This Flashing Circuit used to be a real ‘wowser!’ with students before LEDs become commonplace (around 1995). You’re going to build a circuit that has a control knob that will allow you to set the flash speed of the LED. You can try different LEDs or mini-lamps to see what kind of an effect you get. Are you ready?

NPN and PNP transistors are similar in that when current is applied to the base, electricity flows through them.  But, they way they are used is different.  NPN transistors are often used to control whether a circuit is completed by connecting it to ground or not, where PNP control the positive current going into a device (or portion of a circuit).  NPN transistors are often used where larger currents need to be controlled, because it’s easier for a transistor to control the ground side of a circuit than the plus power side of it.

So, why does the LED flash?  Remember, a capacitor is like a storage tank for electricity.  You fill it up, then empty it out.  But, it takes time to fill up and empty.  This circuit uses the time it takes to fill and empty as a delay for turning the LED on and off.  How fast it fills up depends on the value of the resistor that is connected to it.  We’re using a variable resistor, so we can adjust how fast it fills up, and thus adjust the flash rate.

Click here for Unit 14 (Lesson 1) schematics.


  • breadboard
  • AA battery case
  • 2 AA batteries
  • LED
  • potentiometer
  • transistor (2N2222A and 2N4403)
  • 5.6k resistor
  • 1k resistor
  • 10k resistor
  • jump wires
  • wire strippers

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16 Responses to “Flashing Circuit”

  1. Yes, it actually will change how your circuit works to swap out components! It’s actually kind of fun, and near the end of the videos in this section, I give suggestions for how to do it.

    For your specific question, I am not sure about the “B” when you say “B1k” potentiometer. Did you get the kit from us for this experiment? (It’s in the Diamond Science Mastery program.) If so, then you’ve got the right one. If not, don’t worry – you can still get it to work just fine. a 1k potentiometer should work just fine, but I am wondering about your speaker. Is it 8-ohm? Can you send me a picture of your project so I can help more? It’s hard when it’s just written in text like this. Send me any details/photos/videos to

  2. taniacprezas says:

    We think we are using the wrong potentiometer, as with the one we are using the led just dims. We tried with a speaker as well, and the frequency of the sounds does change but not that much.

    Ours say B1k is it supposed to be that one? does it matter?

  3. The type of LED you have will have a different amount of resistance, and by moving where that resistance is in the circuit will have a different effect on ho wit works. Basically, all LEDs are not the same… so you have to be creative and flexible in your electronics design. Does that help?

  4. Evangeline Gregory says:

    at one point in the video`you said if the LED didnt light up, move the resistor to the positive side of the breadboard. I did, and it worked, but i dont understand why.

  5. Did you get the resistors in the right spot? Also make sure that your transistor is in correctly (and didn’t get hot when you had your circuit on). Watch the video again and trace through your circuit step by step, and let me know if you still have trouble!

  6. Kristiann Saunders says:

    The only thing our potentiometer does is turn the light on, turning it doesn’t effect the light at all. I’ve added a few different resistors, but it either goes all on with one resistor, or all off with another one. Any suggestions?

  7. No, not easily. A transistor is one of the basic electrical components that make up other components, like integrated circuits.

  8. Yvonne DAugherty says:

    can you make a transistor?

  9. Both schematics are in the same file. The light flasher is on page 3 and has two transistors and a potentiometer, while the light actuated needs to sense light so that’s where you’ll find the CdS cell. The name for each circuit is printed at the bottom of the page when you hold it right-side up. Does that help?

  10. Carey Clark says:

    sorry for another question is the Schematics sopposed to have a cds cell in it I looked at both the light acuated circut and the flashing circut and it looks like there both the same.

  11. Sure – how do you think you’d add them in?

  12. Yes, and you’ll learn how to do that in the more advanced lessons in Unit 14. You will need a wall adapter that changes AC current to DC and steps it down to the voltage that you need for your circuit so it doesn’t fry your components.

  13. Carey Clark says:

    is there a way to plug this into the wall

  14. Carey Clark says:

    is there a way to make this project with more than one LED like christmas lights?

  15. These transistors are so versatile, I am not surprised. Thanks for the share! I’ll ask my electronics techie and let you know what he says…

  16. Glen Larson says:

    we found that our 4403 transistor worked the same both ways in the first bit of the experiment, unlike what you said. We were wondering if maybe it had something to do with the fact that we only had a 100-ohm resistor, not a 1-k-ohm, connected to the base. -Evan Larson