Electronic Design

Window Comparator Drives Bicolor LED

Using just one bicolor LED, this window comparator can display the status of an incoming voltage (see the figure). The LED is green when the voltage is within limits, red when it's low, and flashing red when it's high.

In the setup, U1 is wired as a conventional window comparator. Typically, both of U1's outputs are low when the voltage is in tolerance. As a result, there's a high at U2A-3 and a low at U2A-4, lighting the green LED in D3. The flash clock, U2D, remains disabled because pin 13 is low.

If Vin falls below the acceptable limit, the output of U1B goes high. This flips the states of both U2A and U2B via D2, lighting the red LED in D3 to indicate an undervoltage condition. The flash clock still remains disabled. If Vin rises above the limit, U1A-1 goes high instead. This also flips the states of U2A and U2B via D1, lighting the red LED. However, U2D-13 also goes high and starts the flash clock, causing the red LED to blink. This signifies an overvoltage condition.

R1, R2, and R3 are selected to give the desired window. Note that, for the flash clock to work, U2 must be a Schmitt-trigger device. D1 and D2 can be any small-signal diodes.

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