Temperature Range Indicator Using a Window Comparator

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, lighting 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 low (Over Temp LED) while the
other two LEDs remain off.

Light Level Indicator Using a Window Comparator

The second example below uses a LDR (light dependent resistor) to indicate
some desired light level. The LDR has a large dynimic range and varies in
resistance from less than 100 ohms on a cloudy day to over a megohm in total
darkness. A 2K pot was used to adjust the window range for usual room light
conditions. This setup might also be used to indicate sunrise/sunset conditions.

The proto board picture below shows the circuit wired to measure light
level using a LDR and 2K potentiometer. The green (window) LED is lit
indicating the light level is about right to take the picture.

More information on voltage comparator details can be found here: Voltage Comparators