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city Dogstar Digital, LLC

  • Alternating LEDs From Single Wire of Flip-Flop

  • Created: Mar 12, 2017

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The intent of this circuit is to alternate between 2 LEDs with a single "wire" depending on the state of the Q output of a Flip-Flop IC. This method allows the assignment of one task for each output, and reduces their individual current requirements.


The circuit was initially designed to be used with a Flip-Flip in toggle mode that is controlling two "devices" - an analog switch IC, as well as a dual colored LED.

Operation: While Q' is low, the switch is off, and with Q high, the transistors are biased, and only the red LED illuminates to indicate the switch's off state. When Q' goes high, the switch turns on, and the Q output goes low, grounding the transistor bases, turning off the red LED, and turning the blue on.

Q=0 => L1 (Blue)
Q=1 => L2 (Red)

A D-Type or JK Flip-Flop IC may be used for this application. The "devices" attached to the Q outputs can be swapped depending on device requirements (toggle or set/reset states). This circuit will also work with other logic devices with a High/Low Q output capability.

LED colors can be changed, and the current limiting resistors (R3 & R4) may be adjusted to suit. Whatever the case, L1 is lit while Q is low, and L2 is lit when Q is high. The values chosen above were intended to greatly reduce the LED current for 1/8W SMT resistors, as well as lower and balance the perceived LED brightness to fit to the application for which it was originally designed.

If 220k resistors are not available, alternatives can be used for R1 & R2, but values that are too low may cause faulty or undesired results. This circuit operated correctly with values as low as 100k. Using values as great as 1M also worked, but further reduced the LED brightness.

If your system voltage is different, all R values may need to be adjusted accordingly. This circuit was only tested at 9vdc.



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