Picture of Light activated blinking LEDs
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.
brsram5 years ago
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?
sabre (author)  brsram4 years ago
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...
viper5184 years ago
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
PoisonMondo5 years ago
ry shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome
PoisonMondo5 years ago
ry shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome
PoisonMondo5 years ago
try shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome
PoisonMondo5 years ago
try shorting the ldr make sure the pots away from flammable stuff and the smoke and smell sets of smoke alarms buts its awsome
ashikch5 years ago
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??
ashikch5 years ago
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 !! 
dave926 years ago
you never said in the parts list that you need a 330uf capacitor
EngrJayze6 years ago
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
sabre (author)  EngrJayze6 years ago
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.
EngrJayze sabre6 years ago
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
sabre (author)  EngrJayze6 years ago
also make sure your transistors pins arent mixed up and check for anywhere that you might have short circuited your transistor.
sabre (author) 6 years ago
due to lack of funds, the IR versions arrival has been postponed.
tchiseen6 years ago
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?
sabre (author)  tchiseen6 years ago
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.
blast24696 years ago
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.
sabre (author)  blast24696 years ago
Thanks for commenting. I'll see if I can add a few more steps for multiple LEDs in the next couple of days.
bobbyk8816 years ago
What would happen if the led lash activated the flash again?
sabre (author)  bobbyk8816 years ago
Actually, all the LED would do is turn it off. The circuit turns on in the absence of light.
bobbyk881 sabre6 years ago
I thought it would be cool if if flashed long enough to trigger a second flash. so basicly the first flash would go off then the same LED would flash twice as fast.