Introduction: Simple Blinking LED Circuit

Picture of Simple Blinking LED Circuit

A very simple circuit that you can build to blink or flash LEDs. The circuit is built using transistors, resistors, capacitors, and LEDs. Of course you will need a breadboard, wire jumpers, and a power source. The parts list includes:

  1. PNP Transistor, P/N 2907A, qty: 2
  2. Resistor, value 470 Ohms, qty: 2
  3. Resistor, value 100k Ohms, qty: 2
  4. Capacitor, 10 uF, qty: 2
  5. LED, Qty: 2
  6. Breadboard
  7. Jumper wires

Let's get started:

Step 1: Add the Transistors

Picture of Add the Transistors

Add the two PNP transistors and the jumper wires from the power BUS to the emitter of each transistor. Because of the way I inserted the two transistors the emitter is on the left side of both transistors.

Step 2: Add the Capacitors

Picture of Add the Capacitors

Connect the two capacitors to the circuit. Connect the positive lead of the first capacitor to the collector of transistor 2. Next connect the negative lead of the same capacitor to the base of transistor 1.

Repeat the above process for the second capacitor. Connect the positive lead of the second capacitor to the collector of transistor 1. Connect the negative lead of the same capacitor to the base of transistor 2.

Step 3: Add the 100K Resistors

Picture of Add the 100K Resistors

Next connect the 100k resistors to the transistors. One lead of the resistor connects to the Base of the transistor, the other lead connects to ground. Do this for both transistors.

Step 4: Add the LEDs

Picture of Add the LEDs

Finally add the two 470 Ohm resistors along with the two LEDs. I added a picture of a transistor to identify the Emitter, Base, and Collector.

Connect one wire of the first resistor to the collector of transistor 1. The the other resistor wire then connects to the positive wire of the first LED. The negative wire of the LED is then connected to ground.

Follow the same steps for the other resistor and LED. Connect one wire of the second resistor to the collector of transistor 2. The the other resistor wire then connects to the positive wire of the second LED. The negative wire of the LED is then connected to ground.

Step 5: Supply Power and Watch the LEDs Blink

Picture of Supply Power and Watch the LEDs Blink

The last step is to supply power and watch the LEDs blink. I use a 9 volt battery and it worked fine.

For fun you can try other capacitor values to change the rate at which the LEDs blink.


SergeyS85 (author)2017-07-06

Thanks a lot for the nice guide!
And here is our circuit we've built today with my 7 yo daughter (she helped me a lot); we used different capacitors to make LEDs blink faster.

AshishA5 (author)SergeyS852017-12-19

Nice work

george1121 (author)2017-08-28

If I wanted to make the circuit without using a breadboard, where would I connect "ground"? thansk

george1121 (author)george11212017-08-28

This is what I got on internet search--so, the "ground" is the negative terminal of the battery?

"In most circuits these days, "ground/reference" is the most negative terminal of the power supply, so voltages elsewhere are positive relative to "ground".

dbsamurai made it! (author)2017-06-29

Accidentally used NPN transistors for mine, but I still managed to get it to work. Just had to change things around a bit, had to feed the positive power into the C and have the LED positive go into the E, with the LED negative going into the B (for anyone who makes the same mistake I did) but it turned out pretty good. There's a noticeable delay in the discharge rate, too, the blue light I used is on for slightly less time than the red one despite using identical resistors and capacitors for the two of them. Still this setup works REALLY WELL with LED strips.

lmcelhiney made it! (author)2017-06-10

Having no separate power supply which would output to a breadboard, I used the 5V GPIO pins from a Raspberry PI board and standard Female to Male jumpers. With the lower Vcc, I used 220 ohm current limiting resistors. (The time constant R and C remained the same.) Of course, I am writing this using the same RPI board running PIXEL.

RobertF264 (author)2017-06-02

hmm, my post vanished after deleting, hope this doesn't duplicate. Is there a way to make this setup without a breadboard and maybe streamline the wiring? I am looking into wiring up some LED lights to my bike and I wanted to use a hub to power them. Looks like the hub produces 6v 3w and the lights require 4.4 w for 15ft of continuous light length. I wont be using that much length so I was kind of lost on what the calculations would be and was going to wing it with the power match up considering the total produced is a bit less than whats needed. Anyway, it would be nice to get these lights lit up enough to help me stand out because I have a recumbent trike that sits roughly 26" from road to top of seat off the ground and I would like to be way more visible. I like tinkering but I was hoping you guys might be able to steer me (no pun intended) in the right direction. I understand the laws of thermodynamics (I think..) which states you cannot make more energy than the energy it takes to produce said energy (or something to that effect). Like taking a solar powered car and adding wind turbines on them. You would far sooner drain the battery trying to reach a speed high enough to reclaim anything substantial with the turbines. Thats at least the way I understand it, but back to the question at hand. Is something like this possible? Any help with this would be appreciated.

Mishainabox (author)2017-04-20

If I wanted to add 2 more LED's where would I place them?

aravindk39 (author)2017-04-03

Using audrino is easy

AnilS28 made it! (author)2017-03-21

My 12 year son made this, thank you for the detailed instructions

RichardO82 (author)2017-02-10

Does anyone know if i replace the leds with a buzzer will it work? Im trying to get a buzzer to buzz on and off instead of a constant buzzzzz.

keys-da (author)2017-02-02

This is a really useful circuit. Do you know what modifications I would need to make to get it to work using an npn transistor? Thank you.

shiny301 (author)2016-05-12

what is this for anyway

tgray1 (author)shiny3012016-07-21

We call them BLFNARs (pronounced "Blif-nars"), which is Blinky Lights For No Apparent Reason. A vital part of any electronics project!

CsabaB9 (author)tgray12017-01-10

Mwhahaha this answer made my day! Quite agreed!

Eh Lie Us! (author)tgray12016-09-28

Sir/Madam, that is a fantastic answer.

rashton2 (author)2017-01-01

What if I don't have the requisite transistor? Can I use a PNP transistor with a different P/N? And if so, what is the theory behind choosing a similar transistor and/or changing to different resistors and capacitors to make the whole thing work? Just a basic idea so that I can figure it out for myself with the materials I have.

bobdabiulder (author)2016-12-08

Is this the same as ?

JefeV1 (author)2016-11-14

Mine also won't blink. I've rechecked the wiring; as far as I can tell it's all good. If I remove the capacitors they stay on. These are "10uF 35v 20% radial-lead electrolytic" capacitors. Is that ok? Seems they have no bearing on the circuit? Thanks.

WKChai (author)2016-10-30

Does 25V 10uF capacitor ok to use? Mine does not blink driven with 4.5V.

XimoneW (author)WKChai2016-11-01

that capacitor value should be fine, though you may need to check your components/connections (specifically, that you're using a PNP instead of a NPN)

dug1000 (author)2016-04-16

This is a great Intractable! I note that you are using a 9V power supply and I would like to scale it down to use a 3V coin cell battery. Could you help me with the math?

XimoneW (author)dug10002016-11-01

most coin cell batteries are 3.3V, though that won't make a large difference here. The circuit in its current configuration should work for a coin cell battery, but the lights may be a bit dim--you can decrease the resistance of R2 to make the LEDs brighter (but be careful that you don't go over the LED's current rating).

KaseyW11 (author)2016-08-25

What is that you are hooking everything up it looks familiar

MuhammadI193 (author)2016-08-18

Excellent Work. Made it in first attempt.

PranavR9 made it! (author)2015-12-01

It's working

EmmyG3 (author)PranavR92016-05-09

How can I make two led flasher circuit works on 5 volt using variable resistor also sir

emre2563 (author)EmmyG32016-08-03

You can use same schematic but capacitor will be charging longer

Venkatesh BharadwajS (author)2016-06-26

I've connected the circuit exactly as explained above but still, both of them are just glowing. not blinking. What to do?

Be sure that you wait long enough or you can decrease capacitor size for less waiting.

HugoR17 (author)2016-07-27

I tried using and it worked. I change the capacitors to 100uF do increase the interval duration. But I made it with try and error. I know the time constant equation to a capacitor in parallel. How can I calculate the time that the a capacitor will be fully charged or discharge in series? Someone could "blink" me a light?

WilhelmH (author)2016-06-03

When voltage is first applied to the circuit the first capacitor starts
to charge and its voltage increases. When the voltage reaches the
amount needed to cause the transistor to conduct it's like toggling a
switch to short the other capacitor to ground. Then the other capacitor
starts to charge until its voltage is high enough to switch its
transistor and it shorts the other capacitor, which drops the voltage
low enough to shut off the other transistor. Back and forth the
capacitors short each other out by flipping the others switch.

ruliano (author)2016-01-20

May I ask if there would be a problem if I added additional LEDs?

tmichlovitch (author)ruliano2016-01-21

Shouldn't be a problem, as long as you have enogh voltage to drive the LEDs.

ruliano (author)tmichlovitch2016-05-16

Finally got around to building this. I have the correct interval of blinking, my voltage, and amount of LEDs. What I see though, is residual light on off cycles. What can I do to eliminate this? Diode? Thank you.

ruliano (author)ruliano2016-05-16

I am using 4001 transistors though. Should this matter? After I soldered the circuit, they will not blink now with 9V. ...hrrmm

shiny301 made it! (author)2016-05-12

it worked

errolc2 (author)2016-03-03

also whats the interval of two leds blinking?

errolc2 (author)2016-03-03

hello may i ask if im about to use a 555 timer instead. with 1 capacitor two leds and two resistor.. thanks

DeamonS (author)2016-02-04

I tried this but with 560 Ohms instead of the 470 ohms because I didn't have any 470 ohm capacitors. However, I got no blinking. I did get 2 constant led's (one was white and one was red). Should I expect blinking with 560 ohm capacitors?

DeamonS made it! (author)DeamonS2016-02-08

I got the right resistors and still no blinking. I swapped out my LEDs with some LEDs that came from the same package and it's blinking. Now I am going to get the kids to build it. Thanks.

tmichlovitch (author)DeamonS2016-02-08

Glad to hear you got it working. Have fun building it with your kids.

DeamonS (author)tmichlovitch2016-02-10

So, my 9 year old put this together on a breadboard I bought for her super cheap on eBay. She put it together, we apply power, only one LED turns on. I swap out every component, same thing. Switch to another old breadboad I have and it works fine. Stick back on her breadboad, nothing. Ever heard of a faulty breadboard? I am able to get constant LED with resistor to work on her breadboard, but not this particular circuit. What ups with that? Other 2 breadboard, circuit works fine. Her's constant lit LED.

DeamonS made it! (author)DeamonS2016-02-10

Got it. Had to jumper half the board power rails to the other. Interesting.

tmichlovitch (author)DeamonS2016-02-10

I remember having to learn that one the hard way too. Usually when I can't get something to work righ I break it down into smaller sections and make use of a multimeter to help trouble shoot.

rockfordray (author)2016-01-24

can you just eliminate the second circuit and the caps to make a single LED blinker?

tmichlovitch (author)rockfordray2016-01-24

The second circuit needs to stay. However you can replace the LED with a wire.

SudarshanG (author)2015-11-19

Anybody pls explain the working of this circuit??? I want in-dept working knowledge of this circuit..

BTW its great that 2 Blinking LED working in alternate. Superb..

tomatoskins (author)2015-03-02

These look awesome! Electronics are great! I've never thought to use this type of circuit before.

tmichlovitch (author)tomatoskins2015-03-02

Thank you for the feedback. It's definitely fun to see what can be done with electronics.

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