Instructables
Picture of 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

Picture of 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)
 
switch

Step 2: Circuit diagram

Picture of circuit diagram
2n3904-2n2222apinout.gif
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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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wliu34 months ago
Next time solder the transistors at last as they are quite heat sensitive.
clayman9871 year ago
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!:)
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)
Can I use BC547 instead of 2N2222A ?
Any NPN will work for the most part.
clayman9871 year ago
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,
Thanks,
Clayton
Derrick9112 years ago
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
sooraj619 (author)  Derrick9112 years ago
1) yep,
2) use a 10k variable resistor adjust its value for a 3v supply and reduce value of 390 ohm to lower values
3)you can use a 10k variable resistor in place of 10k resistor or change the capacitor value to change the flash rate
jimmy5582 years ago
can i operate this with aproximately 7 volt battery??
sooraj619 (author)  jimmy5582 years ago
yes
jimmy5582 years ago
what is the timing of leds turning on & off...??
sooraj619 (author)  jimmy5582 years ago
about 1 second, you can increase or decrease the fading effect by changing the values of both the capacitors
kyooby2 years ago
thank you.i remember toose days when i maded this stuffs.cool
sooraj619 (author)  kyooby2 years ago
thanks
maybe this is better
circuit-diagram.jpg
sooraj619 (author)  therookie1873 years ago
ok
gzaloprgm3 years ago
Just one correction: It should be called Astable, not FlipFlop
actually chief this is called a flipflop http://talkingelectronics.com/projects/5-Projects/Projects16.html
An astable multivibrator and a flip flop are basically the same type of circuit. However flip flops generally require an input or timer on a gate to make the state change whereas an astable multivibrator is not stable in either state and will constantly change between states without an input or timing circuit.
magic monkey You are Correct!! Normally a Flip flop needs a clock pulse to change it's output Logic state 1 or 0. Where as an astable Multivibrator uses FEEDBACK to sustain oscillation( LED FLASHING)
sooraj619 (author)  gzaloprgm3 years ago
ok
We made this at school and made christmas lights. When we made it the curket dyagram was called a FlipFlop.
chipper353 years ago
Not bad at all! .....would've like to've maybe seen a few more steps/photos, though.
sooraj619 (author)  chipper353 years ago
thanks
ray_Bond3 years ago
good work sooraj bhai :)
keep it going
god bless you .
sooraj619 (author)  ray_Bond3 years ago
thanks a lot
axeman9113 years ago
Good Job. Looks pretty cool!