The final transistor is not saturated, it is a current regulator.
If the base-emitter voltage of the final NPN is 0.7V and its base voltage is 1.25V regulated by the two diode-connected transistors then the 12 ohms emitter resistor regulates the collector and LED current at (1.25V - 0.7V)/12 ohms= 46mA.
The circuit is used to regulate the current because different color LEDs have different forward voltages so a simple resistor to limit the current cannot be used.
Wait a minute! Purple is red and blue at the same time. Cyan is green and blue at the same time. Yellow is green and red at the same time. White is all 3 colors at the same time. So all they needed was one current limiting resistor for each of the 3 color groups.
You're mistaken Ian, it's a constant current source, which can be switched ON/OFF by the LH transistor. The two diode connected transistors provide the voltage reference for the constant current source, and the emitter resistor sets the current (to about 60mA).
I doubt that an LCD screen with LEDs for its backlight uses white LEDs and color filters because a white LED is actually a blue one with a yellowish phosphor on top and does not produce all the colors that are in real white light.
I have place two diodes on the base of my BC182 with a 10k to 5v I have place a 10 ohm resistor on the emitter and now the current is limited to 34mA... The screen looks bright enough so It'll do!!
Because it's best to feed LED's with a constant current, and it makes it easily switch-able as well (and presumably PWM could be used to vary the current via the same transistor).
Because it's best to feed LED's with a constant current, and it makes it easily switch-able as well (and presumably PWM could be used to vary the current via the same transistor).
Thanks for your help Nigel.... First I tried a lone 15 ohm resistor... About 91mA..... I quickly disconnected it and did the current sink..... As I said its running on about 34mA.... Three channels... so about 11mA per channel....