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12 January 2010
Author: Giorgos Lazaridis
Miniature 2.4 Volts LED Flasher

The tiny LED flasher under heavy test. The circuit runs day and night with two 1.2volts rechargeable batteries, since the 11th of January 2010, 20:00.

First of all, something about the 3V LED flashers. As everybody knows, AA and AAA size batteries have 1.5 Volts per cell. Thus, two of them connected in series will deliver 3 volts. Using normal or alkaline batteries, all 3 volts LED flashers should operate normally. If you happen to have a rechargeable battery on your hand, it is most likely that it does NOT have 1.5 volts per cell. Instead, they usually come at 1.2 volts. This is rather annoying especially when a 3 volts flasher needs to be powered. Usually they do not work. I tried 3 different circuits. Either they did not oscillate at all, or the LED power was rather low. So, i decided to make another circuit that operates also at 2.4 volts.

The circuit is ideal for safety rear flashing light on a bicycle. This is the main idea after all. That's why i used 7 high brightness LEDs. You can connect of course as many LEDs as you want in parallel, or even use one 10mm ultra bright LED, but the 7 LEDs can make a nice long flashing line. It can be seen from 100 meters away very clearly! Of course, it could be also used for decorating purposes or for security light when you go for a walk at night.

The Circuit

8 components! The complete circuit is made of 2 transistors, 5 resistors and one capacitor. It can be made extremely portable. The batteries will be significantly larger than the PCB itself. For more portability, coin-type batteries can be used instead. The power that this circuit needs is extremely low, and even very small batteries can have quite a long life.

On the 11th of January 2010, i turned on this circuit with a pair of rechargeable batteries, 2500 mA fully charged. I run this circuit day and night to see how long the batteries would last. The circuit kept running until the 23rd of March! That is 71 days of continuous operation, more than 1700 hours! Of course the LEDs were very dim the last weeks and could not be sued for security light, but it still flashed.

To make this application legal for on-road use, i had to check the Vienna Convention on Road Traffic. For the rear signaling light it says:

A light without a steady mode is considered approved only if it flashes at a constant rate of between 60 to 240 flashes per minute and has a luminous intensity of at least 4 candela.

The second pard is covered by far, as each LED is ultra bright with luminosity 11000 mcd. As for the first part, it describes the pulsing frequency. It must be between 1 to 4 Hz. My first design was around 1.2 Hz pulsing frequency. I chose such a low frequency, because i wanted to extend the battery life as much as possible. The result was rather... boring... The LEDs were flashing once per second. And believe me, it really does not sound as slow as it is. So, i decided to increase the pulsing rate up to 2.6 Hz, a little bit faster than the car's flashing direction lights.

Another circuit parameter that i experimented was the pulse positive period. This is the time that the LEDs will remain ON. This was rather difficult to choose. I had to find a setting that would keep the LEDs ON long enough to be seen, but not too long because they would drain the batteries faster. I ran some experiments in a dark road about 100 meters long. After many tries, i decided to keep the positive period between 10 to 14 mSec.

Bill Of Materials
 Resistors R1 Resistor 220 Ohm 1/4 Watt 5% Carbon Film R2 Resistor 220 Ohm 1/4 Watt 5% Carbon Film R3 Resistor 2.2 KOhm 1/4 Watt 5% Carbon Film R4 Resistor 4 MOhm 1/4 Watt 5% Carbon Film R5 Resistor 1 Ohm 1/4 Watt 5% Carbon Film Capacitors C1 47 nF Ceramic Capacitor Transistors T1 BC327 Switching and Amplifier Applications PNP Epitaxial Silicon Transistor T2 BC547 Switching and Applications NPN Epitaxial Transistor

Relative pages
• Basic transistor circuits
• The LED theory
• The transistor theory of operation
• How to make a light / dark activated switch - 3 different circuits under the microscope