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The third project  on the board . . .


Build your own Flip Flop circuit and learn how it works 
This is Project #3 on the PC board in "5-PROJECTS"

In this project we examine one of the most valuable circuits to be invented - the flip flop. Originally it was designed with VALVES, along with its simpler version (without the two capacitors - called a bi-stable Multivibrator), it was realised it could store a "bit" of information. The bi-stable Multivibrator circuit required an input pulse to the left side of the circuit and the load (say a globe) stayed ON when the signal was removed. A pulse to the other side of the circuit turned the globe OFF. This was the first time an electronic circuit had stored a "piece of information." This was the beginning of the COMPUTER AGE. 
When you realise each letter on this page requires 8 circuits like this to store the "bits" you can see how little each "storage element" can hold. That's why you need millions of cells similar to the Flip Flop circuit to hold data for even the simplest application. 

The Flip Flop is a symmetrical arrangement using two transistors with cross-coupling. Each transistor has a base bias resistor (10k in our case) and a LED with 470R resistor in the collector lead to form the collector load.

The circuit consists of two identical halves and is called a Flip Flop because one half is ON while the other half is OFF. The ON half is keeping the OFF half OFF but it cannot keep it off indefinitely and gradually the OFF half turns ON via the 10k base-bias resistor.
This drives the ON side OFF and the circuit changes state. In other words it flips over. The same events occur in the other half of the cycle and the circuit eventually flops back again. 

This sounds very complicated but in reality the circuit is quite simple in operation as one half is exactly the same as the other and there's only 5 components in each half. 

The circuit is self-starting and only one LED is on at a time. It is a free-running multivibrator (this means it does not stop) and we will describe its operation in a non-technical way. A free-running multivibrator is also called an astable multivibrator (meaning is has no stable states) and that is why it flips from one state to the other continuously. 

The standard way to draw this type of multivibrator is to show the two capacitors crossing at the centre of the circuit, this also gives the circuit symmetry and makes it easy to recognise.

The other way to identify an astable multivibrator is knowing that it has two capacitors. (The monostable multivibrator has one capacitor and the bistable multivibrator has no capacitors.)

In simple terms, the astable [pronounced (h)ay-stable] multivibrator has two states. When one transistor is turned on it operates (supplies current to) a LED (or other device) in its output line and at the same time keeps the other transistor off. But it cannot keep the other off forever and eventually the other transistor begins to turn on. When it does, the action turns the first transistor off slightly and a change-over begins to occur. This produces the flip action.
After a short period of time the other half of the circuit cannot be kept off and the whole arrangement flops back to the first state.

The components that determine the frequency are the electrolytics and two base-bias resistors. If these values are changed, the frequency will alter.
For instance, if the electrolytics are reduced in value, the frequency will increase and if the resistors are decreased, the frequency will increase. 

If you increase the frequency of this circuit to more than 20 cycles per second, it will appear as if both LEDs are on at the same time. But the fact is the circuit will be operating faster than your eye can see and that's why we have chosen large values of capacitance to slow it down. 

When the electrolytics and resistors are made equal value (as in our case), each LED flashes for the same length of time. This is called an equal mark-space ratio: (50%:50%). This means the flip time is the same as the flop time. 
Theses components can be changed to any ratio, to give different effects.


The animation above shows the Flip Flop circuit in action with the red and green LEDs. 

As each step of the construction is completed, the (  ) should be ticked.
(   ) The four resistors fit flat against the board. To make them sit neatly, bend the leads to 90 with a sharp bend and push them up to the board before soldering. 

(   ) The two 100u electrolytics are next. The positive hole is marked on the board for each electro. This is the longer lead. The negative lead is marked on the component with a black stripe.

(   ) Fit the two NPN transistors. We have used BC 547 but any general-purpose NPN  low-power transistor will be suitable. They are pushed to the board so that the transistor matches the "D" outline on the board. If the transistors supplied in the kit are different, a modification sheet will come with the kit. 

(   ) The red and green LEDs can be fitted to either position on the board. The short lead is cathode (k) and this is the bar on the symbol. 

(   ) The project is now ready to turn on. 

The Flip Flop components added to the board

We have already explained how the circuit works already but there are a few terms that can be gone over again to explain the condition when a transistor is conducting and when it is non-conducting (turned off).
We can also talk about the electrolytics, as they are experiencing a voltage change on their leads that is not obvious at first glance.  
We can also mention that a conducting transistor is equivalent to a very low value resistor (we are talking about the resistance between the collector-emitter leads). In fact we can think of it more accurately as a very low voltage drop, in the order of about 0.35v.

A transistor that is OFF is called CUT-OFF and one that is fully turned ON is called BOTTOMED or SATURATED. 

These are the two states for the transistors in the Flip Flop circuit. One transistor is CUT OFF while the other is SATURATED.

With these facts in mind we can again go through how the circuit works. When the power is applied, the slight difference in characteristics between the two transistors and electrolytics causes one transistor to turn on faster than the other. Suppose Q1 turns on faster via
the uncharged 100u electrolytic C1, LED2 and the 470R resistor.

The voltage on the collector of Q1 will drop to about 0.35v and LED1 will light up. The positive lead of capacitor C2 will have 0.35v on it and this voltage will also be on the base
of Q2. Transistor Q2 will be turned off by this action but LED2 will come on for a short time while C1 charges.
C2 begins to charge in the reverse direction (electrolytics can do this provided the voltage is not too high) and as the voltage rises above .6v, Q2 begins to turn on. This lowers the voltage on its collector and begins to turn on LED2. 

The positive end of C1 is also connected to the collector and as the voltage drops, this effect is transferred to the base of Q1 via C1. This action begins to turn off Q1 and its collector voltage rises.
Since C2 is connected to this point, the base of Q2 will see a rising voltage and it will turn on harder. In a very short time the two transistors have changed state.
There's a little more concerning C1. 

An electrolytic can be considered to be a rechargeable battery and when C1 is charged at the beginning of the cycle, it will have about 5v across it (for a 9v supply). 

If we change this to a 5v rechargeable battery the explanation will be easier. The positive terminal of the battery will be connected to the collector of Q2 and when the transistor turns ON, the collector will be .35 above the negative rail. (the zero rail). 

This means the negative terminal of the battery will be 4.85v BELOW the zero rail. In other words the base of Q1 will see a negative voltage of 4.85v. 

And this is exactly what happens. The energy in the electrolytic will now be removed by the 10k resistor and after a short time the base will see a positive voltage of .6v and Q1 will begin to turn on and change the state of the circuit.

This is how the delay is created for each of the cycles. 

Before we leave the multivibrator there's an important concept that should be explained.

Since each transistor is either ON or OFF, the circuit is classified as DIGITAL, since it has only two states and the time to change from one state to the other is so fast that we do not take it into account.

If we take the collector of one of the transistors, say Q1, it will be either HIGH or LOW and never part- way between. 
These digital states will be very important later in our course, when we connect transistors to integrated circuits. 

Integrated circuits are digital devices with inputs that only accept either HIGHs or LOWs. The transition time between these two states must be very quick to prevent noise getting in. If noise were to get in, the circuit would not work. Many IC's are counting devices and noise will cause them to count at maximum speed. Others will create excessive noise if the input line is at about mid-rail voltage. It takes a small period of time for the chip to start to produce counting
or noise and if the transition is fast enough, it does not get the opportunity to start-up.

The astable multivibrator is also called an oscillator and when it is connected to an IC it will provide pulses called clock pulses. These clock pulses enable the IC to count or perform other functions such as division etc. 

The flip flop is also called a square wave oscillator and either the same circuit or a similar circuit is now available in an IC to produce clock pulses. 

1. What type of multivibrator is presented in this project?

2. Name the two other types of multivibrator.

3. How many capacitors does a monostable multivibrator have?

4. Give another name for a fully conducting transistor.

5. What is the value of base voltage when a transistor begins to turn ON?

6. When a transistor is fully conducting, what is the voltage between collector-emitter?

7. What is another name for a transistor that is switched OFF?

8. What is another name for an astable multivibrator?

9. If the electrolytics in the Flip Flop are replaced with 47u, what will happen to the frequency of the circuit?

10. Can both LEDs be ON at the same time?


1. Astable multivibrator. 

2. Monostable multivibrator, Bistable multivibrator.

3. One. 

4. Saturated, bottomed, output 0, output LOW. 

5. .6v 

6.  .35v 

7. Cutoff, output HIGH, Output 1. 

8. flip-flop 

9. It will increase in frequency. 

10. No. Only one LED can be illuminated at a time. 


Click HERE for Interactive Flip Flop circuit 

Click HERE for Memory Test


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