Thermistors, Light-Dependent Resistors and Sensing Circuits · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Sensors that split voltage

Science · Energy · ages 15-16
Name ______________________   Date ____________

  1. More light reaches a light-dependent resistor. What happens to its resistance?

    • It stays constant
    • It decreases
    • It increases

  2. Which trend belongs on a graph of an NTC thermistor's resistance against temperature?

    • Resistance falls as temperature rises
    • Resistance rises as temperature rises
    • Resistance stays constant as temperature rises

  3. In an unloaded two-resistor divider, Noor says the resistor with greater resistance has the greater voltage across it. Is Noor right?

    Circle one:   True   False

  4. A dark-activated circuit contains a fixed resistor, an LDR and a lamp. Which component senses the room's light level?

    • A fixed resistor
    • An LDR sensor
    • A filament lamp

  5. An unloaded divider has a 12 V supply, a 3 kilo-ohm upper resistor and a 1 kilo-ohm lower resistor. What is the output voltage from the midpoint to 0 V?

    • 4 V
    • 9 V
    • 3 V

  6. An unloaded divider has an LDR above a fixed resistor. The supply stays steady. As the room darkens, what happens to the output voltage measured from the midpoint to 0 V?

    • It decreases
    • It stays constant
    • It increases

  7. Kim connects a load across the lower resistor of a divider. The supply voltage and upper resistor stay unchanged, but the output voltage falls. Why?

    • It adds a series path, raising the lower branch resistance
    • It adds a parallel path, lowering the lower branch resistance
    • It leaves branch resistance unchanged but raises the supply voltage

  8. A controller for a cold switch turns on when divider output rises above a reference. Output is measured across the lower resistor. Which unloaded divider makes output rise as the room cools, with light and supply voltage unchanged?

    • Upper NTC thermistor; lower fixed resistor
    • Upper fixed resistor; lower light sensor LDR
    • Upper fixed resistor; lower NTC thermistor
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Sensors that split voltage W1-mt_d92v1CiqIG-s1

  1. It decreases · An LDR has lower resistance when more light reaches it.
  2. Resistance falls as temperature rises · An NTC thermistor has lower resistance at a higher temperature, so its resistance-temperature graph has a falling trend.
  3. True · The same current passes through both resistors. Their voltage shares are proportional to resistance, so the greater resistance has the greater voltage.
  4. An LDR sensor · The LDR senses light through a change in resistance. The lamp is the controlled load.
  5. 3 V · The output is across the lower resistor: 12 x 1 / (3 + 1) = 3 V.
  6. It decreases · Darkness raises the upper LDR's resistance. Its voltage share grows, leaving less voltage across the lower fixed resistor, where the output is measured.
  7. It adds a parallel path, lowering the lower branch resistance · The load is in parallel with the lower resistor. This lowers the lower branch's effective resistance and its share of the unchanged supply voltage.
  8. Upper fixed resistor; lower NTC thermistor · Cooling raises NTC thermistor resistance. In the lower position, this raises output voltage and can turn on a controller with a suitably set reference.
Worksheet · LightMySky