Physics and Electronics

Electronic systems

Electronic systems are made up of 3 parts: 1) An INPUT SENSOR – these detect changes in the environment Examples: 2) A PROCESSOR – these decide what action is needed Examples: 3) An OUTPUT DEVICE – these are controlled by the processor Examples:

Logic gates

Logic gates are the basics behind any kind of processor. There are 3 that you need to know : AND – “the output is on if A

AND B are both on”

OR – “the output is on if A

OR B are on”

NOT gate – “the output is

NOT what the input is”

Input 0 1 Output 1 0 Input A 0 0 1 1 Input A 0 0 1 1 Input B 0 1 0 1 Input B 0 1 0 1 Output 0 0 0 1 Output 0 1 1 1

NOT gate:

Logic gate symbols

AND gate: OR gate:

Some problems to solve

• The pump on a central heating system is switched on at room temperature if the system is switched on (with the slide switch). When the temperature rises the pump needs to be switched off.

• Design a circuit that will sound a buzzer if the temperature of a hot radiator falls during the day ONLY. Include a test switch to check the operation of the buzzer.

• Design a circuit for a gardener that will warn them of cold conditions at night. The alarm should be able to be switched off.

Relays

Circuits such as those containing logic gates only take very ______ currents. These circuits may be needed to operate a device that takes a much _____ current, e.g. a _________. To do this the circuit would need a RELAY switch, a device made of an _______ that can operate a switch. A relay switch is activated by the small current and the switch part is placed in the circuit needing a large current: M Words – motor, larger, electromagnet, small Symbol for relay:

The Resistor Colour Code

We would use this to determine the value (in



) of a resistor:

E.g this resistor would have a value of 42 x 10 = 420 of 5%  and a tolerance Significant figures 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 0.01

0.1

1 10 100 1k 10k 100k 1M 10M 5% 1% 2% Tolerance

(no band = 20%)

Multiplier

Drawing electronic circuits

12V V This voltmeter will read 12V 0V V IN This bulb and resistor are in parallel 0V

Drawing electronic circuits

1) Two resistors in series V IN 2) Two resistors in series with one bulb in parallel V IN 0V 3) A voltmeter measuring the voltage across two bulbs in series V IN V 0V 0V 4) An ammeter measuring the current through a resistor V IN A 0V

V IN

Potential Dividers

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

12V 0V

Some example questions

50V 100  10  V OUT 100  75  0V 0V V OUT 0V 3V 0V 75  25  1.5V

V OUT 0V 0V 50  45  V OUT 0V

Practical applications

V in Here’s a potential divider that is used to control light-activated switches… V OUT 0V When the light intensity on the LDR decreases its resistance will ________. This causes V of the whole device.

OUT to _______ so the processor and output will probably turn _____. The variable resistor can be adjusted to change the ________

Words – decrease, sensitivity, increase, off

Transistors

A transistor acts like a switch: Collector Base When a SMALL current flows through the base emitter part of the transistor a different current is switched on through the collector emitter part.

Emitter

6V

A light dependent switch

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 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

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 6V 2) Adjust this resistor to vary the sensitivity 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

6V R

Time delay circuits

Power supply Output device 0V

6V R

Time delay circuits

Power supply 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

System

Electronic systems

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