Build Electronic Circuits

Electronics explained in a simple way

Blinking Christmas Lights

By 43 Comments

blinking-christmas-lights-finishedChristmas is coming up, so why not set aside an hour or two to build this blinking Christmas lights circuit?

This circuit is easy to build and it’s something you can put to use right away. I built this and hung it in the window, something my girlfriend loved!

The blinking part of the circuit is made up of only 4 components. Then you’ll add as many lights as you want.

How To Blink Lights

There are several ways to blink lights. I’ve previously written about how to blink a light using transistors or a relay. Another method is to use a microcontroller.

But, I think one of the easiest way, at least in terms of the “easiness” of building the circuit, is using a 555 timer.

555-chip

The 555 timer is a chip that you can use for creating a signal that turns on and off repeatedly. That’s exactly what you need for blinking Christmas lights!

Just connect this signal to some LEDs and a resistor to control the LED currents.

Blinking Christmas Light Circuit Diagram

In this circuit you’ll only need 4 components + the LEDs and the resistor.

To get the blinking effect, you need to set the 555 Timer in astable mode. Then it’s output will change between 9V and 0V repeatedly.

Here’s the full circuit (see further down for parts list and explanation):

555-christmas-leds

blinking christmas light components

Parts List

Part Value Description
U1 LM555 555 timer
C1 4.7 µF (10V and upwards) Capacitor
R1 10k Ohm Resistor
R2 100k Ohm Resistor
R3 68 Ohm Resistor
L1 – L8 Standard Output LED Light-Emitting Diode

Setting The Blinking Speed

Two resistors and a capacitor is all you need to set the blinking speed. You can calculate the number of “blinks” per second with this formula:

Blinks = \frac{1.44}{(R1 + R2 + R2) \times C1}

But, an even easier way is to use this calculator. Then you can try with whatever values you have at home and see what result you can expect.

With the values I’ve chosen, you should get about 1.5 blinks per second. And the time the LEDs are on should be about the same as the time they are off.

Setting The LED Brightness

When you light up an LED you should always use a resistor, so that you can control how much current that goes through it.

10-15 mA is usually a good current amount for standard output LEDs. In this circuit, you have eight of these in parallel. That means you need 80-120 mA in total for the eight LEDs. Which also means you want this much current to flow through the resistor. The current going through the resistor will be the amount of current that will be shared among the LEDs.

If this reasoning does not make sense to you, I recommend you read my article What You Need to Know About Current, Voltage and Resistance.

The LEDs I used had a forward voltage of about 2V. So, a 68 Ohm resistor is a good match. You can learn how to calculate the correct resistor value for any LED in my article about current limiting resistors.

If you’re thinking about adding more LEDs, keep in mind that the 555 timer only supports around 200 mA of current. For example 20 LEDs with 10 mA.

Soldering the LEDs

I used a cable from some old headphones to connect the LEDs. Then I used heat shrink tube to cover up the solder joints.

blinking christmas lights circuit

My Blinking Christmas Lights

If you built this circuit, let me know in the comment field below. Also, any questions about modifications or anything, use the comment field below.

Reader Interactions

Comments

  1. Hi Oyvind, thanks for this tutorial. I love the way you explaned things, practically, from the basics, with tools very helpfull (pictures, web sites…, circuit…). Have a nice – blinking – day !

    Reply

  2. The LED current limiting resistor was determined assuming that all 8 LEDs are connected. If a wire breaks i.e. all 8 are not connected you could destroy those lit. My apologies if I missed this caution.

    That being said I admire your efforts to educate! Keep up the good work!

    Reply

    • This is actually a strip board and not a breadboard. So I’ve soldered the components onto the board (which makes the project more permanent).

      I’ve replied to your email with an image, but not sure if it helps…

      Best,
      Oyvind

      Reply

  5. Good article for beginners, thank you!

    But what about values used in the formula for resistor and capacitor? Were they in kOhms or Ohms and mF or nF? I’ve used online calc you provided – it show absolutely different value.

    Reply

    • Hey,

      You can see the values in the parts list. kOhms for R1 and R2. µF (microfarad) for the capacitor. It gives you a frequency of 1.462. I just tried the caclulator and got this value.

      Maybe you used milli instead of micro for the capacitor?

      Best,
      Oyvind

      Reply

      • Hi.
        Using “http://www.ohmslawcalculator.com/555-astable-calculator” i also got 1.462Hz. But i was using formula you posted “=(1.44)/((10+100*2)*4.7)” and IT gives me “0.001458967” as a result.

        Please advise.
        Thank you.

        Reply

          • Oh…
            I’ve got it! Thank you very much! That is much handy than online calculator – formula is always with you :)

  7. Hi,

    I’m still new to this and I was wondering how do you determine the resistor values and capacitor value? The LEDs I am using for this have a forward voltage of 3V and I’m still not sure how many LEDs I am going to use. Is there a formula to find the values?

    Thank you.

    Reply

    • Hey Cinthia,

      For values that set the blinking speed, check the “Setting The Blinking Speed” section in this article.

      R3 sets the current for the LEDs.

      Let’s say you want three LEDs. Each LED needs current. How much depends on your LED, but 10mA is normal, so let’s go with this.

      3 LEDs with 10 mA is 30 mA.

      You need 30 mA through the resistor R3.

      You get 9V out from the 555 Timer. 3V of those 9V will drop over your LEDs. That means you have 6V left to drop over the resistor R3.

      Ohm’s law says R = V/I

      That’s 6V / 0.03A = 200

      So you need a resistor of 200 Ohms.

      Learn more about this calculation here:
      http://www.build-electronic-circuits.com/current-limiting-resistor/

      Best,
      Oyvind

      Reply

  8. Dear Oyvind,

    I have made the Christmas lights whitout problems and learned from it.
    I really like your tutorials and clear explanarion.
    Thank you so much.

    Happy Christmas to you and all the readers.

    Yvonne

    Reply

  9. hi oyvnd smashing circuit i replaced the nine volt battery with a phone charger it works great and i spread the leds further apart and built them into a christmas wreath for on my front every one is asking me where i bought it from great idear

    Reply

  10. This content is really helpful to make such a this type of light, because when we buy it, so its costly and when we make this light by self, it look more beautiful.

    Reply

  11. LED driving voltage ranges depend upon color. These voltage ranges do not necessarily overlap and probably depend upon the manufacturer as well. So if the voltage on a parallel design is too low for some LEDs the higher voltage ones will not light. If driven at the higher voltage, the lower voltage ones quickly burn out. So, you cannot necessarily drive a color mix with the circuit described. One internet source sells a selection of 5 LED colors. The voltage range for yellow and red is 1.8 to 2.3 volts and the range for blue, green and white ones is 2.8 to 3.3 volts. This circuit will not drive a mix of colors drawn from these two groups.

    Reply

  14. Thanks for the circuit! As one of the earlier posts suggests, the LEDs have to be the same colour (or at least similar forward voltage) to work welll.
    Is there a way use the 555 timer and separate the LEDs into two groups so that they’re on alternate cycles? (What I mean is group1=on & group2=off; group1=off & group2=on; and repeatedly cycling in that pattern.)
    Thanks!

    Reply

  16. GREAT ARTICLE
    NOW ASSUMING I WANT THE LED S TO BLINK IN A RANDOM ORDER OR OR RISING ORDER CAN I DO IT WITH 555 TIMER AS WELL?

    THANKS IN ADVANCE.

    Reply

    • Yes you can. But you can also just change the value of R3 to give you the current that you need for the number of LED you have.

      Best,
      Oyvind

      Reply

  19. Quick question. I’m still a bit new to this but how do you know the limit of LEDs you can use? For example, I have 8 LEDs that have 24mA each and I am planning on using a 9V battery. Each has a forward voltage of 3V.

    Thank you.

    Reply

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