Circuit drives and quantities LEDs

Posted on Jul 27, 2012

The circuit mixes LEDs of different forward-bias requirements, yet keeps the loads reasonably well-balanced through use of a current mirror comprising transistors Q1 through Q4. It also eliminates the need for a separate current-limiting `ballast` resistor on each LED or string of LEDs and provides a common control point (IC1`s ADJ pin) for adjusting the LED intensities. Transistors Q2 through Q4 mirror the current in the diode-connected transistor, Q1. Note that the Q1 current-set string (LEDs D3 through D5) should have an equal or larger voltage than that of subsequent LED strings.

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