There are two main areas of electronics: ANALOGUE and DIGITAL.
Analogue is mainly audio and radio, while Digital is mainly computers.
The two are completely different but you have to learn BOTH because there is often a small amount of analogue in a digital circuit and maybe a digital section in an analogue project.
In simple terms, a DIGITAL circuit consists of signals that are either HIGH or LOW. A torch is digital. The globe is either ON or OFF.
We are going to start at the beginning and
show DIGITAL ELECTRONICS started.
Here is the transistor connected to a 5v supply. In simple
terms the base is the INPUT and the collector is the OUTPUT.
|TRANSISTOR TESTER - 2
Here is a very simple transistor tester.
This is basically a high gain amplifier with feedback that causes the LED to flash at a rate determined by the value of the 330k and 10u electrolytic.
The transistor tester allows you to test a
transistor and find out if it is NPN or PNP and identifies the leads.
Simply connect both transistors to the circuit and the LED will flash. Replace one of the transistors with an unknown transistor and try all the leads around every different way. If the circuit does not work. Try the other 3 pins.
digital transistor has two resistors included inside the case
It is the components in the circuit and the signal entering the
circuit that create a DIGITAL STAGE or ANALOGUE STAGE. The transistor
can also operate at high frequency or only at low frequencies and it can
be a high voltage circuit or only a low voltage circuit.
Firstly we will cover how to make a digital circuit with a transistor. In other words we will turn a transistor ON FULLY or OFF FULLY.
We call this a DIGITAL CIRCUIT or DIGITAL BUILDING BLOCK and the transistor is referred to as a DIGITAL TRANSISTOR.
It is actually an ordinary transistor in a digital circuit but in "electronics language" we say "Digital Transistor."
When we use the word "TRANSISTOR" we are talking about one of the first types of transistor to be invented. It has the technical name BIPOLAR JUNCTION TRANSISTOR or BJT, but we will just call it a Transistor as this is the normal way of talking about this simple transistor. All the other transistors have special names such as Field Effect Transistor, Uni-Junction Transistor, orr Silicon Controlled Switch, and many others.
TURNING a TRANSISTOR
ON or OFF
It is easy to turn a transistor OFF. No voltage is applied to the BASE.
No current will flow through the collector-emitter leads and the voltage on the COLLECTOR will be HIGH. This is shown in Fig 1.
To turn a transistor ON, a voltage of 0.6v is applied to the base.
The transistor turns ON and the collector voltage will be very close to 0v. (maybe about 0.2v to 0.4v)
In other words, the transistor INVERTS the incoming voltage. It is an INVERTER. (when the base is "HIGH" the collector is "LOW."
We also say the transistor is a switch. It "switches" from one state to the other.
In a digital circuit the transistor changes from one state to the other very quickly. If it changed slowly, we would call it an analogue transistor.
Later in the course we show how transistors can be placed in a package called an INTEGRATED CIRCUIT (also known as a CHIP) and these chips operate on a voltage from 4.5v to 5.5v (and other packages work to about 16v).
To compare the transistor circuit with a "chip" the supply will be 5v for all circuits in our demonstrations.
Fig 1. Turning a Transistor ON and OFF
|The transistor is turned ON
when a voltage above 0.6v is applied to the base and the voltage on
the collector drops to less than 0.4v.
The transistor is turned off when the voltage on the base is less than 0.5v and the voltage on the collector is very close to RAIL VOLTAGE. (5v)
The animation shows the transistor turned ON and OFF.
These two states are called DIGITAL STATES and when the transistor is operating in these two modes, it is called a DIGITAL TRANSISTOR.
Fig 2. The Base Resistor
If we connect a resistor to the base, the input voltage can
be higher than 0.6v and the transistor will not be damaged.
A HIGH on the input creates a LOW
on the output.
Fig 3. Turn a Lamp ON
|We can use the transistor to
turn ON a lamp. (globe, torch globe or a motor)
The circuit uses an input (from 0v to 5v) to create an output of: 5v to 0v.
The transistor passes NO CURRENT when the lamp is OFF. The transistor remains COLD.
When the lamp is illuminated, maximum current flows but the voltage across the transistor is VERY SMALL and this is why the transistor does not get hot.
Heating is combination of voltage across a device and current flowing through it.
A single DIGITAL TRANSISTOR does very
little. It just turns ON or OFF.
Here are the two states for a DIGITAL TRANSISTOR:
Fig 4. The two States of a Digital Transistor
transistor is used in a DIGITAL CIRCUIT, the voltage on the
collector will be very close to rail voltage when the transistor
is off and this is called a
HIGH. Or the collector voltage will be very close to
0v when the transistor is turned ON. This is called a LOW.
A DIGITAL TRANSISTOR has only two states. It is either fully turned ON or fully OFF.
When it is ON, we say the transistor is SATURATED or BOTTOMED.
When it is OFF we say the transistor is CUT-OFF.
|But when a DIGITAL TRANSISTOR is connected to another DIGITAL TRANSISTOR via "cross-coupling" - an amazing thing happens. The circuit "remembers" or remains in a state called MEMORY. The result is a MEMORY CELL and the circuit can store a piece of information. This piece of information is called a BIT and is the basis of memory in a computer.|
Fig 5. The "MEMORY CELL"
When Switch A is pressed, the voltage on the base is removed and transistor A turns OFF.
Transistor B turns ON via resistors R1 and R2 and the LED is turned ON.
When the switch is released, the voltage on the collector of transistor B is less than 0.6v and the two transistors remain in this state.
Pressing switch B turns the LED OFF. (Transistor A turns ON via R3, R4 and the LED - very little current flows through the LED and you can hardly see it glowing). The voltage on the collector of transistor A is less than 0.6v and the two transistors remain in this state.
Cell consists of two cross-coupled transistors that hold their state ON or OFF after one of the switches is pressed.
LED remains illuminated after switch A is pressed.
Pressing Switch A briefly turns the LED ON and pressing switch B turns the LED OFF.
The circuit "remembers" or remains in each state called a stable state.
The technical name for this circuit is:
BISTABLE MULTIVIBRATOR or BISTABLE SWITCH or BISTABLE LATCH..
Fig 6. "Set" and "Reset"
|Here is another way to operate the bistable latch
and another name for the circuit.
The circuit is called ";Set" and "Reset" or RS Latch.
The circuit shown is manually operated to show the basics of a LATCH.
Fig 7. The FLIP FLOP
This circuit is said to be UNSTABLE. In other words, it does not stop (with one LED illuminated). It FLIPS and FLOPS from one side to the other. That's why it is called an ASTABLE MULTIVIBRATOR. (pronounced ay' stable)
|When we add 2
capacitors to the circuit above, another amazing thing happens. The
circuit operates ALL BY ITSELF.
It FLIPS and FLOPS and is the basis of an OSCILLATOR.
Because each transistor is ON or OFF, the circuit is said to be a DIGITAL CIRCUIT and the transistors are said to be CROSS-COUPLED.
The next digital building block we
can describe is called a GATE. It is called a GATE
because the output changes ONLY when a particular
condition is delivered to the input.
In this case the GATE causes (or supplies or delivers) INVERSION. This means the output is the opposite of the input.
One of the natural things a transistor does is provide INVERSION.
A transistor will INVERT a signal, turning a LOW input into a HIGH output or a HIGH input into a LOW output.
With a transistor we can create a NAND Gate, NOR Gate and an INVERTER - (called a NOT gate).
The technical word for Inversion is NOT. It is simplified to the letter "N." Thus the word NAND means NOT-AND and NOR means NOT-OR.
A NAND gate is an AND gate with an inverted output.
A NOR gate is an OR gate with an inverted output.
Fig 8. The "NAND" GATE with switches and a transistor
|INVERSION with a
Switch A and Switch B provide a HIGH into the base of the transistor.
When Switch A and Switch B are pressed, the output goes LOW.
For the NAND GATE: close switch A PLUS switch B for the the output to go LOW
Fig 9. The "NOR" GATE with switches and a transistor
Switch A and Switch B provide a HIGH into the base of the transistor.
When Switch A or Switch B is pressed, the output goes LOW.
For the NOR GATE: close switch A or switch B for
the output to go LOW
Fig 10. The "NOT" GATE with a switch and a transistor
A single switch and transistor produces a NOT GATE.
This is simply an INVERSION.
The output of the transistor is HIGH when the switch is NOT pressed. The switch delivers a voltage to the base and the output goes LOW.
To get a transistor to work in
Digital Mode, it needs a resistor on the base and one on
the collector. We will not be discussing the value of these
resistors as a simple experiment will determine the value of the
base resistor is about 1k to 10k and the load resistor on the
collector is 100 ohms to about 1k.
The value of the resistors allow the transistor to turn OFF fully when no base voltage (current) is applied and turns the transistor ON fully when base voltage (current) is applied.
|To describe how a gate
works we use mathematical expressions. This is simply a sentence
with letters and symbols.
These sentences were first described by an inventor called BOOLEAN and that's why they are given the name BOOLEAN EXPRESSIONS.
A Boolean Expression is an answer being TRUE or FALSE. In electronics, we say:
TRUE = "1" and FALSE = "0."
These words give rise to a table we call a TRUTH TABLE
The TRUTH TABLE for the 5 gates we have covered are:
Fig 11. Truth Tables
Fig 12. IC Packages
Fig13. Gate Symbols
All gates have a few things in common.
They have a high resistance input (or reasonably high).
They have only one output.
The voltage on an input line must be above about half rail voltage for the gate to detect a HIGH.
The voltage on an input line must go below about half rail voltage for the gate to detect a LOW.
The output will be HIGH or LOW according to the type of gate and the state of ALL the inputs.
Inputs MUST NOT be left "floating." They must be connected to positive rail or 0v rail.
There are basically two groups of gates. TTL and CMOS.
TTL require 5v supply and they consume a high current when operating.
CMOS can be operated from 3v to 15v and take very little current.
Fig 14. Connecting a Gate
The symbol of the gate you need is added to a circuit diagram and the power rails can be omitted as they are part of the "chip."
The inputs can be connected to a switch, or the output of another gate or any circuit that "swings" from almost 0v to almost rail voltage. Inputs MUST NOT be left "floating."
A Gate will not work if the signal is very small in amplitude.
The output of a gate will deliver about 10mA and this is suitable for driving a LED.
The diagrams show two gates with switches on the inputs to make the outputs go HIGH. The shape of the gate indicates how it will process the information.
Fig 15. Connecting two gates
|Sometimes a gate needs to be directly
connected to another gate to produce a different result, such as
inversion or buffering.
Gates are designed to be connected directly together without any "interfacing" (joining) components.
In figure 15, an AND gate is connected to an INVERTER.
The INVERTER is called a NOT gate and has the letter "N" It turns an AND gate into a NAND gate.
The second diagram shows a NAND gate. It has the two features inside the gate.
You will need to refer to the TRUTH TABLES above to see when the LED is illuminated.
Use the tables to prove the two circuits operate the same.
Fig 16. Waveforms for the Gates
Some gates have only single feature, while other
gates can be used in many different circuits.
A gate with a Schmitt Trigger input can be used as an oscillator as well as detecting a voltage in a timing circuit and it can also be used to remove noise from an input. These features are due to the fact that the HIGH and LOW detection-points on the input are WIDE APART.
The inputs on a NAND gate and most of the other gates has a very high impedance. This similar to the input being connected to "nothing" inside the chip. This is called a HIGH IMPEDANCE LINE or HIGH IMPEDANCE INPUT and the chip detects when this line is HIGH or LOW. But when the voltage on it is about half-rail voltage, the gate does not know if the line is HIGH or LOW and the output jumps up and down very quickly. This is called SELF OSCILLATING and the chip heats up very quickly when doing this.
This will occur if the voltage on the input rises very slowly and to avoid this, the voltage must rise and fall very quickly.
In other words, the input voltage must be DIGITAL. A DIGITAL voltage rises and falls very quickly.
It can remain HIGH or LOW for a long period of time but the TRANSITION from one state to the other must be very quick.
This oscillation occurs because the gap between the chip seeing a HIGH and LOW is very small. To fix this problem, the gap is made very wide. This is covered in the next panel:
Fig 17. Input voltage levels
|We mentioned a gate needs 0v to detect LOW
and 5v to detect HIGH.
These are ideal voltage levels and many gates will detect HIGH at 4.6v and LOW at 4.4v
These levels are very close to each other and if the input voltage is passing from LOW to HIGH very slowly, the gate will start to oscillate when the voltage is moving though this narrow voltage-gap.
This is because the circuit inside the chip cannot work out if the voltage is HIGH or LOW.
To overcome this problem, a special circuit is included that widens the gap. The LOW is 1.6v and the HIGH is 3.3v.
This gap is called the HYSTERESIS GAP and prevents the gate oscillating as well as removing noise from an input voltage.
The circuit is a SCHMITT TRIGGER.
The noise produced by a normal gate
Fig 18. The Schmitt Trigger Gate
|The Schmitt Trigger Gate
The animation shows the gate starting to oscillate when the input voltage is at mid-rail.
To prevent this, the Schmitt Trigger circuit has its detection-point at 66% when going HIGH and 33% when the input is falling.
The second animation shows exactly when the gate changes state.
The electrolytic charges to about 66% of rail voltage, then the output goes low and capacitor discharges via R to about 33% and the output goes HIGH to repeat the cycle.
Fig 20. The 74c14 used in a handy circuit
The Schmitt Trigger is a very versatile gate. It can do many things.
It can be used as an oscillator, a delay, an inverter or a buffer.
The animation above shows the capacitor charging and discharging via the resistor and the output changing from HIGH to LOW to HIGH and the circuit above shows 4 of the gates used to create a handy timer.
Fig 21. The 74c14 as an Oscillator
|The diagrams show
the 74c14 Schmitt Trigger gate as an oscillator.
The output is a square wave and only two components are needed.
Diagram A produces a high frequency and diagram B uses an electrolytic to produce low frequencies.
The output of the oscillator can be seen on a CRO (Cathode Ray Oscilloscope) and it consists of a HIGH and LOW. The length of the HIGH is called the "Mark" and the length of the LOW is the "Space."
We talk about the mark:space ratio of the waveform. In this case it is 50:50 (or 1:1)
The time taken for the waveform to rise from a LOW to a HIGH is called the "rise time." You can see the time is very short (possibly nanoseconds).
Fig 22. The charging of the capacitor is not a linear function (linear graph) but that does not matter because we only need to know the time when the voltage is 33% of rail and 66% of rail.
The output is DIGITAL because it rises and fall very quickly.
Fig 23. Gating the 74c14
The 74c14 oscillator can be turned ON or OFF by an action called GATING.
This is done with a diode on the input and we have seen how a diode can deliver a current when it is connected around the correct way.
The current delivered by the diode keeps the capacitor charged and prevents the gate changing states.
The diode can also be connected so it keeps the capacitor discharged and this prevents the gate from oscillating.
When the diode keeps the capacitor charged, we call the action JAMMING THE OSCILLATOR.
The type of gating depends on if you want the output HIGH or LOW when it is frozen (when in the jammed state). Circuit A keeps the output HIGH and Circuit B jams the oscillator with the output LOW.
In circuit A the jamming diode is connected to a Schmitt gate wired as an inverter. When the output of this inverter is HIGH, the diode does not have any effect (no control) over the charging and discharging of the capacitor. That's because it is said to be "reverse biased."
In simple terms it means the cathode (k) is positive and the anode voltage is always lower than rail voltage (it is between 33% and 66%). By referring to Fig 32 above, we see the diode is connected "around the wrong way" and it will not pass any current. If it does not pass any current, it cannot charge the capacitor and thus it has no effect on jamming the oscillator.
Circuit B works in exactly the same way, but this time the diode does not have any effect on the operation of the oscillator when the output of the inverter is LOW (when its anode is low).
Fig 24. Changing the "Mark:Space" ratio
|The Mark:Space ratio of
the output can be altered
by adding a diode to the charging or discharging path.
In diagram C, the diode is in series with discharging resistor R2 and if this resistor is smaller than R1, the capacitor will discharge faster than it is charged.
This means the LOW time of the output will be shorter than the HIGH time.
In diagram D, the diode is in series with charging resistor R2 and if this resistor is smaller than R1, the capacitor will charge faster than it is discharged.
This means the HIGH time of the output will be shorter than the LOW time.
Fig 25. Adding a BUFFER
|The output of a Schmitt
Trigger will deliver about 10mA. If you try to take more than
10mA, the output will not rise to rail voltage and the gate will
be overloaded. The oscillator may "freeze."
The answer is to add a Transistor BUFFER stage.
The base resistor can be 220R to 4k7.
The resistor does two things:
It allows the output of the Schmitt Trigger to go HIGH and it limits the base current to between 1mA and 10mA.
Without the base resistor we know the base of the transistor does not rise above 0.6v and the oscillator will not work.
Many circuit requires the
combining of ANALOGUE
Here is a gate used to detect a voltage on a TIME DELAY. The rising of the voltage on the mid-point of the two Timing Delay components is analogue (gradually rising and not at a constant rate) and the DIGITAL GATE detects 66% of rail voltage to change the state of the gate. The gate changes very quickly and this is DIGITAL.
Fig 26. The DELAY
The Schmitt Inverter can be configured as a DELAY.
A Delay is a circuit that produces an output after a period of time.
Or it may produce an output for say 3 minutes then turn OFF.
The capacitor sits in charged condition and the output of the circuit is LOW.
A short HIGH on the input of the circuit makes the output of the first inverter LOW and this discharges the capacitor fairly quickly.
This makes the output of the circuit HIGH and the capacitor starts to charge. When the input of the second inverter detects 66% of rail voltage on the capacitor, the output goes LOW.
We mentioned at the beginning that electronics has two main branches: Analogue and Digital.
Sometimes these come together and you have to know the concepts of both to design a particular circuit.
There are many different types of "pick-up" devices and these are often referred to as TRANSDUCERS.
These include microphones, coils, light-detecting devices, switches, and magnetic detectors.
There are also many different types of output devices, including relays, speakers, coils (solenoids), globes, LEDs, sirens, and others.
In most cases you cannot connect any of these devices directly to a circuit. This is because the signal emerging from the device (the transducer) is not suited to the signal required by the input of the circuit. The signal from most transducers is very low (very small voltage) and needs to be amplified.
And with an output device, the circuit will only be able to supply a low current and the output device will require a high current. A CURRENT AMPLIFIER is needed.
In simple terms, the signal from the device to the circuit or from the circuit to the device must be the same amplitude. In addition, the current requirements must also be met.
Getting these two quantities to "match" is an art and is called MATCHING or INTERFACING or simply: JOINING.
When the requirements are met, the maximum amount of energy will pass from one item to the other - one stage to the other.
A STAGE is a self contained section, often called a BUILDING BLOCK and it has an INPUT and an OUTPUT. Quite often a capacitor (electrolytic) will be connected to the input and output to separate the voltages. This allows the signals to pass but keeps the voltages apart as they are generally different values.
Almost all the input devices we have listed are analogue and if you want to connect them to a digital circuit, you will need an ANALOGUE INTERFACING STAGE. It is called an Analogue Interfacing stage because it amplifies a voltage. It is a VOLTAGE AMPLIFYING STAGE
Connecting output devices to a digital circuit will require a AMPLIFYING STAGE. The amplifying stage is actually a CURRENT AMPLIFYING STAGE.
Here is a VOLTAGE AMPLIFYING STAGE
Here is a CURRENT AMPLIFYING STAGE
The micro will only deliver 10mA and the relay needs 100mA (or
more - depending on the type of relay). This is the main reason
for the transistor. It is a CURRENT AMPLIFIER.