucet using the system shown in Figure 28. The user sets the desired tem-perature between 30°C and 50°C by a sliding knob attached to a 1-kft variable resis-tor. The user sets the water volume by manually adjusting the cold-water valve.
The hot-water valve is controlled by the computer to provide the selected temperature. The particulars are that a two-channel multiplexer is attached to an 8-bit ADC with a 5.0-V reference. The temperature sensor is a thermistor with a characteristic given by Figure 29 and a 5-mW/°C dissipation constant.
The control valve op-erates from a 0.0-5.0-V signal from an 8-bit DAC. ADC input is via PORT 301, DAC output is via PORT 302, and control signals are exchanged via a bidirectional PORT 300, as shown in Figure 28. The port-select lines go low when that port is addressed. It has been determined that PI control is needed with gains of K,, = 2.5%/% and IC/ = 0.08%/(%-min).
a. Design the appropriate analog hardware for getting the thermistor temperature measurement and the user temperature setting into the computer via the ADC.
b. Complete appropriate digital connections in Figure 28 for control interface of the ADC, DAC, and multiplexer.
c. Prepare a flowchart of the software required to take samples every second.
d. Develop the control equations for implementation of the error detector and specified PI control mode. Remember that the thermistor resistance change is nonlinear with temperature.
e. (Optional) Write a program in assembly language, C, or any other appropriate language to implement the design.
