The post explains a simple LED flashing beacon circuit which exactly simulates a rotating police beacon light by generating alternating pulses of suddenly rising and fading illumination effect on the connected LED. The idea was requested by Mr. Ankit Agarwal
The Circuit Request
Can you advice on how to make the following circuit A single flashing LED circuit, in which the LED flash simulates that of beacon ie when the LED flashes, first it glows slowly, then to full intensity, and then fades.
This gives the effect of a rotating beacon OR just like the lights on an aircraft tail.The circuit is intended to be used in the car (as a flashing rear light) so the circuit may function on 12v and the LED may be bright enough to be visualized clearly so it may be 1 watt smd led.
The Circuit Design
The proposed idea of a single LED rotating beacon lamp simulator can be implemented using the above shown circuit.
Here the IC 4017 and the IC 555 together are configured to generate a sequential chasing high logic across the 10 pinouts of the IC 4017.
The IC 555 is wired as a standard astable which feeds the clock or the flashing signal at pin14 f the IC 4017.
The IC 4017 responds to these clocks and generates a shifting high logic across its 10 outputs from pin#3 to pin#11.
These pinouts are integrated via individual diodes, and the common terminal can be seen connected with the base of a TIP122 transistor.
This transistor includes a 1 watt LED across its base emitter points which enables it form an emitter follower configuration with the LED.
This means that the LED will be supplied with a voltage level that may almost equal to the base voltage of the TIP122, and if this varies, the LED supply can be expected to vary accordingly.
The resistors connected across all the shown outputs of the IC 4017 is selected with an incrementing order or in way such that it forms an incrementing potential divider with reference to the preset resistance which may be seen across the base and ground of the TIP122.
Therefore as the 4017 IC generates the shifting or chasing high sequence across its pinouts, the resistors with reference with the preset resistance value generates a correspondingly increasing or decreasing potential difference at the base of the TIP122 transistor.
This effect in turn allows a varying potential difference to develop across the LED, which responds to this and produces the required sudden rise and decay effect and viec versa on the LED simulating a rotating beacon light.
The speed at which this happens can be set or adjusted with the help of R2.
The light intensity on the LED can be adjusted by suitably setting up the preset at the base of the TIP122 transistor.
The values of the resistor across the pinouts of the IC 4017 may be selected and swapped as per the user preference for generating different random flashing effect with due experimentation.
A rough simulation effect of the above explained rotating beacon simulator circuit using LED may be witnessed as given below.