Transistor Bistable Flip Flop

In this article we learn a couple of transistorized flip flop circuits which can be used for toggling a load ON/OFF through a single push button trigger. These are also called transistor bistable circuits.

What is Bistable

The term bistable refers to a state of a circuit where the circuit with an external trigger is able to render itself stable (permanently) over two states: ON state and OFF state, hence the name bistable meaning stable on either ON/OFF states.

This ON/OFF stable toggling of the circuit alternately could be normally done through a mechanical push button or through a digital voltage trigger inputs.

Let's understand the proposed bistable transistor flip flop circuits with the help of the following two circuit examples:

Simulation and Working

In the first example we can see a simple cross coupled transistor circuit which looks quite similar to a monostable multivibrator configuration except the the base to positive resistors which are missing here intentionally.

Understanding the transistor bistable functioning is rather straightforward.

As soon as power is switched ON, depending upon the slight imbalance in the component values and the transistor characteristics, one of the transistor will switch ON completely rendering the other to switch OFF completely.

Suppose we consider the right hand side transistor to conduct first, it will get its biasing via the left hand side LED, 1k and the 22uF capacitor.

Once the right hand side transistor has switched completely, the left transistor will switch OFF completely since its base will now be held to ground via the 10k resistor across the right transistor collector/emitter.

The above position will be held solid and permanent as long as power to the circuit is sustained or until the push-to-ON switch is depressed.

When the the shown push button is pushed momentarily, the left 22uF capacitor now will be unable to show any response since its already fully charged, however the right 22uF being in a discharged state will get an opportunity to conduct freely and provide a harder biasing to the left transistor which will instantly switch ON reverting the situation in its favor, wherein the right hand side transistor will be forced to shut off.

The above position will be held intact until the press button is yet again pressed. The toggling can be flipped alternately from left to right transistor and vice versa by actuating  the push switch momentarily.

The connected LEDs will light up alternately depending upon which transistor is rendered active due to the bistable actions.

Circuit Diagram

Transistor bistable flip-flop circuit using a relay

In the above example we learned how a couple of transistors can be made to latch in bistable modes by pressing a single push button and used for toggling relevant LEds and the required indications.

In many occasions a relay toggling becomes imperative in order to switch heavier external loads. The same circuit which is explained above can be applied for activating a relay ON/OFF with some ordinary modifications.

Looking at the following transistor bistable configuration we see that the circuit is basically identical to the above except the right hand LED which is now replaced with a relay and the resistor values have been adjusted a bit for facilitating more current that may be required for the relay activation.
The operations of the circuit is also identical.

Pressing the switch will either switch OFF or switch ON the relay depending upon the initial condition of the circuit.

The relay can be flipped alternately from an ON state to OFF state simply by pressing the attached push button as many times as desired for switching the external load connected with the relay contacts accordingly.

Bistable Flip Flop Image

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gps s said…
Dear, Sir
I made the above circuit with relay but the circuit remains power on and push button does not work. where the fault can be ??
Swagatam said…
GPS, try slightly higher values for the 22 ohm resistor until the relay responds correctly.
gps s said…
Sir, I tried higher values for the 22 ohm resistor but when i pushed the button relay activates and deactivates after releasing the button. But it must remain activate once press and deactivate when pressed again.. plz help.
Swagatam said…
gps, you will need to adjust the resistor value to some optimal value to make the relay function in the bistable modes

if it appears difficult, then probably you should quit this and go for an IC based design.
ravindra metri said…
Sir, above i want to use 1 watt LED plz tell resistor value
Zaeem Ahmad said…
Hey swagatam, can u please tell me the effect of capacitor value in this circuit.
Also tell me the effect of resistor value that is between base of one transistor and collector of other transistor.
Swagatam said…
Hi Zaeem, the base capacitors help the transistors to flip and switch ON instantly, until the capacitor charges and stops the process, and once this happens the parallel resistor make sure that the position is held intact by sustaining the required minimum base drive to that particular transistor

the values may depend on many factors such as the load current, supply voltage, transistor HFe etc.
kuntal karmakar said…
Sir i have made the ckt wid bc548(coz i had these earlier) but every time when power is on one particuler led is on and does not change state either even after pressing the sw

now what should i do
Swagatam said…
Kuntal, try changing the 100 ohm resistor increasing or decreasing and check the effect
Vishal Ahuja said…

Can you please tell a simple working flip flop circuit to drive a relay. Any circuit tested by you.

Swagatam said…
Vishal, check out the first circuit in the below link:

you can use the IC1 section for the flip flop function.

trigger at 14 will alternately switch the relay ON/OFF

make sure to include R5 and C3

Vishal Ahuja said…
Thank U Sir. Will try that.
Vishal Ahuja said…
As suggested by you I had a look on the circuit but I think to trigger pin 14 it will require positive input and in your above circuit it needs negative input trigger.
This one was perfect for me as I want to add it to a circuit that give negative trigger.
But the above circuit was not working when I made it, when I powered it, it activated the LED as well as the relay and triggering it didnt do anything.
I had altered the 22 ohm and 1k resistance to few nearby values but it doesnt make any difference.

I will be very thankful to you if you please help me in this same circuit.

Swagatam said…
for a negative trigger you just have to include T2, R4 in the linked 4017 IC circuit, a negative trigger at R4 will toggle the circuit ON or OFF

the above transistor circuit is a very crude design and might require a lot of adjustments, so will be difficult for me to suggest without practically checking it.
Ajeesh Alath said…
boss i want to make a relay based automatic on off circuit, i need minimum 5 second delay in switching on and off. just like a flashing circuit for flashing a 230 v bulb continuously.. can you please help me
Swagatam said…
you can use any of the circuits from the following link:

remove D2 for continuous flashing.
Dear Guru,
If we take two identical transistors say 2N3055 and connect them in series, then the equivalent voltage becomes 120V 15A.

Now are my queries:
1. Does the equivalent voltage becomes 120V 15A? Am i right?
2. What happens to the GAIN VALUE of the above configuration?
3. Could you suggest a circuit so that both transistors can conduct freely?

I would request you to do the same for parallel connection and help me with the above queries.
Swagatam said…
Hi Sherwin, yes the voltage spec will increase to 120V since each transistor is rated to handle 60V, and the current will not change and stay fixed at the rated 15Amp.

the gain will not change, and will remain at the rated 20 or 25 hFE value.

if you apply correct biasing voltages to the bases of both the transistors when both will conduct freely.

for parallel connection, the C/E voltage rating will become 60V, and the current will become 30amps.
for more info you can refer to this post
V.Ramakrishnan said…
Dear Sri.Swagatham ji,
After a gap of around 3 years I am once again reaching you for help.
That is:
How i can use your first circuit with 12 Volt supply in a model rail road signal application; that is if the model train passes over a reed switch on the track and activated by a small magnet embedded underneath the chassis of model train the signal lights are to be switched to red if it is in green initially.
What are the alterations needed in the circuit.
Please help since dasara is nearing before which I had to fix this in the model rail road in the house.
Swag said…
Dear Ramakrishnanji, If you wish to have a hassle free, clean and perfectly operating circuit then I wouldn't recommend the above circuit, instead you could try a 4017 IC based circuit as shown in the below article, with the following modification.

ignore everything that's connected at the right side of the IC. remove them all

disconnect the 10K end which is connected with pin#10, and connect it with pin#4.

Connect one LED with a 1K resistor from positive to pin#2 and the second one from pin#2 to ground.
replace the push button with the reed switch.

that's all.
V.Ramakrishnan said…
Dear Sri.Swagatham ji,
Glad you have given a suggestion which I will try and post you the outcome of it.
Swag said…
You are most welcome VRK ji :)
Greetings for the day
I am looking for a circuit
1 Push to On/Off at any duration
2 if ON then switch off after 25 mins
3 Use of 3.7v lithium ion battery
4 load uses 3.7v so we use z44 for load this circuit can handle load (load is 90 laser diodes 650nm)
5 circuit to charge the battery too while the switch is ON....

I have a prototype imported can share video of the same
Swag said…

you can try the following timer circuit for your 1st requirement

for charging the cell you can try the following concept:

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