Question

Please answer the problem below for all parts. Please show all work and write clearly. Thanks.

The circuit below is a temperature range indicator using a Window Comparator circuit. Discuss when you would use this type of analog circuit and when you would use a microcontroller based circuit like that of Lab 4 shown below. A window comparator usually employs 2 comparators with one output indicating the input is somewhere between two limits. In these examples, a third comparator is added to display all three conditions where the input is in the center range, or higher, or lower. The first example uses a thermistor to indicate the temperature is near 68 degrees within about +/- 5 degree tollerance. The thermistor measures 33K at around 68 degrees and varies about 3570 ohms over a range of 10 degrees. Using a 12 volt supply, the thermistor voltage will be 6 volts in the center of the range. As the temperature increases 10 degrees, the total resistance falls 3750 ohms, the current will be 12/ (66K-3750) =193uA and the thermistor voltage will be 193u * (33k-3750) = 5.65 volts. This represents a voltage change of (6-5.65) 350 millivolts for a 10 degree change. The center resistor of the window voltage divider must then drop 350 millivolts. Using 20K resistors on the top and bottom of the window voltage divider produces a current of (6 - (.350/2)) / 20K = 291uA, and the center resistor is .350/291u 1.2K When the temperature is in the center of the window range, the voltage at pins 5 and 6 will be 1/2 the supply voltage, or 6 volts in this case. The voltage divider (20K, 1.2K, 20K) produces a voltage of around 5.8 at pin 4 and 6.2 at pin 7. Since the voltage at pin 5 (6 volts) is more positive than the voltage at pin 4 (5.8 volts), the output at pin 2 will be a high level. At the same time, the voltage at pin 7 (+ input) is higher than pin 6 (- input) causing pin 1 to also be a high level. This condition produces a high (12 volt) level at pin 10 (input) which produces a low level at pin 13, 1ighting the window LED indicating the temperature is in the window range. As the thermistor voltage moves above the upper 6.2 limit, pin 1 will switch low, extinguishing the window LED and illuminating the (Low Temp) LED. Similar action happens as the thermistor voltage moves below the lower 5.8 limit causing pin 2 to switch 1ow (Over Temp LED) while the other two LEDs remain off 33K 20 K 20 K LED LED K LED LM339 1.2K 10 N914 12 Thermi st or 33K at 68 degrees 20 K Bill Bowden 03-28-12 Lab 4 Circuit 5 volts from PICkit 3 14 2 O 13 lk 4 0 330 LED Place Thermistor away from other components so it can be heated and cooled. LED 330

Answer 1

Circuit 1 with OP AMP consumses more power compared to Circuit 2 with PIC.Hence circuit 2 can be used in low power requirements.

Circuit 2 is simple in construction.