This circuit was designed on request to drive a Light-cluster formed by several LEDs that can be mounted in the vehicle as a tail and brake light.
When SW1 is on, the cluster will illuminate at medium brightness. When brakes are operated, SW2 will be closed and the cluster will shine at maximum brightness.
These two brightness levels of the cluster are obtained by a constant current source drive formed by Q1 and Q2. The two constant current levels are set by R2 and R3 values.
The cluster can be formed by up to 12 LEDs as shown in the circuit diagram. Common cluster types usually range from 5 to 10 LEDs.
Using the values shown above, stand-by current was 1mA; SW1 on = 20mA, SW2 on = 60mA.
Constant output current value can be changed by varying R2 and/or R3.
The formula is: R = 0.6/I (in Amperes).
Use high brightness, high efficiency red LED types of suitable size and change R2 and R3 values to suit LED’s Absolute Maximum Ratings.
Any Schottky-barrier type diode can be used in place of the 1N5819: the BAT46 type will be a very good choice
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