Simple Blinking LED Circuit





Introduction: 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

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

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

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

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

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.

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3 Questions

Could you add more LEDs? Per say 8-12? How would this change this circuit in terms of power supply and other components?

Thanks a lot for your time sharing that cool project. ¿Could you suggest another type of transistor that one can use insted?

Hi. My first ever post on this forum, having just joined because I saw your Instructable.
I want to make a simple flashing on and off LED sign but using 187 LEDS (an 11x17 LED pattern).
The LEDs would be mixed white LEDs and blue LEDs.
Would it be easier for me to attach the sequence of LEDs to a simple off/on switching connector (and if so, how would I make such a connector, do you know, please)?
Thanks for bringing me here with your Instructable.


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.

2 replies

7 year old ! Re you kidding!! I'm struggling at 18 :(

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

1 reply

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".

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.

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

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.

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.

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

Mwhahaha this answer made my day! Quite agreed!

Sir/Madam, that is a fantastic answer.

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.

Is this the same as ?

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.

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

1 reply

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)

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?