third project on the board . . .
Build your own Flip Flop circuit and learn how it
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.
RECOGNISING A FLIP FLOP
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
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 FLIP FLOP IS A FREE-RUNNING
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.
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.
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
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.
that determine the frequency are the electrolytics and two base-bias
resistors. If these values are changed, the frequency will
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
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.
components can be changed to any ratio, to give different
THE FLIP FLOP IN ACTION
The animation above shows the Flip Flop circuit
in action with the red and green
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
( ) 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
HOW THE CIRCUIT
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.
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.
transistor that is OFF is called CUT-OFF and one that is fully
turned ON is called BOTTOMED or SATURATED.
the two states for the transistors in the Flip Flop circuit. One
transistor is CUT OFF while the other is SATURATED.
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
the uncharged 100u electrolytic C1, LED2 and the 470R
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
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
There's a little more concerning C1.
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
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.
how the delay is created for each of the cycles.
we leave the multivibrator there's an important concept that should
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 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
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.
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
1. What type of multivibrator is presented
in this project?
2. Name the two other types of
3. How many capacitors does a monostable
4. Give another name for a fully
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
7. What is another name for a transistor
that is switched OFF?
8. What is another name for an astable
9. If the electrolytics in the Flip Flop are
replaced with 47u, what will happen to the frequency of the
10. Can both LEDs be ON at the same
2. Monostable multivibrator, Bistable
4. Saturated, bottomed,
output 0, output LOW.
7. Cutoff, output
HIGH, Output 1.
9. It will
increase in frequency.
10. No. Only one LED can be
illuminated at a time.
HERE for Interactive Flip Flop circuit
HERE for Memory Test