Flip Flop LED Circuit





Introduction: Flip Flop LED Circuit

This is a simple circuit based on transistor 2N 2222A and some resistors, there are 2 LED s and when one is on other will be off and this will be repeated in equal intervals of time  

Step 1: Parts List

2 x  2N2222 transistor

2 x 10k resistor (brown black orange)

2 x 390 ohm resistor( orange white brown)

2 x  100uf electrolytic capacitor

1 x blue LED (or other colors u like)

1 x red LED (or other colors u like)

battery ( 3 - 9v)

Step 2: Circuit Diagram

complete the circuit according to the circuit diagram,here i have used a common circuit board, there is a simple way to find the polarity of the LED ie, by looking for the flat part in the diameter of the LED the leg in that side will be negative 

Step 3: Conclusion

 use safety scissors to decrease the length of the legs of components, do not use more than 9v which will make the transistor hot and some times burned,use alligator clips while soldering the transistors which will reduce the heat of the transistor thus saving it,comment if u have any doubts



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    Can somebody please tell me about the dependence of frequency on the capacitance used. does it vary as the root of the capacitance or is there a linear relationship??

    Would someone please explain in detail exactly how that schematic works to produce the flip-flop effect?


    6 replies

    hi friend,

    its called an oscillator circuit.

    the capacitors are the real key to this circuit. The caps alternatively charge and discharge, which causes the two transistors to alternatively turn on and off.

    check this circuit here. for better understanding.

    in this circuit, can i give a supply of 3.3 V or 5V? if I can then i have to change the value of capacitor and resistor?

    I do appreciate your help, but instead of detail you have given me an overview and an animation. The animation shows what is happening in the circuit except nothing about the orientation or state of the capacitors. It doesn't tell me what the graphs at the bottom mean. What it really doesn't show is why these things are happening. Why does one capacitor charge while the other discharges? Why are both transistors not "ON" at the same time?

    I feel this understanding is important to my continued advancement in the electronics field. To understand how is meaningless if you don't understand why.

    I built this circuit and it works, but I still don't understand it.

    Which capacitor is charging when the left led is on? Thanks

    consider the circuit from the link in my previous reply. c1 is discharged when left led is on. change the simulation speed(top-right corner of page) in the link for better understanding.

    I am wondering are the transistors 3904 or 2N2222A because all I have is 3904. And for the 390 ohm resistor can I use 330 ohms, sorry I don't have many parts!:)

    4 replies

    Yes, I did that... I thing is for the LED's so I think you can use down to 220 Ohms

    This is a late post but for anyone in the future, yes, you can substitute all of those parts and it will work.

    Those should work, as long as its NPN, (to an extent)

    hello..can this circuit use 3v of voltage?

    1 reply

    Next time solder the transistors at last as they are quite heat sensitive.

    Any NPN will work for the most part.

    I replaced an led with a piezo buzzer and I used new batteries, and suddenly only the other led will turn on when its powered up. How can I change my circuit so the piezo and led blink opposite? It has worked a while ago but after recovering the project, with new batteries, it doesn't work. Help please,

    I have a few questions;
    1.) are the 10k resistors that are wired to the capacitors for discharging them?
    2.) if using a 3 volt supply should i use a weaker resistor as opposed to the 390 ohm?
    3.) how fast does your run, im looking for police light for an rc car fast