LED Obstruction Light Circuit

Obstruction lights are warning lights that we see at the top of tall structures like towers and skyscrapers, installed for indicating the aircrafts and other flying objects about these obstructions.

These lights alert the flying aircrafts regarding the minimum height they should maintain above these tall structures to avoid a possible collision and accidents.

Obstruction lights are mostly red in color so that they can be visualized from maximum distance and even during foggy conditions. These can be a continuously illuminated type of lamp or a flashing, revolving beacon type of lamp.

In this this article we discuss about an easy construction of a powerful LED based obstruction light system, using minimum parts, and efficient working.

The idea was requested by Mr. Jerry as given below:

Circuit Specifications

I have a medium intensity obstruction light that has gone faulty. It’s input voltage is 48 VDC and it’s power is 60 W. It has four circuits that has 12 LEDs per circuit. It also has an LDR which is supposed to switch the light off during the day time and ON during the night.

Now because of the damaged components which I couldn't find their ideal numbers, I want you to design another circuit for me that will be able to perform the same function as before, remember that it flashes (it goes on and off) flip flop. The four different circuits have their supply from the 48VDC.

The four circuits I guess work in two ways: The upper part and the lower part. Two circuits controls the upper part while the other two controls the down part.

The flash should be about 2 sec interval (on and off) that should be continuous, it has a photocell also.

Design a circuit that will be able to control the upper and lower parts of the system at the same time and make a provision should there be need to separate the upper part from the lower part. The power is 60W/48VDC.

Circuit Analysis

Analyzing the above description we are able to conclude the following assumptions.

It seems the 4 circuits are 4 separate but identical LED drivers, employed for controlling current for the 4 LED groups separately. The separate drivers ensure that all LEDs together can never fail in case of a malfunction.

The 60 watt power is for all the LEDs combined, therefore each 12 LED group should be rated at 5 watts. In other words the current through each 12 LED string can be 0.12 amps, or 120 mA.

The inclusion of an LDR and also a photocell appear confusing, so we'll ignore the photocell and use only an LDR for the required automatic day night switching.

Circuit Design

As explained above, the 4 circuits can be 4 LED drivers, or to be precise current controller circuits for safeguarding the LEDs from over current.

However, a deeper analysis shows that a 120 mA LEDs may not require a special current controller, and a resistive current limiting may be quite sufficient. We consider the input supply 48V DC to be relatively constant.

The LED that we can select for this obstruction light circuit project are 2835 SMD LEDs for optimum brightness. The technical details can be studied from the from data:

2835 SMD LED Specifications

Forward Current: 120 mA to 150 mA
Forward Voltage: 3.1 V DC
Luminous Flux: 10 to 15 LM
Power: 0.5 watt

Calculating Current Limiting Resistor

The current limiting resistor for each of the series 12 LED group can be calculated from the following formula:

R = Vs - Total FWD Drop / Limiting Current

where Vs is the supply voltage = 48 V
Total Fwd drop = 12 x 3.1 = 37.2
Limiting current: 0.12 amperes

Therefore,

R = 48 - 37.2 / 0.12 = 90 Ohm

Wattage of the resistors will be (48 - 37.2) x 0.12 = 1.2 watts or 1.5 watts rounded of.

Using a Transistor Astable for Flashing the LEDs

Since the obstruction light LEDs need to be blinked in a flip flop mode, a transistorized astable circuit appears to be a good choice. This is because a transistor based astable offers two alternately oscillating transistor outputs which could be used for blinking two sets of LEDs separately.

Complete Circuit Diagram can be seen below:

Parts

R1, R4 = 22 k Ω
R2, R3 = 78 k Ω
R9, R10, R11 = 6k8
R12 = 100 k preset
R5, R6, R7, R8 = 90 Ohms 1.5 watt
C1, C2 = 1 μF/60 V
T1, T2, T5 = BC547
T3, T4 = IRFD110
D1, D2 = 1N4148
LDR, photoresistor = typically, 30 k in day light under shade
LEDs = As discussed above, 48 nos.

How it Works

The proposed LED obstruction light circuit working can be understood with the following point:

The 4 resistors at the center, along with C1, C2 and T1, T2 form a basic transistorized astable multivibrator circuit. The main feature of this astable is its low cost, and quick failproof functioning as soon as it's powered. Once switched ON, T1 and T2 alternately begin switching at a frequency rate determined by the base resistors R2, R3, and the capacitors C1, C2.

These specific components can be changed as desired for altering the switching rate of the T1 and T2. Higher values will produce slower switching rates and vice versa.

Another advantage of this astable is that it can be dimensioned to operate at higher voltages, such as 48 V here, without incorporating special voltages regulator stages. Furthermore, we are able to achieve a two alternately switching outputs, which may not be possible with IC based astables, unless an external BJT is applied.

The MOSFETs T3, T4 are used for switching the LEDs in accordance with the blinking signals from the respective astable BJT collectors.

The LEDs are divided into 2 groups of 24 LEDs each, which can be configured on top and bottom of the obstruction light cabinet. These groups of the LEDs then go on flashing flip flop continuously as long as they are powered.

The T5 stage is the day night automatic switcher circuit. When sufficient light is available during day time, T5 gets biased through the LDR low resistance, and keeps the two MOSFETs switched OFF by grounding their gates.

As darkness falls, the LDR resistance increases, which gradually depletes the base bias from T5, ultimately switching it OFF.

When this happens, the MOSFETs become enabled and they begin switching the LEDs alternately, quickly serving the intended function of an obstruction lamp.

During day time the maximum consumption of the circuit is not more than 5 mA.

Circuit Specifications