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Grade 9 Technology Project Term 3 Mini PAT

Mini PAT 1: Situations where electronic circuits control electric circuits

Grade 9 Technology Project Term 3 Mini PAT An electronic circuit differs from an electric circuit in that it uses very little current and electronic control devices such as thermistors, LDRs, diodes, and transistors.

Many household appliances use electronic circuits to control larger current electric circuits.

The two devices listed below are used inside the electric switchboard (or distribution board) of every building that is safely connected to electricity.

• Ordinary circuit breakers: Shuts off a circuit (for example the circuit supplying all the lights in a house) when the current becomes too big (if the current is too big for the thickness of wire used, the wire will overheat).
• Residual-current circuit breakers: Switches off the main power supply if it detects a leakage of power, such as when a person accidentally touches a “live” electrical wire or contact and the electricity is then conducted through his or her body. This device has to cut the current very quickly; otherwise the person can die due to electric shock. Therefore it switches off the power even when it detects only a small amount of leakage of electrical current.

The following household appliances use electronic circuits to control them:

• ovens: to control the temperature,
• radios and other music appliances: to control the volume of the speakers,
• some energy-saving lights: to switch off automatically when there is enough natural light, and
• kettles: to switch off when the water boils.

Questions:

1. Give two examples of situations or applications where electrical circuits are used.
2. Give two examples of situations or applications where electronic circuits are used.
3. Give three examples of situations or applications where electronic circuits and electric circuits are used together.

Mini PAT 2: Investigate: A circuit with an input sensor, control knob, transistor and output device.

A sensor is a control device that can have a variable effect. A switch can only be open (infinitely large resistance) or closed (zero resistance), so a switch is not a sensor. Devices such as thermistors and LDRs can have different resistances, depending on the temperature or amount of light. They can therefore be used as sensors. A device that can generate a voltage, such as a photovoltaic cell, can also be used as a sensor. A sensor “senses” something such as temperature, or light, just as your body’s senses do. A variable resistor is also a control device, but it is not a sensor, because it is a device for which the user can set the resistance.

The circuit for the fire alarm that you built in Chapter 5 can be used for different applications where a small input current from an input sensor has to switch on a circuit with a larger current for an output device. There is also a variable resistor so that the user can determine at what level of light or temperature (for example) the output device should be switched on or off.

Questions:

1. Name three input components that you know of.
2. Name three output devices that you know of.
3. Name a device that uses a control knob to set the level of something.

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