**The Request**

Hi sir,

I would like to build a brake lamp project using 1 watt high power leds, consisting of 12-15 leds.

the leds would light on dim parking lights and would light full on brake pedal press. kindly provide me a circuit..

kind regards,

Awan

**The Design **

LEDs are a lot economical than ordinary incandescent lamps or even the modern halogen lamps in terms of their efficiency, luminance and life.

Therefore even in automotive field we can now witness a rapid transition from the old filament type bulbs to the modern high bright LED lamps.

These are normally being implemented as brake lights and head lights in most modern and the new generation vehicles.

In the proposed automotive brake light circuit 1 watt high efficiency LEDs are employed for executing the ultra high intensity illumination.

We all know that basically today's all modern high watt LEDs require two crucial parameters in order to function correctly and safely, namely a current controlled supply and thermal or heat controlled assembly.

The first criterion can be implemented by using any modern sophisticated linear IC such as a LM338, I have discussed it elaborately in one of my previous articles high watt LED current limiter circuit.

For the second condition one can simply use a special aluminum base PCB mounted on a heatsink for assembling the 1 watt LEDs.

The circuit for the LED brake light may be witnessed above, and it looks pretty straightforward.

The LM338 is configured as a current limiter, where Rx determines the maximum allowable amps to the connected LEDs. It may be calculated using the following formula:

Rx = 1.25/LED current

When LeDs are connected in series there effective current consumption

is always equal to the rating of the one individual LED. Therefore in

the diagram each string would consume 350mA since this is the rating of

each 1 watt LED.

Combined current for all the three strings would be 3 x 350mA = 1050mA or approximately 1 amp

Substituting the above parameter in the formula we have:

Rx = 1.25/1 = 1.25 Ohms

Wattage = 1.25 x 1 = 1.25 watts

The resistors Ry which can be seen connected in series with the LEDs are actually optional, these may be included only for assisting the IC and providing proper balance across the LED strings.

It may be calculated using the following formula:

Ry = (Supply - LED total FWD voltage) / LED current

Since here the LEDs are specified with a forward voltage of 3.3V and 3 nos

are arranged in the series, the combined forward voltage becomes 3 x 3.3

= 9.9V

For reducing full loading of the LEDs, we can take the current at 300mA instead of the specified 350mA

Therefore Ry = (13 - 9.9) / 0.3 = 10.33 ohms or simply 10 Ohms

wattage = (13 - 9.9) x 0.3 = 0.93 watts or 1 watt

It seems we missed an important inclusion in the above diagram, it's the dimmed LED feature during the normal course of the vehicle and while the brakes are not applied.

The following diagram suggests how simply this may be implemented using a parallel connected resistor Rz, with Rx.

Here the values of the Rx and Rz may be identical but twice that of the above calculated value that is 1.25 x 2 = 2.5 Ohms. This would allow a 50% dimming of the tail lights while the brakes are in the released position.

If one desires to obtain further dimming of the LEDs Rx may be increased to 3 ohms or 3.5 Ohms, this would also mean lowering the Rz value proportionately such that the parallel value of the two resistors constitutes 1.25 Ohms.

Awan says

sorry to bug you, can you please explain me this

Here the values of the Rx and Rz may be identical but twice that of the above calculated value that is 1.25 x 2 = 2.5 Ohms. This would allow a 50% dimming of the tail lights while the brakes are in the released position.

If one desires to obtain further dimming of the LEDs Rx may be increased to 3 ohms or 3.5 Ohms, this would also mean lowering the Rz value proportionately such that the parallel value of the two resistors constitutes 1.25 Ohms.

Swagatam says

for optimal brightness the Rx + Rz combination should be = 1.25 ohms, therefore since both are in parallel there effective resistance must be 2.5 ohms.

but this will allow only 50% dimming, for more dimming you can use the following formula and work out the values of the respective resistors:

1/R = 1/Rx + 1/Rz

that is 1/1.25 = 1/Rx + 1/Rz

Swagatam says

In the above comment I meant to say,

optimal brightness when brakes are applied….and 50% dimming when brakes are released…

Awan says

if i use 13 or 14 leds, how would this work out then..

Swagatam says

you can use either 12 or 15 LEDs… not 13 or 14.

12 is already shown, for 15 add one more string, and calculate Rx, Rz as explained in the article and the comments

Awan says

What is 6A4 sir

Swagatam says

it's a 6 ampere rated diode

Awan says

thank you for the diagram and details, appreciate your passion for helping us world wide,

i will get back to you for any help or query.