Light Activated Blinking LEDs





Introduction: Light Activated Blinking LEDs

About: Student @ University of North Texas working towards a Mechanical and Energy Engineering Major. I do all kinds of fun little projects. I'm currently trying to scavenge old pictures so I can publish a few more...

In this Instructable I will show you how to put together a flashing LED circuit that turns on when you wave your hand over it, blinks for a second or two, and then fades out.

I got the schematic for the flashing circuit from steven123654's instructable which can be found at:

I have since modified the circuit to include a light dependant resistor that is used as a momentary switch.

Step 1: The Idea

Besides being a fun project, and a great first LED project, if you make 5-10 of these circuits and put them together, it creates a pretty awesome wave effect as you pass your hand over each module.

I found the idea for this circuit on youtube. someone had made a table of LED modules that flashed as they cast shadows on the table. It looked pretty cool but I decided that it would be more fluid looking if there were more individual modules instead of large squares of LEDs all activating at once so I went to work and came up with this circuit.

Here's a list of the recommended parts:

-a breadboard
-100K ohm potentiometer
-a 3904 NPN transistor
-22 micro Farad capacitor
-1000 micro Farad capacitor (minimum, I use two caps and one is a 2200 uf)
-an LED
-a light dependent resistor
-two 1K ohm resistors
-one 100 ohm resistor
-a 555 timer IC
-a bunch of wires to connect it all

I'm pretty sure all of these parts can be bought at RadioShack.

If you already have the stuff check out the schematic on the next page to get started.

Step 2: The Flashing Circuit

This page shows the basic timer circuit used. It's a bit less complex than the full schematic and you may find it helpful to take it one step at a time, just to make sure you get it right.

Step 3: The Schematic

This is the schematic for my modified flashing circuit. as you build it, feel free to change out parts for others of different values. The potentiometer will definitely need to be adjusted, but after some tinkering you will get it right.

Make sure that it is in moderate to bright light when your playing with it. I'm not sure if the potentiometer can adjust far enough to allow it to work in darker rooms.

You may have to refresh the page to see the notes on my schematic.

Step 4: The Challenge

Hope you guys have fun with the project. If you come up with any cool uses/variations for it I'd love to know!



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    Sorry, I am confused. Is it 330 uF or 2200 uF or 22 uF? The image notes says 2200 and 1000 uF, the list of recommended parts says 22 and 1000 (in parenthesis it says (I use 2 caps and one is 2200 uF) while the schematic shows 330 and 1000 uF. Which one is it? Also, what is the voltage used?

    1 reply

    sorry I didn't get back to you sooner. Tha actual values are those shown on the schematic. The other values mentioned were alternates. I just failed to mention that I didn't use them all at the same time...

    Can anyone tell me the parts that I need and how to hook up 10-15 leds in a row and have them light 1 at a time down through the row of leds and to vary the speed. Thanks

    ry shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome
    ry shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome
    try shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome

    try shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome

    dude! its working but the thing is LEds are blinking when there is shadow but not after the shadow is removed as given in the schematic ??how to make that blinking after the shadow is removed??

    dude! i tried this circuit but LEds are not blinking!! :(
    but LEds are glowing when there is a shadow on the LDR ..I tried a lot with pot and thn removed a resister with 7th pin but still there is no blinking Im thinking to change 1000uF capacitor to 2000uF ?? pls help me !! 

    you never said in the parts list that you need a 330uf capacitor

    i am using this circuit for my project...i get a flashing LED all the time if i put a power supply to it..the LDR doesn't seem to do anything..can you help me? I need to finish this as soon as possible.. I can't understand how to connect the Potentiometer and LDR..can you provide a high quality image of your actual breadboard so that i can see where did i go wrong..hope to hear from you soon..thank you

    3 replies

    sorry I took so long getting back w you. I really don't think a picture of the assembled product is the best way to fix any problems, but here are some suggestions. 1. make sure you have a high resistance LDR 2. check connections 3. are you using an npn or pnp transistor? 4. make sure you are only using 2 if your pots pins If you think your using to low resistance of an LDR try shining a bright light on it. If that makes it stop blinking, its an LDR problem. If all else fails, try taking it apart and putting it back together again.

    Thank you for answering my inquiries..I opted to change some values to make my circuit work.. I also figured out that the LDR I used is only 10mm with 15K resistance..I also have some issues with the Pot's connection..but all are fixed now..It's already my summer vacation..thanks for your circuit

    also make sure your transistors pins arent mixed up and check for anywhere that you might have short circuited your transistor.

    due to lack of funds, the IR versions arrival has been postponed.

    Do you think it would be possible to use instead of a LDR, A PIR Motion sensor, to make sometihng like the Interactive Coffee table?

    1 reply

    Funny you should mention it, my next plans include using an IR LED and IR detecting tranistor to activate the circuit. Although it will be slightly more complex, it will be much more reliable. I will try to get a schematic for an IR circuit up within the week.

    I used your schematic and it worked perfect for me, no errors in drawing cause it worked, lol. Would like to see an add for different rates of flash for multiple LEDs in the project. very nicely done, it was easy to follow and a nice layout.

    1 reply

    Thanks for commenting. I'll see if I can add a few more steps for multiple LEDs in the next couple of days.

    What would happen if the led lash activated the flash again?