Schematic Reference

LEDs are used more and more in motor vehicles, replacing the standard incandescent lamps because they are more energy efficient and have a much longer life expectancy. In this article we describe a simple LED tail light that has been specifically designed for motorcycles, scooters and mopeds. There appears to be a significant need among motorcyclists for rear lights with LEDs, as evidenced by the many messages on this topic that turn up in various internet forums. The circuits that accompany these messages are often very rudimentary and therefore not very robust.

After reading some of the literature concerning the use of LEDs in motor vehicles, it appears that the most common reason why LEDs still fail is the incorrect and/or insufficient use of series resistors.

In poorly implemented circuits there are often a number of LEDs connected in parallel which are all fed from a single series resistor. Because of small variations between LEDs, one LED can quickly give up the ghost. This causes an increase in current through the remaining LEDs and can easily lead to a domino effect, ultimately resulting in the failure of the entire circuit.With high-intensity LEDs, a small variation in current is immediately obvious as a large variation in light output.

This has to be taken into account when designing a circuit. This is important because when the engine rev speed goes up, the on-board voltage increases significantly. It would appear that you were braking when you actually opened the throttle instead.LEDs need mainly a constant current.That is why most circuits choose to drive LEDs from a constant-current source.

This circuit has been designed to operate both as a motorcycle rear light and as a brake light. This requires two different currents. Because the voltages measured on the author’s motorcycle varied from 10.5 to 15 V and because two different currents are required for the total of 17 high-intensity LEDs it was not possible to use only one constant-current source.

The idea was to turn the strongly varying DC voltage into a nice constant voltage first and then turn that into a constant current through a number of series resistors. The problem that is highlighted in many forums is the fact that the signal for the brake light is a positive voltage. It would require a lot of work on the motorcycle to change this. That is why the decision was made for a de sign that regulates the voltage on the chassis side, with the aid of a negative voltage regulator, a 7908. The disadvantage of this arrangement is that an additional chassis wire is required; normally the minus side of the lamps is directly connected to the chassis of the motorcycle.

However, the advantage is that both the + from the rear light as well as the + from the brake light can be directly connected to the LEDs.The ‘lamp’ con sists of a centre part with nine round, red,5-mm LEDs (HLMP EG08 Y200) wi th positioned around that eight oval ,r e d L E D s HLMP AD61 of 5 mm.The round LEDs D12 through D20 which have qui te a narrow radi ation angle are connected in series in sets of 3. Three of the se ‘strings’ are connected in parallel and each string has its own series resistor.

The oval LEDs D4 to D11 which have a wide radiation pattern are connected with two in series, so there are therefore four strings connected in parallel. These ensure with their wide radiation angle of 110 degrees that the rear/brake-light is also clearly visible from the side.The oval and round strings are connected to the brake contact via diodes. When the brake is operated all the strings are presented with the +12V from the battery via the series resistors. The light intensity therefore depends on the current that flows as a result of the series resistor (and the voltage drop across the diodes).

When the brake is not operated, the LEDs strings are still connected to the positive voltage of the battery, but this time via additional resistors R1 and R2.Because of the value of these resistors,the current is much lower and therefore also the light intensity. The intensity of the brake light can be adjusted using the series resistors (R3 to R9) in each of the individual strings,the brightness of the rear light is selected with the additional series resistors R1 and R2.Diode D1 has been added to protect the circuit from reverse connection of the power supply voltage.Electrolytic capacitors finally provide filtering for the fairly large varying,and not so clean, voltage.

The circuit was built into a silver coloured tube by the author. The electronics are mounted on two pieces of prototyping board, one behind the other,in the tube. The front (visible) PCB holds the LEDs and the series resistors. The LEDs are arranged as indicated next to the schematic. The 9 round LEDs are mounted in the middle of the rear light in a square pattern. The oval LEDs are mounted in a circle around the square.

The second PCB contains the remaining parts and the regulator.You can modify the circuit to your heart’s content by adding more strings, each fitted with its own diode and two resistors (a series resistor such as R3) and a resistor to +12 V (such as R1).The total current (when braking), must not exceed the maximum rating of the voltage regulator, this amounts to 1 A.

Author : Marcel Ulrich Copyright : elektor