Sensing circuits

1.2 Circuit concepts

1.4 Output circuits

Sensing circuits

  1. Resistors

    1. Changing resistance

      1. Resistance can be increased by connecting resistors in series: R = R1 + R2
      2. Resistance can be decreased by connecting resistors in parallel: 1R = 1R1 + 1R2

    2. Types

      1. A light-dependent resistor (LDR) is a resistor whose resistance decreases in a non-linear fashion as light intensity increases
      2. A “negative temperature coefficient” thermistor is a resistor whose resistance decreases in a non-linear fashion as temperature increases

    3. Practice

      1. The resistance of a resistor is denoted by bands of colour on them
      2. The first bands of colour represent a value that is multiplied by 10 to the power of the penultimate band to calculate the total resistance
      3. The final band represents the resistor’s tolerance

      4. Colour code:

        Colour Value
        Black 0
        Brown 1
        Red 2
        Orange 3
        Yellow 4
        Green 5
        Blue 6
        Violet 7
        Grey 8
        White 9
        Silver ±10%
        Gold ±5%
      5. The E24 series is a logarithmic scale of 24 resistance values to 2 significant figures with a 5% tolerance, from which resistors should be chosen from
      6. In practice, when a resistance is needed, the least resistant resistor with a resistance higher than the required resistance is used to limit the current

  2. Potential divider

    1. A potential divider circuit is used to produce an output voltage that is a fraction of the input voltage: potential divider circuit diagram
    2. The potential divider rule states: Vout = R2R1 + R2⋅Vin
    3. Fixed resistors can be used in a potential divider to produce an output voltage that is a known fraction of the input voltage
    4. A variable resistor can be used in a potential divider to produce an output voltage that is controlled by changing the resistance in the variable resister
    5. A potentiometer a three-terminal resistor with a sliding contact that forms an adjustable potential divider

  3. Switches

    Type Function
    Push It causes a temporary change in state whilst physically actuated
    Toggle It is manually actuated by a mechanical lever, handle or rocking mechanism
    Reed It is a pair of contacts on ferrous metal reeds that is actuated in magnetic field
    Micro It is actuated at very high speed by very little physical force but separating the contacts requires significant force in the opposite direction
    Tilt It is actuated using a small amount of liquid mercury, or ball bearings, which falls onto the contacts under gravity when the switch is tilted to an appropriate angle
    Rotary It is operated by rotation and is often used when more than two positions are needed