Bi-Color Indicator Circuit

Posted on Aug 6, 2012

With SI open, base bias is supplied to Q2 through a voltage divider (formed by R2 and R3), thus turning on the green element in the LED. That indicates that power is being supplied to the project. If you close SI, current through R1 biases Ql on, thereby grounding the voltage divider and turning off Q2. That reverses the flow of current through the LED, which causes its red element to light. That indicates that the circuit is under power and SI (really a DPDT switch), whose remaining section controls another circuit, is active.

Bi-Color Indicator Circuit
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In this circuit, a bi-color LED is used to indicate when a circuit is under power and the status of SI. In that way, the LED does the job of two indicators.

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