Simple Flip Flop Circuit

In this Instructable I show you how to make a Flip Flop LED circuit. The special thing about this circuit is that it does not use an IC (Integrated Circuit).

By using different resistors, capacitors and transistors, I succeeded in this project to make a circuit that lights the LED lights one by one.

I will explain how the circuit works in the last step.

Video Tutorial

Supplies

• 2 LED lights (any color you want)
• 2 BC547 transistors
• 2 470 Ohm resistors
• 2 10K resistors
• 2 100 uF capacitors

Step 1: Prepare 470 Ohm Resistors

For this step you need 2 470 ohm resistors. Bend one pin of both resistors 90 degrees. Then solder the bent pins together.

It doesn't matter how a resistor is placed, it works the same in both directions.

Step 2: ​Mount the 10K Resistors.

For this step you need the 2 10K resistors. Cut a piece off the pin on one side of both resistors. Then solder the resistors to the 2 resistors from the previous step.

The resistors from this step do not necessarily have to be 10K. As long as they are anywhere between 10K and 50K it is fine. (speed of changeover is therefore also different)

Step 3: Mount the First Transistor

Bend the collector and emitter terminals of the BC547 transistor 90 degrees. Solder the Collector to the 470 Ohm resistor and the Base to the 10K resistor.

The last image is the pinout of the BC547 transistor.

Step 4: Mount the Second Transistor

Bend the collector and emitter terminals of the BC547 transistor 90 degrees. Solder the Collector to the 470 Ohm resistor and the Base to the 10K resistor.

Step 5: Mount the LED Lights

Bend the connectors of the LED lights so that they are at an angle of 90 degrees. Solder the anode (+) of the first LED to one of the pins that are still free from the BC547 transistors (emitter).

Then solder the anode of the second LED to the other transistor.

Connect the cathodes (-) of both LED lights together by soldering them.

Step 6: ​Mount the Capacitors

For this project I used 100 uF capacitors. Between 10 uF and 100 uF works in any case (interval of the LED lamps changes with it)

Mount the first capacitor with the + connection between "transistor 1 and the 470 Ohm resistor" and with the - connection between "transistor 2 and the 10K resistor".

Mount the second capacitor with the + connection between "transistor 2 and the 470 Ohm resistor" and with the - connection between "transistor 1 and the 10K resistor".

Step 7: ​Mount the Power Supply

The circuit is finished so far. Only the power supply needs to be connected. I used a 9V battery for that.

Connect the + connection of the battery clip to the connection of the 4 resistors.

Connect the - connection of the battery clip to the connection between the 2 LED lamps.

Connect the 9V battery to the connector.

Step 8: Ready

The Flip Flop circuit is ready!

How it works

The base of both the transistors are connected to + connection through resistor. Any one transistor will get activated first. (this is because no transistor is exactly similar to other one). Lets assume transistor 1 turns on first. When the transistor turns on the collector pin gets grounded and led at its collector lights up. Also the capacitor starts to charge through ground provided by 1st transistor and the 10k resistor at the base of 2nd transistor. Since the 1st capacitor is charging it is taking current from the 10k resistor which is at the base of 2nd transistor. The base of 2nd transistor has no current to activate its led. At some point 1st capacitor will charge up. And when it does so, there is no more current flowing through it. Now there is enough current that can flow through base of 2nd transistor. The 2nd transistor gets activated. (Including the led connected to its collector).

Video Tutorial

I am curious about your creations, comments and additions!

See you soon.

New Pew

Comments

[chrisnorris1981]

How did you determine the 470ohm resistor or what would change by changing this

[Al Magic]

Fresh of the bench. Going to bread board it and change a view values. Aiming to make a set of tower structures with a circuit like these. Great instructable New Pew

[fulviot66]

And what about electric diagram?

[yamms2]

Is this not an astable multivibrator circuit? Our Electronic Technician apprentices used this to learn electronics and fault finding techniques.

[frarugi87]

Yes, this is not a flip-flop but an astable vibrator

[Drakkar02]

Nice ! and how to for 12 volt ?

[taki2007]

Just adapt the 1k resistors value so as to limit the LED's current (rule of thumb makes me say, 1.5k should be OK). And check the voltage rating for the capacitors and transistors is over 12V (OK for BC547 transistors, but double check for the capacitors since 6.3V do exist. I would suggest to go for 25V rated capacitors.)

[Drakkar02]

Ok ! Thank you for taking the time to respond. Have a good day.

[BobW125]

Simple art and tech craftsmanship at it's finest, well done> liked how you used the 3rd finger to assist in the soldering, you made it look ez, make vid on how to solder>

[Al514]

Hello, I really like these flip flop circuits, I was wondering could you explain how to make this into 5 or 10 or more LEDs ? It's a great circuit when you don't need any IC's. Thank you for your time.

[taki2007]

You can do this by daisy-chaining each "half" of the circuit (that is, one 1k resistor, the transistor to which it is connected, the LED, the 10k resistor connected to the transistor's base, and the capacitor going from its collector to the other transistors base). Intead of looping the 2nd "half circuit"'s capacitor back to the 1st transistor's base, just connect it to a third transistor in the same configuration (the one I described in brackets). And so on, the capacitor going from T3's collector should be connected to T4's base; when you got the number you like, just loop it back to T1's base. I don't think there is a theoretical limit to that, since every elementary circuit is working in parallel. It makes a kind of no-IC LED chaser

When I did that with 3 transistors, I noticed it would sometimes get stuck in one state at power-on, so I also added a push-button in parallel with one of the capacitors (which one does not matter) to "jump start" the oscillation.

[ELECTRONFLYER1]

DO I HAVE TO USE LEDS CAN I USE LIGHT BULBS---- LIKE TWO 12 V CAR TAIL LIGHTS CAN I USE 12 VOLTS

[mstratman]

Not without major modifications. This uses a simple RC oscillator, and the power to light the LEDs is stored in the capacitors. Car tail lights will require far more than can be stored in those tiny electrolytic caps.

[taki2007]

Actually the power for the LEDs is NOT stored in the capacitors. It is supplied on demand by the 9V power supply when the transistor is "closed" (in saturation mode). The 2x"capacitor+10K resistor" are 2 R-C oscillators that trigger the transistors "on" or "off". Provided you chose the transistor appropriately (base/emitter current vs collector/emitter current when saturated), the power rating of the 10k resistor and of the capacitor is irrelevant.
BTW the circuit in this instructable is not state of the art for transistor circuits... LEDs should be in the same circuit branches as 1k resistors, so as to minimize base/emitter current. This is true with NPN and PNP transistors (but with reversed emitter and collector positions).
So to respond to O.P., yes, you could use it to drive bulbs, but you should modify the circuit a bit : replace 1k resistor by the bulb (if the bulb's rated power is lower than the transistor's rated current), remove LEDs, and "short" emitter to negative terminal of the power supply. Or, if the bulb's rated power is greater than the transistor's, drive it using a relay or (even better) a higher power npn-transistor/N-channel MOSFET whose base/gate you should connect to the collector of the original transistor.

[ShannonW49]

Just a thought - if you hooked up relays instead of LEDs, the relays could drive your lights. (Cars use relays themselves to handle turning lights -heavy current items- on and off to be able to use cheaper switches, lighter wiring, computer control, etc)

In this manner, your circuit is only dealing with small currents, while the relays handle the big stuff. It would take a little experimentation, but I think it would work.

[Arthur HarlemanH]

you would need to put a diode between the bulbs and the ground and lose the resistors. then play around with the capacitor sizing. Also make sure your transistor can handle 12v.

[RobertS854]

If you would like a Circuit Diagram, you can visit the same project at CircuitsDIY.com and search for LED Flip Flop Circuit using BC547 Transistors by Shagufta Shahjahan

[checker280]

What is that soldering technique? Load the solder onto the iron first?

Newbie solderer

[ShannonW49]

You should get the wires hot (put the soldering iron on the joint where the wires come together, touch both wires) then touch the solder to the wires at the same place. I usually put the iron on, then hold the solder in contact until it melts (on the other side of the wire).

You want the wires to melt the solder, not the iron. Inside the solder (the core) is flux. This is something that allows the solder to "flow" better and into tiny spaces. By having the solder melted by the wire, the flux "pulls" the solder onto the wire (instead of the iron).

When soldering, the smoke you see is actually the flux burning off. The flux also can leave a blackish residue on your hardened solder. This residue can be larger than your solder point on a board. This residue can also conduct small amounts of current. Because of this, especially when soldering on boards not open air like this, I recommend getting rid of the residue. Luckily it is hard and fragile. It scrapes/pops right off with something pointy like a toothpick. Just don't damage the board or part.

[spkrrpr]

Can you please credit the music? thank you. It is very pleasant.