BASIC 
ELECTRONICS COURSE 
Page 25 INDEX

DIGITAL CIRCUITS
Up to now we have described circuits without talking about categories. Circuits can be divided into TWO CATEGORIES:  ANALOGUE  and DIGITAL.  
If a circuit turns on and off GRADUALLY, it is classified as an ANALOGUE circuit - such as audio amplifiers, radio circuits and sinewave oscillators,
If the circuit turns ON and OFF VERY QUICKLY, it is classified as DIGITAL - such as multivibrators (square wave oscillators), circuits for turning on a LED or globe, and storage circuits. The main reason why a globe is turned on and off very quickly is so the transistor does not get hot. This is one of the advantages of a digital circuit. We will explain this in a moment. There are other advantages including:
1. It is very reliable in operation (it is either on of off - and cannot have any "half-on" values)  
2. It can store a "bit" of data,
3. It can be made very compact.

In this chapter we will concentrate on the difference between digital and analogue circuits. 
The main characteristic of a digital circuit is it turns on and off very quickly. This gives the output condition: OFF or ON. The OFF condition = 0 (zero) and the ON condition  = 1. We also say OFF = LOW and ON = HIGH. 
The Pulse Generator circuit is a typical DIGITAL CIRCUIT. The output is either OFF or ON and it changes from one state to the other VERY QUICKLY. The animation below shows how the voltage across the load INCREASES when the circuit turns ON. 
When the output of circuit is LOW, the load is ACTIVATED. The load may be a globe, motor, relay, siren, speaker, or any other device. The second circuit shows the voltage across all the components is very small when it is active so that most of the rail voltage appears across the LOAD.  
There are lots of different DIGITAL CIRCUITS but they have two things in common. 
1. The all switch from HIGH to LOW very quickly,
2. They all deliver RAIL-VOLTAGE when turned ON. In other words the load sees RAIL VOLTAGE (or very close to full rail voltage) when the circuit is turned on. 
The animation below shows these two features:

HOW THE CIRCUIT TURNS ON AND OFF
Firstly you have to see how the circuit works without 10u electrolytic. Without it, the circuit turns on immediately and remains ON. 
The diagram on the left makes it easy to see how this happens. The BC 557 transistor is a PNP device and operates as a "mirror image" of the NPN device. The BC 557 is turned on via the 330k base resistor and this reduces the resistance between the collector-emitter leads so that the BC 547 transistor is turned on via the 1k base resistor. The load sees rail voltage and is activated. Fitting the 10u electrolytic changes the operation of the circuit completely.

It produces the amazing feature of "pulsing." The circuit delivers a short pulse to the load. 
When power is applied the circuit does not turn on immediately. The 10u is charged via the 330k (and 1k) and when 0.7v appears across it, the BC 557 transistor turns on. This causes the BC 547 transistor to turn on and rail voltage is delivered to the load. 
The action of the BC 547 brings the negative end of the 10u towards the 0v rail and it continues to charge via the base-emitter junction of the BC 557. The charging current keeps the circuit hard ON but after a short period of time the capacitor is nearly charged and the charging current is reduced. This lower current turns the BC 557 off slightly and and this action turns the BC 547 transistor off slightly too. The voltage on the collector rises and this is passed directly to the base of the BC 557. The slightly lower current into the base of the BC 557 is multiplied by the gain of the two transistors and this causes a reduction through the load. The voltage across the load is proportional to the current through it and as the current reduces, so does the voltage. 
In a very short period of time the two transistors are fully off and the charge in the 10u is gradually removed by the 330k. This is a slow process and that's why the off time is considerably longer than the on time. 

 SQUARE WAVE OUTPUT
The output of the Pulse Generator is a SQUARE WAVE. A square wave does not have to have an equal On and Off time. It gets its name from the square corners of the wave.  

As shown in the animation, the HIGH and LOW times are different but the change from one state to the other is very fast.  

In the animation below we see how each transistor in the MULTIVIBRATOR (covered in the beginning of this course) turns ON and OFF. When the transistor turns ON the voltage across it is LOW and the LED is illuminated. The LED bargraph indicates a low voltage on the collector of the transistor when the LED in the circuit is illuminated. The graph also indicates the voltage on the collector during this time. 


The circuit above is a FREE RUNNING MULTIVIBRATOR or ASTABLE [(h)ay stable] MULTIVIBRATOR  and indicates the multivibrator has NO stable states. In other words it oscillates (operates) continually and does not remain permanently in either state. 
There are two other types of multivibrator. They are MONOSTABLE and BISTABLE. 
The MONOSTABLE  has one stable state. It is triggered into its unstable state. The circuit remains in this state for a period of time then falls back into its stable state - ready for re-triggering. 
The BISTABLE has two states and must be triggered from one state to the other. The circuit is not capable of changing states by itself. It remains in either state until triggered into the other state. 

Not long after the introduction of circuits such as the multivibrator, it was realised the advantage of the bistable arrangement. It was capable of holding a "bit" of information. About the same time the US launched its space project and poured millions of dollars into electronics research. The result with was the perfection of the "chip." (The INTEGRATED CIRCUIT)
Some of the first chips were simply transistor stages that produced an output (HIGH) when the input was HIGH. Other chips produced an output when two lines were HIGH or when three lines were HIGH. They called these chips "GATES" (not after Bill Gates of Microsoft) and created the very first "decision making" circuits. 
On the next page we will cover GATES and show they forged the way to the introduction of the first "computer."

Question 112: Name a difference between an analogue circuit and a digital circuit:

Ans: Analogue rises and falls SLOWLY. Digital rises and falls QUICKLY. 

Question 113: Give two names for a HIGH output:

Ans: High = 1 = ON = Active 

Question 114: Name the three types of multivibrator circuits:

Ans: Monostable Multivibrator, Bistable multivibrator, Astable multivibrator.

Question 115: Name the multivibrator with No stable states:

Ans: Astable multivibrator.

Question 116: Name the multivibrator with one stable state:

Ans: Monostable multivibrator.

Question 117: Name the multivibrator with 2 stable states:

Ans: Bistable multivibrator.

Question 118: When a circuit changes state very quickly, is the output a sinewave or square wave?

Ans: Square wave. 


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