Circuit switches bipolar LED

Posted on Oct 24, 2012

The circuit in Figure 1 represents one method to switch a bipolar, two-color LED using an SPDT mechanical switch or relay. This circuit wastes power and does not work properly if the power-supply voltage is not substantially more than the sum of the LEDs` forward voltages. The circuit is, therefore, marginal, to the point of being unusable, with a 5V supply and a red or green LED, which typically has a total forward voltage of 4V. You can use a circuit resembling a flip-flop (Figure 2) that doesn`t suffer the disadvantages of the circuit in Figure 1. It adds only one VCE(SAT) voltage to the VF of each LED, so plenty of headroom exists with a 5V supply and a series resistor to control the LEDs` current.

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