Build and draw electronic circuits


Figure 1: A part of the circuit for a radio


Figure 2: All of these appliances contain electric circuits.

LEDYou will need:
  • an LED,
  • a 470 resistor,
  • a switch,
  • four 1,5 V cells in series, or a 9 V battery, and
  • electric conducting wire with crocodile clips for connections.
  • Tech2_gr9_ch5_fig3.tif 40044.jpg 

    Figure 3: A circuit with an LED, a battery, a switch and a resistor

    You will need:
  • an LDR,
  • four 1,5 V cells in acell holder, and
  • a buzzer.
  • Tech2_gr9_ch5_fig4.tif

    Figure 4: A circuit where the current is regulated by a light-dependent resistor

    40044.jpg   output device on and off without using a switch. Instead of a switch controlled by hand, this type of circuit uses an input sensor in combination with a transistor to switch the output device on or off automatically, depending on the measurement of something by the input sensor.control circuit since one circuit controls another circuit. In the case where a transistor is used with a sensor such as an LDR, the base-emitter current controls the larger collector-emitter current.

    Figure 5: The circuit diagram for the control circuit

  • If a decrease in resistance of the input sensor should switch on the output device, then resistor 2 and the input sensor should be arranged as in Figure 5. Look back at the circuit for a day/night switch using a light-dependent resistor (LDR) on page 48.
  • If an increase in resistance of the input sensor should switch on the output device, then resistor 2 and the input sensor should be arranged in the opposite way of Figure 5. Look back at the circuit for a heat-activated switch using a negative-temperature coefficient (NTC) thermistor on page 51.
  • Tech2_gr9_ch5_fig6.tif 

    Figure 6: A systems diagram of a control circuit


    Figure 7

  • a battery consisting of 6 cells in series,
  • an input sensor to measure the temperature,
  • a variable resistor to set the temperature at which the alarm should go off,
  • an output device to make noise when it gets too hot, and
  • a transistor to switch the output device on when it gets too hot.
  • 35587.jpg 

    Figure 8: A circuit diagram showing the different components in a fire alarm

    1 has already been explained. The purpose of the other two resistors is difficult to explain. It has to do with the minimum current to the base of the transistor that is needed to allow current through from the emitter to the collector of the transistor. If you choose to study more electronics in FET or at university, you will learn about the purpose of these resistors, and how to calculate their resistances.specifications for the resistances of components.
  • R1 = 700 to 1400 kΩ (variable resistor)
  • R2 = 820 Ω
  • R3 = 1 kΩ
  • input sensor: 10 kΩ
  • You need the following materials to build the circuit:
  • a 9 V battery and a connection clip with red (+) and black (-) wires,
  • conduction wires with crocodile clips,
  • a 10 k NTC thermistor,
  • a 700 to 1 400 k variable resistor;
  • a 820 and a 1 k resistor,
  • an npn transistor, and
  • a buzzer.
  • Troubleshooting
  • test whether the battery is flat or not,
  • test all your connections again,
  • follow the flow of the current on your board with your finger, to check whether you connected the components the right way, and
  • check that you connected the transistor the right way round.
  • You will need:
  • four 1,5 V cells in series, or a 9 V battery,
  • two LEDs,
  • a 470 Ω resistor,
  • a 1 000 µF capacitor, and
  • an SPDT switch.
  • Tech2_gr9_Ch5_fig9.tif
    Figure 9: A time-delay circuit

    If you try to build a more complicated circuit by connecting components using conducting wire and crocodile clips, many wires will cross one another and the circuit will be messy, looking like a tangled bunch of ropes.

    To make a complicated circuit in a neater and smaller way, most circuits are built on boards such as "bread boards", "strip boards", or "printed circuit boards" (PCBs).

    Figure 10 below shows a simple LED circuit, such as the one you built in section 5.1, but here it is built on a strip board. Notice that there are no connecting wires used to build this circuit! This is because at the bottom of the strip board there are parallel copper strips connecting the holes in each column. This makes it possible to construct a circuit without using wire.


    Figure 10: A simple LED circuit built on a strip board


    Figure 11: One possible layout of the simple LED circuit on a strip board

    Figure 11 shows one possible plan of how to arrange the simple LED circuit on a strip board. The copper strips are at the bottom of a strip board, and not visible from the top. Therefore the copper strips on the drawing of the layout were drawn with hatching, to show that you cannot really see them from the top.

    The arrows on Figure 11 are drawn to help you understand how current flows through the copper strips at the back of the strip board. The current flows in the direction of the arrows.

    The connectors of the components are soldered to the copper strips at the bottom of the strip board. This is to ensure that they make proper electrical contact with the copper strips.

    Soldering is done with lead, because lead is a good electrical conductor and has a low melting point, so it is easy and quick to melt it with a soldering iron.

    Bread boards and printed circuit boards are other types of boards used to build complicated circuits. They also have copper connections at the back, but these connections are arranged in a different way than on a strip board.


    Figure 12: Soldering components onto the back of a strip board

    Tech2_gr9_ch5_fig13a.tif Tech2_gr9_ch5_fig13b.tif

    Figure 13: The front and back of a bread board

    With a breadboard it is not necessary to solder connections, since each hole in the breadboard has a spring that grips the wire tightly to make proper electrical contact.

    Almost all manufactured electronic devices use printed circuit boards, where the copper connections at the back can be made in any pattern. This makes it possible to make complicated circuits that are very small.

    Tech2_gr9_ch5_fig14a.tif Tech2_gr9_ch5_fig14b.tif

    Figure 14: The front and back of a printed circuit board

    The next chapter is your Mini-Pat for this term. You will learn how an electronic circuit can be used to control another circuit with a much bigger current. You will build a device using both circuits and then test it.