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Electronic Analog Vs. Digital 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. x x x x x x No current flows x x 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. x Current / A Current flows in the direction with low resistance but is not proportional to the voltage. Voltage / V Diodes Anode Wired towards the positive terminal Cathode Wired towards the negative terminal Which way through ? This is the direction which current will easily pass through a diode. Diode calculation 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 = IR = 5.0 A x 5.0 = 25 V 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. So that we can get an output voltage. The diode conducts in this direction. I I t t INPUT The diode will not conduct in this direction. OUTPUT DIODE 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. VIN Potential Dividers R1 VOUT R2 0V 0V The Potential Divider equation: VOUT VIN x (R2) (R1 + R2) Some example questions 12V 50V 100 100 0V 0V 3V 75 0V VOUT 0V 1.5V 75 25 0V 10 VOUT 50 VOUT 0V 45 0V VOUT 0V Practical applications Here’s a potential divider that is used to control light-activated switches… Vin VOUT 0V When the light intensity on the LDR decreases its resistance will ________. This causes VOUT 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: Collector Base Emitter When a SMALL current flows through the baseemitter part of the transistor a different current is switched on through the collectoremitter 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 Power supply Output device 0V 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 VOUT 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 We could modify this circuit (if we wanted to…) 1) Swap these two around and the output will now switch on when it becomes LIGHT, not when it becomes dark 2) Adjust this resistor to vary the sensitivity 6V 0V 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 P.d. Increase resistance or capacitance Time Time P.d. Increase resistance or capacitance Time Time Time delay circuits 6V Power supply R Output device 0V Time delay circuits 6V Power supply R Output device “1” 0V 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. 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. INPUT A OUTPUT Q INPUT B Symbol for AND gate A B Q 0 0 0 0 1 0 1 0 0 1 1 1 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. 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. AND A OR A Q B Q B RULE: Q = 1 if A AND B =1 A B 0 XOR A B RULE: Q = 1 if A OR B =1 Q Q A B 0 0 0 1 1 1 Q = 1 if A OR B =1, but NOT both A B 0 0 0 0 1 0 1 0 1 0 1 0 1 1 1 1 1 NAND A Q Q NOT NOR A Q B Q A Q B RULE: Q = 0 if A AND B =1 A B 0 Q RULE: Q = 0 if A OR B =1 Q RULE: Q = 0 if A =1 A B A 0 0 0 0 0 1 0 1 1 1 0 1 0 1 1 1 1 Q LOGIC GATES AND A OR A Q B Q B RULE: Q = 1 if A AND B =1 XOR A Q B RULE: Q = 1 if A OR B =1 Q = 1 if A OR B =1, but NOT both A B Q A B Q A B Q 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 0 1 1 1 0 0 1 0 1 1 0 1 1 1 1 1 1 1 1 1 0 NAND A NOT NOR A Q B Q A Q B RULE: Q = 0 if A AND B =1 RULE: Q = 0 if A OR B =1 RULE: Q = 0 if A =1 A B Q A B Q A Q 0 0 1 0 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1 1 0 0 1 1 0 1 1 0