Mini-pat electronic systems and control

Tech2_gr9_ch6_fig1a.tif   Tech2_gr9_ch6_fig1b.tif 

Figure 1: Many everyday devices use electronic control circuits.

electronic circuits to control electric circuits with bigger currents.

An electronic circuit is different from an electric circuit because it only uses a very small current, and because it uses electronic control devices such as thermistors, LDRs, diodes and transistors.

  • 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.
  • Tech2_gr9_ch6_fig2.tif

    Figure 2: An electrical distribution board with circuit breakers

  • 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.
  • 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.

    Figure 3: The control knob of a stove plate is connected to a variable resistor. This controls the current through the heating element. The bigger the current, the hotter the plate will be.


    Figure 4: The control circuit that you built in Chapter 5 for a fire alarm

    The scenario for the Mini-PAT

    Figure 5

    Tech2_gr9_ch6_fig6a.tif Tech2_gr9_ch6_fig6b.tif

    Figure 6: An electric door lock

    Look at the brown part on the right-hand side of the lock mechanism above. This is the part that moves in or out to open or lock the door. This part is called a "latch".


    Figure 7: A 3D assembly drawing of the parts inside an electric door lock

    Design briefSpecificationsConstraintsDesign sketches28198.png Week 2

    There is no such thing as a perfect design! For example, you can make a complicated design that will work very well, but that will be expensive and difficult to build. Or you can make a simple and cheap design that works, but is not strong enough.

    Learn from the different designs that different people made
  • First generate ideas, sketch and explain them,without anyone saying anything negative aboutthe ideas.
  • Once you have several ideas on the table, startthinking about how and whether the differentideas will work or not. Don't talk about "Mary'sdesign" or "Sipho's design". Rather talk about"Design C" or "Design B". Once someone has puta design on the table, you talk about the design.You do not talk about the person. You evaluatethe designs. You do not evaluate yourself orsomeone else.
  • If someone makes a negative remark at this stage, you should say "Red flag! No negative remarks at this stage."

    Saying "Mary's made a bad design" or "Sipho's is much better", for example, will hurt someone's feelings or make others feel proud or arrogant. If someone says "Mary's design ...", you should say "Red flag! We call that Design C."

    28198.png 28198.png 28198.png 28198.png If you do not do this, you won't be able to build a model of your design.prototype.

    Designers and engineers usually make many prototypes before the design is good enough to start manufacturing and selling it. Each prototype is an attempt to improve on the previous one.

  • You brought all the materials needed to make a model of your design.
  • You accurately made the model according to your design drawings.
  • You successfully built the electric output device.
  • You connected your model to the simple circuit with the output device,and used a good method to test it.
  • After you tested your model for the first time, you made a list of all thepossible reasons that your model is not working or why it is notworking well.
  • You used the list to fix or improve your model.
  • You tested your model again, writing down the problems, and going backand fixing or improving your model until it worked, at least one more time.
  • Your model worked, or you wrote a good explanation and made sketches of what you still need to change on your model to make it work.
  • Week 4ons (90 minutes)
  • You were well prepared for your presentation.
  • You explained how you made progress during the design process.
  • You looked at your audience and spoke clearly.
  • An alternative to the kettle swicth project: Designing and building a circuit continuity tester

    Your teacher may decide to let you do the following project instead of designing and building an automatic kettle switch.

    Often when people have to connect wiresin electric circuits, there are so manywires that it is difficult to know whichtwo wire ends are of the same wire.

    It would help to have a device thatshows whether two wire ends areconnected or not. This is whata "circuit continuity tester" does.


    Figure 8

    A circuit continuity tester is actually an open circuit. The circuit can only be closed by the two wire ends thatyou are testing. Use the two test leads of the circuitcontinuity tester to touch the two wire ends that youwant to test. If there is a path for current to beconducted between the two wire ends, this willcomplete the circuit and a light or a buzzer on thecircuit continuity tester will be activated.

    Safety warning:

    First switch off the power supply before you do a test such as this one.

    Note that a circuit continuity tester cannot tell you whether the two wire ends are of the same wire. It can only tell you whether there is a path for current to be conducted between the two wire ends, in other words whether the two wire ends are electrically connected. But if you know that there are no splitting or joining of wires in between the two wire ends, then the wire ends can only be electrically connected if they are of the same wire.

    If you design and build a circuit continuity tester as your project, think about the following:

    A few ideas for building a circuit continuity tester are shown in the photos below.


    Figure 9


    Figure 10