Alternating LED Flasher

We came across a bunch of simple and cheap LED flasher kits at a surplus sale fair recently.

The Alternating Dual-LED Flasher Kit shown here is from Kemo Electronic GmbH.

This do-it-yourself hobby electronics kit is a 2-LED alternating flasher.

In this little kit, two differently colored light emitting diodes (LEDs) will flash alternately.

The flashing speed of the LEDs (flashing velocity) is adjustable through a small onboard potentiometer. 

A 9V battery is required to run the kit.

Naturally this inspired us to go back to the basics of simple LED flasher circuits based on common transistors, because it is not only fun, but also a good learning experience for many beginners and students.

Now you can use a small perfboard to build your own alternating light emitting diode flasher.

Here is its circuit diagram (click to enlarge):

See the list of suggested components below.

• D1: 1N4007 Diode
• C1: 100uF/25V Electrolytic Capacitor
• LED1: 5mm or 10mm Red LED
• LED2: 5mm or 10mm Yellow LED
• T1 & T2: BC547 NPN Transistors
• C2 & C3: 10uF/25V Electrolytic Capacitors
• P1: 20KΩ Trimpot
• R1 & R4: 620Ω Resistors
• R2 & R3: 33KΩ Resistors

Note that these part numbers are not very critical. Therefore you can change the values of the resistors and capacitors (R1-R2-R3-R4 and C2-C3) to alter the flashing velocity and/or the operating voltage.

This LED flasher can be used in flashing adornments, nameplates, signboards, miniature toys, models etc. 

Although a 9V battery is preferred, it will work from any 6 to 12V dc power supply. The total power consumption at 9V dc input is around 20mA only.

The little circuit is called an astable multivibrator (AMV) as it does not remain in one of its two well-defined states.

That is, the circuit oscillates between one of two states where the right LED only lights up when the right transistor is active. Likewise, the left LED turns on whenever the left side transistor is active.

So, the circuit switches between these two states to an indefinite extent as long as it is powered!

Using the same values for C2 and C3 and R2 and R3 results in symmetric on and off times for both LEDs.

In principle, you can work out the on/off times of the LEDs (T) by using the formula:

T = R x C = 33KΩ x 10uF  =  33000Ω x 0.00001F  = 0.33s (neglecting the potentiometer's resistance).

So, each LED in this circuit will stay on and off for approximately 0.33 second.

OK, now you have a simple alternating flasher circuit which is a fantastic project for getting started with electronics engineering.

Not to mention it comprises only a handful of commonly available and inexpensive components.

The circuit might seem trivial at first, but upon closer inspection, it introduces a fantastic playground for figuring out and practicing computations in real-world electronic circuits. Have fun!