Physics and Electronics

1. Physics and Electronics

2. Electronic systems Electronic systems are made up of 3 parts: • 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:

3. Logic gates NOT gate – “the output is NOT what the input is” 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”

4. Logic gate symbols NOT gate: AND gate: OR gate:

5. 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.

6. Relays M Symbol for relay: 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: Words – motor, larger, electromagnet, small

7. The Resistor Colour Code Significant figures Tolerance (no band = 20%) Multiplier 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 and a tolerance of 5%

8. Drawing electronic circuits This voltmeter will read 12V This bulb and resistor are in parallel V VIN 0V 12V 0V

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

10. Potential Dividers VIN R1 VOUT R2 0V 0V The Potential Divider equation: (R2) VOUT VIN x (R1 + R2)

11. Some example questions 12V 3V 1.5V 50V 100  10  50  75  VOUT VOUT VOUT VOUT 75  45  100  25  0V 0V 0V 0V 0V 0V 0V 0V

12. Practical applications Vin VOUT 0V 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 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

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

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

15. A light dependent switch • When the light on the LDR decreases its resistance _________, which will decrease the ________ across the variable resistor • 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 • The resistor limits the amount of current flowing into the transistor, to avoid __________ it • When the transistor detects the current at its _____ it will “switch __” the collector-emitter current • A small current will then flow through the _______ • The relay will then switch on a _____ current in the output circuit • 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

16. A light dependent switch 6V 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 0V We could modify this circuit (if we wanted to…)

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

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

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

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

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

22. Electronic systems