Electronic

Analog Vs. Digital

Analog –Continuous –Can take on any values in a given range –Very susceptible to noise Digital –Discrete –Can only take on certain values in a given range –Can be less susceptible to noise

Analog versus Digital Analog systems process time-varying signals that can take on any value across a continuous range of voltages (in electrical/electronics systems). Digital systems process time-varying signals that can take on only one of two discrete values of voltages (in electrical/electronics systems). –Discrete values are called 1 and 0 (ON and OFF, HIGH and LOW, TRUE and FALSE, etc.)

Benefits of Digital over Analog Reproducibility Not effected by noise means quality Ease of design Data protection Programmable Speed Economy

Diodes

diodes

Diodes symbol A diode is another semi-conductor device. A diode will only conduct electricity in one direction. They are useful for changing a.c. into d.c – this is called rectification.

A diode is a component that allows a current to flow in one direction only. It has a low resistance in one direction and a very high resistance in the other. Current flows in the direction with low resistance but is not proportional to the voltage. If the voltage is reversed or the diode is connected the other way around, the high resistance of the diode ‘blocks’ the flow of current. Current / A Voltage / V x x x x x x x x x No current flows

Diodes Cathode Wired towards the negative terminal Anode Wired towards the positive terminal

Which way through ? This is the direction which current will easily pass through a diode.

A diode has a current of 5.0 A running through it and a resistance of 5.0 . What is the potential difference across the diode? V = I R = 5.0 A x 5.0  = 25 V Diode calculation

Example circuits An electric current can pass through the diode on the left, allowing the lamp to light. The diode on the right stops the current and hence the lamp remains out.

Changing a.c. to d.c. tt II INPUTOUTPUT DIODE The diode conducts in this direction. So that we can get an output voltage. The diode will not conduct in this direction. This means that there will be no output voltage.

Half wave rectification

Smoothing Although the above produces a direct voltage and current, it is not the steady sort of dc we get from a battery. To ‘smooth’ the voltage we add a capacitor. The capacitor, labelled C, is placed in the half-wave rectifier circuit as indicated below:

The effect of the capacitor on the voltage across R is represented below:

Light Emitting Diodes A LED is a type of diode designed to emit light. The light can be visible such as a laptop light. It can also be infra red such as on a remote control. Here is its circuit diagram symbol. Circuit symbol for an LED

Quick graph quiz ? Fixed resistor at constant temperature ? ? Filament lamp Diode

Initially the voltage across the resistor rises to its maximum, and the capacitor charges to its maximum, the top plate becoming positive. Without the capacitor, the voltage across R then drops to zero and stays there for half a cycle. But now, as soon as the voltage starts to fall, the capacitor start to discharge through the resistor – this maintains the voltage across it close to its maximum until the next cycle starts. Thus, the voltage across the resistor and the current though it are smoother than without the capacitor.

Potential Dividers 0V V IN V OUT 0V R1R1 R2R2 (R 1 + R 2 ) V IN x (R 2 ) V OUT The Potential Divider equation:

Some example questions 0V 12V V OUT 0V 100  0V 1.5V V OUT 0V 50  45  0V 50V V OUT 0V 10  75  0V 3V V OUT 0V 75  25 

Practical applications 0V V in V OUT Here’s a potential divider that is used to control light-activated switches… When the light intensity on the LDR decreases its resistance will ________. This causes V OUT to _______ so the processor and output will probably turn _____. The variable resistor can be adjusted to change the ________ of the whole device. Words – decrease, sensitivity, increase, off

Reed switch

Reed switch in a Burglar alarm

Transistors A transistor acts like a switch: Base Collector Emitter When a SMALL current flows through the base- emitter part of the transistor a different current is switched on through the collector- emitter part. It conducts between C and E when the voltage between B and E is above +0.6V.

A Frost alarm

A light dependent switch 6V 0V Power supply Output device

A light dependent switch 1)When the light on the LDR decreases its resistance _________, which will decrease the ________ across the variable resistor 2)This will cause V OUT to ____. The____ gate will recognise this as a “0” and convert it into a “1”, i.e. a current will flow into the resistor 3)The resistor limits the amount of current flowing into the transistor, to avoid __________ it 4)When the transistor detects the current at its _____ it will “switch __” the collector-emitter current 5)A small current will then flow through the _______ 6)The relay will then switch on a _____ current in the output circuit 7)The “reversed biased” diode is also placed in the circuit to act as a “_______” to prevent current flowing back into the transistor when the relay is switched _____ Words – base, buffer, on, increases, damaging, relay, off, larger, voltage, drop, NOT

A light dependent switch 6V 0V We could modify this circuit (if we wanted to…) 2) Adjust this resistor to vary the sensitivity 1) Swap these two around and the output will now switch on when it becomes LIGHT, not when it becomes dark

The Capacitor A capacitor is a device that can store charge (it has a “capacity”). It is basically made of two plates: …or… Charge builds up on these plates and the voltage between them increases until it reaches the supply voltage.

Charging and discharging a capacitor P.d. across capacitor Time P.d. Time P.d. Time Increase resistance or capacitance

Time delay circuits 6V 0V Power supply Output device R

Time delay circuits “1” 6V 0V Power supply Output device R

Time delay circuits 1)When the switch is closed the capacitor is being short circuited so no charge builds up on it 2)This means that the input to the NOT gate is __, so the output is 1 and the output device is ___ 3)When the switch is released the capacitor starts to ________ up 4)When the voltage across the capacitor reaches a certain level the input to the NOT gate becomes __ so its output is 0 5)This means that the output device is now switched ___ 6)To INCREASE the amount of time taken to switch the device off you could: 1)Increase the _________ of the capacitor 2)_________ the resistance of the resistor R Words – charge, 1, capacitance, increase, 0, off, on

Gates The most basic digital devices are called gates. Gates got their name from their function of allowing or blocking (gating) the flow of digital information. A gate has one or more inputs and produces an output depending on the input(s). A gate is called a combinational circuit. Three most important gates are: AND, OR, NOT

LOGIC GATES Logic generally has only 2 states, ON or OFF, represented by 1 or 0. Logic gates react to inputs in certain ways. Symbol for AND gate INPUT A INPUT B OUTPUT Q Logic can be used to control devices according to certain conditions, such as “switch on a fan if it’s hot AND the sun is out”. Look at the diagrams below. The AND gate will only switch on its output Q, if Input A is ON and Input B is ON. This can be shown in a Truth Table, 0=OFF and 1=ON. ABQ If both inputs are OFF the output is OFF Even if one input is ON the output is OFF Only if A =1 and B =1 will the output switch on

MORE LOGIC GATES Try and work out the truth tables for these gates. The rule will help you. ABQ NOT OR A B Q NAND A B Q NOR A B Q XOR A B Q AQ RULE: Q = 0 if A OR B =1 ABQ RULE: Q = 0 if A AND B =1 ABQ AND A B Q RULE: Q = 1 if A AND B =1 RULE: Q = 1 if A OR B =1 ABQ ABQ AQ 0 1 RULE: Q = 0 if A =1 Q = 1 if A OR B =1, but NOT both

LOGIC GATES ABQ NOT OR A B Q NAND A B Q NOR A B Q XOR A B Q AQ RULE: Q = 0 if A OR B =1 ABQ RULE: Q = 0 if A AND B =1 ABQ AND A B Q RULE: Q = 1 if A AND B =1 RULE: Q = 1 if A OR B =1 ABQ ABQ AQ RULE: Q = 0 if A =1 Q = 1 if A OR B =1, but NOT both