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08-05-2009

LED driven tail/brake Light Cluster

Constant current circuitry
12V Battery operation

Circuit diagram:

LED driven tail/brake Light Cluster

Parts:

R1

10K 1/4W Resistor

R2

33R 1/4W Resistor (See Notes)

R3

15R 1/4W Resistor (See Notes)

   

D1

1N5819   40V 1A Schottky-barrier Diode (See Notes)

D2--D13

LEDs   High brightness, high efficiency red types (See Notes)

   

Q1

BC547   45V 100mA NPN Transistor

Q2

BC337   45V 800mA NPN Transistor

   

SW1

SPST   Tail Light Switch

SW2

SPST   Brake Light Switch

Comments:

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.

Notes:

  • 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).
  • Please note that the brake current is obtained by paralleling R2 and R3 values.
  • 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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