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Make this 2 Pin Bi-Color LED Flasher Circuit

Make this 2 Pin Bi-Color LED Flasher Circuit

This transformerless mains power supply circuit will allow flashing of a Bi-color 100 LED string, in an alternately switching red, green effect.

Using 2-pin Bi-color LEDs

The proposed circuit can be used as a Bi-color LED flasher, for generating an alternate red, green flashing effect over a string of 100 LEDs.



Bi-color LEDs are available in 3-pin and 2-pin variations, in our project we use the 2-pin Bi-color LED option for keeping things compact and much efficient.


Circuit Operation

Looking at the design shown above, we can see a simple configuration using a push-pull clock generator IC 4047.

The IC is used for generating an alternately switching pair of outputs, from the shown pinout#10 and 11 of the IC.

The frequency of these alternately conducting outputs can be set by appropriately adjusting the pot P1 and by selecting the desired range with C1.

The switching outputs can be seen configured with two oppositely wired SCRs, which are in turn hooked up with the Bi-color LED string across the mains input through a dropping high voltage capacitor C3.

The circuit also incorporates a transformerless power supply stage consisting of C2, D1, C4, Z1, for powering the IC with the required low voltage DC.

When the proposed 2 pin Bi-color LED flasher circuit is switched ON, the IC starts oscillating at the set rate across its pin#10 and pin#11 alternately, driving the SCRs at the same alternating rate.

The SCRs respond to these pulses and conduct accordingly, enabling the Bi-color LED string to illuminate through an alternately green and red color flashing effect.

Caution: The above circuit is not isolated from mains, therefore is extremely dangerous to touch in an uncovered and switch-ON positioned.

Parts list for the above explained Bi-color LED flasher

R1, R2, R3 = 1K

C1, C4 = 100uF 25V

C2, C3 = 0.33uF/400V

Z1 = 12V 1 watt zener

D1 = 1N4007 diode

SCRs = 2nos BT169G

LEDs = 100nos (for 220V input), 50nos (for 110V input) of 2pin, Hi-bright RED, Green Bi-color LEDs

Input: 220V/110V

Correction Update

The design shown above has a serious flaw in it. The SCR1 is configured wrongly and might not conduct as proposed in the explanation.

The following diagram using an DPDT relay appears to be the correct approach for implementing the above discussed Bi-color LED flashing operations:



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

14 thoughts on “Make this 2 Pin Bi-Color LED Flasher Circuit”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hi Swagatam,

    One of my friend request me to repair his bi-color (26 nos) LED series. It is china mfg low cost lighting operated in AC current through 4 diodes. Also there is Resistor attached with each LED. How the thing are working on that High voltage using only 4 diodes and resistors?

    • Hi Sunil, may be the resistor values are very high, could you tell me what value resistors are used? or you can investigate a little more and check what exactly are used in addition to the 4 diodes to step down the input current.

  3. Sir ,
    I did the test twice, i have use bicolour but only shining red color. What will be my fault in this? I'm waiting for your advice.

    • Hi Satheesh, yes, SCR1 is incorrectly configured in the first circuit, therefore the circuit might not work, I have designed another one which is shown at the end of the article, you can try that….or alternatively you can try using a 3-pin Bicolor LED with the first circuit by inverting the SCR1 and a few other minor modifications.

  4. Sir,
    Good day to you I looked at doing this. However, only shining red color. Not two-color fluorescence What is the solution? So please provide you suggestion I have used c1 20k pot

    • satheesh, did you connect the ground of the circuit with the SCR ground?

      and make sure both the outputs of the IC are oscillating.

      and if possible first check the LED series manually by changing the supply polarity with an external DC power supply and check whether or not the LEDs illuminate with their specified twin colors?

  5. Me again,

    What type of modification could be applied to the circuit so that the leds are fading from one color to the other instead of switching ?

  6. Hi, I'm assuming that this circuit is designed for main AC voltage of 220 V, right ? (Roughly 2V per led, 100 of them).

    What is the value of the potentiometer P1 ? As per the provided values, the switch between red and green would occur roughly 2.2 times a second, assuming P1 is at the zero mark. Putting a 10K pot would max out the switching rate to roughly once every 5 seconds or so. Which value are you using or tested with ?

    How much current will this deliver to the leds and how do you calculate it ?

    Isn't connecting the main ac neutral to the circuit ground a risky practice ? What if the mains AC neutral and line wire are reversed ? Wouldn't that expose the user to potential risks ?

    It should be CLEARLY stated that the ground shown on your drawing ISN'T the earth ground from the mains otherwise if the mains wires are swapped, there will be a good surprise when the user will connect his project. It will flash indeed, but not the expected way.

    • I have presented the concept only, rest can be investigated by the interested users.

      I am happy that most people here are courteous and respectful to me for my contributions and I acknowledge their immense love and trust and am thankful to them

      you can refer to the datasheet of the IC 4047 for finding the correct values of the pot and the capacitor.

      the ground in any electronic circuit refers to the negative of the circuit, that's an understood fact.

      I have provided enough warnings in this site regarding the consequences of playing with electronics and electricity without adequate knowledge and ignorance, so that makes everything clear.

  7. Hello sir……if I want to run 50 led Series and another 50 leds series in parallel total 100 leds then it will work

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