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.




Simulation and Working


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:



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