String LEDs are now very popular as decorative lights which are available in different colors at very  low cost. It gives very brilliant colour display sequencing in four channels. The blinking pattern is controlled by a programmed chip and the switching is achieved by a high volt SCR .String LEDs show automatic Multi pattern changing display. Its working is really amazing and breaking all the conventional circuit designs. It shows that how a chasing and blinking light pattern can be created using a very simple method. Thanks to China technology  for giving such a simple and low cost decoration. Let’s open the tiny circuit box and see what is inside.

String LED is low cost versions and each set has four strings each having 25 or 30 LEDs. These LEDs are directly powered from AC and its power consumption is very low compared to the 6 volt bulbs genrally used in sequential displays.


The circuit board of string LED is directly powered by the 230 volt AC and the chip gets DC through  diode rectification and capacitor smoothing. Each string of LED is driven by a high volt SCR and its gate trigger is obtained from the Programmed chip. The most commonly used SCR is PCR 406 which is a high performance planar diffused PNPN device intended for low cost high volume applications. Its peak offset voltage is 400 volts and on state current 0.5 A.


How it works?

The LEDs in each string are serially connected and each has 2M current limiting resistor.

Four strings are independently operating with common Phase line. From the circuit case, 5 wires are coming out. Of these, 4 wires are used to connect LEDS and the 5th wire is common Neutral. That is, the strings of  LEDs are connected to the Cathode of SCR and the Anode of SCR gets Phase. All the strings are then connected to the common neutral. Thus when the SCR triggers by getting pulse from the Chip, current passes through its Anode to Cathode and then through the LEDs and the circuit completes. Since the LEDs are working in AC, noticeable flickering is seen but brightness of LEDs overrides these flickering. Each string has 25 LEDs making the whole set 100 LEDs.


The wires and sleevings are water resistant and can be used in outdoor applications.

19 thoughts on “STRING LED

  1. hello sir/mam!! Thanks for posting.. this is very informative and i got interested to make one of it. Just want to know if there is any alternative component that could be use instead of the CHIP?? by the way what is the purpose of that chip?? thanks.. hope you could reply soon..

    1. Hello Edz
      The Chip is meant for changing the lighting pattern. It is a programmed chip like the COB( Chip On Board) used in toys. There is no substitute for the Chip, but a circuit using Programmed IC will do the same function

      1. thanks sir for your response! I’m about to make a string led for our school project. however, that chip is not available in our area. that’s why i am asking for alternatives. sir if i remove the chip and just make a wiring connected to scr. Do you think string LED will still work? will the blinking effect will be removed?
        thanks again!!

  2. Sir,
    Thanks for article.
    sir, can we replace the PCR 406 transistor with any other?

  3. It’s really a cool and useful piece of info. I’m glad that you shared this useful information with us. Please keep us up to date like this. Thanks for sharing.

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  7. sir your articles are very useful to me plz guide to make aac to dc adaptor range
    i/p 230v o/p36v/1.2amp

  8. Dear vhhp
    Thanks for your feedback. The decorating lamps using Bulbs have the same circuit and working. The only difference is, it uses 3 volt bulbs without the current limiting resistor. Bulbs are serially connected in 4 channels with common neutral. Interestingly it is possible to drive four 60 W bulbs with the same circuit. Remove the bulb strings from the PCB, and connect 4 holders to the 4 channel points( one wire each). Connect the second wire of all the holders to the common neutral. The bulbs show the pattern lighting similar to the string bulbs. It can be used to light X mas stars.

    1. Thank you very much for your comparative statement. Being a professor of Zoology, you have mastered the Electronics in such a way that you are able to guide many students too. Thanks a lot. And coming to my new query, it is clear from your reply that I should be careful of how much wattage the sequential bulb circuits are using. So, please consider giving the WATT Meter circuit also
      Thanks and regards

  9. Dear DMK,
    Another query please! There are many circuits which work on AC. If I want to know what the WATTAGE of those circuit is, I am finding it difficult. Can you please show me a way out, by giving a WATT meter. I suggest, a simple LCD calculator circuit be used which has most of the required circuitary and the calculator too can be used for other calculating purposes of the calculated wattage of the circuit in test. Thank you again for your kind responses.
    V. Hari Hara Prasad

    1. Dear Hari Hara Prasad
      Domestic power supply is 230 volts 50 kHz or 110V 60 Hz in some country. The current consumption depends on the Wattage of the instrument used. Current consumption can be calculated using the formula

      I = W /V

      I is the current in amperes, W is the wattage of instrument and V the 230 volt power supply.

      For example a Compact Fluorescent Lamp (CFL) rated 11 watts consumes 0.04 ampere current in one hour at 230 volts.

      Wattage of an instrument can be calculated using the formula

      W= V x I
      see this link
      I will try to make a Watt meter

    2. Hello Prasad Power consumption depends on the Wattage of a device. This is important in the case of high resistance devices like heater, Iron etc.Watts can be calculated using the load in the watt meter. Simple watt meter circuit for measuring milli watts is available.I think, Clip Multimeters intended for measuring load current can do the job. If the load current is determined, then it is easy to calculate the Watt.

  10. Dear DMK,
    Thanks for your article. Can you write a comparative article on the same String of bulbs from the same China company?

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