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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