Window comparator II LM324
Electronic

In this project, you're going to build a window comparator using an OP amplifier. A window comparator is a comparator that detects levels within a set range or window rather than simply distinguishing between levels above and below a set point.

When you finish wiring, rotate the control volume fully counterclockwise and turn power ON. LED 1 should be lighting. Rotate the control volume slowly clockwise. How has LED 1 changed?

It should have gone out, and again on. Take a look at the schematic. There are three voltage points, VIN, Va, and Vb, which switch the window comparator ON and OFF.

Vb is constant as it is clamped at 4.5 V by a diode, but VIN and Va change with rotation of the control volume.

When the voltage at the (+) terminal of U1 goes above that at the (-) terminal (Va), Q1 turns ON and LED 1 lights. Figure 1 shows the relations of the rotational positions of the control volume, different voltage points, and ON/OFF states of LED 1.