Practical sensing systems use potential dividers to convert environmental changes, such as light or temperature variations, into measurable voltage signals for control, monitoring, and automation applications across electronics.

Designing Practical Sensing Systems

Designing practical sensing systems involves applying the potential divider principle to produce an output voltage that depends on a changing physical condition, such as light intensity or temperature. By combining variable resistive components—commonly light-dependent resistors (LDRs) and thermistors—with fixed resistors, a potential divider can provide a predictable, adjustable output voltage for controlling devices or feeding into measurement instruments like voltmeters or data loggers.

The Potential Divider Principle

A potential divider is a simple resistive circuit that splits an input voltage into portions proportional to the resistance of its components.

Schematic of a two-resistor potential divider showing V_in, resistors R₁ and R₂, and the tap for V_out. The divider produces an output proportional to the resistance ratio, forming the basis of sensor circuits. The image contains only the essential elements required by the syllabus. Source.

EQUATION
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Potential Divider Equation (V_out) = (R₂ / (R₁ + R₂)) × V_in
V_out = Output voltage (V)
R₁, R₂ = Resistor values (Ω)
V_in = Input voltage (V)
—-----------------------------------------------------------------

This equation forms the mathematical foundation for designing sensing systems, as it relates output voltage to the ratio of resistances. In sensor applications, one of the resistors changes its resistance according to an environmental factor, causing V_out to vary correspondingly.

Designing Temperature-Sensing Circuits

Temperature-sensing systems typically employ a negative temperature coefficient (NTC) thermistor, whose resistance decreases as temperature increases.

Potential-divider temperature sensor with an NTC thermistor and a fixed resistor producing a voltage that varies with temperature. Placing the thermistor as the upper or lower element sets whether V_out rises or falls with temperature. The diagram focuses on the divider; any external comparator or controller is not shown, in line with the syllabus scope. Source.

The thermistor can be positioned either as the upper or lower resistor in the divider, depending on the desired voltage-temperature relationship.

Configurations

• Thermistor in lower position (R₂):

  • As temperature increases, resistance decreases.

  • V_out decreases with temperature rise.

  • Suitable for circuits where high temperature must result in a low output signal.

• Thermistor in upper position (R₁):

  • As temperature increases, resistance decreases.

  • V_out increases with temperature rise.

  • Ideal for systems where high temperature triggers a higher control voltage.

In each design, it is important to select a fixed resistor of comparable resistance to the thermistor at the midpoint of the operating temperature range. This maximises sensitivity and ensures a linear response within the desired range.

Designing Light-Sensing Circuits

Light-sensing circuits use an LDR (light-dependent resistor), whose resistance decreases with increasing light intensity. This property allows the circuit to convert variations in illumination into corresponding voltage changes.

Configurations

• LDR in lower position (R₂):

  • Bright light → lower resistance → lower V_out.

  • Suitable for automatic lighting control where darkness increases V_out to activate lamps.

• LDR in upper position (R₁):

  • Bright light → lower resistance → higher V_out.

  • Useful in circuits where high light intensity increases control voltage (e.g., for display dimming systems).

These configurations form the basis for automatic systems such as streetlights, camera exposure controls, and light-activated alarms.

Sensor Characteristics and Circuit Matching

The design process requires understanding the characteristic curves of LDRs and thermistors, which show how resistance varies with the physical variable. These curves are typically non-linear, meaning sensitivity varies with temperature or light level. For optimal operation, the resistor ratio should be chosen so that the output voltage changes significantly within the range of interest.

Key design considerations include:

• Supply voltage (V_in):
  Must be constant for reliable readings.

• Sensor resistance range:
  Select fixed resistors to match mid-range sensor values.

• Linearisation:
  For better accuracy, small operational ranges can approximate linear behaviour.

• Calibration:
  Measured voltage values should be compared against known environmental conditions to ensure reliability

Interfacing with Control and Measurement Devices

Practical sensing systems often interface with microcontrollers, comparators, or operational amplifiers. The output voltage (V_out) can serve as an input to a comparator circuit, triggering a device when the voltage exceeds a threshold. For instance:

• A temperature alarm may activate when V_out falls below a set level.

• A light-activated relay may switch on when V_out rises above a threshold.

For data acquisition, the output may be fed into an analogue-to-digital converter (ADC), enabling digital systems to interpret sensor responses. Proper impedance matching ensures minimal loading effect, maintaining accurate voltage readings.

Practical Design Steps

When designing a sensing circuit, students should follow a structured approach:

• Identify the physical variable to measure (temperature or light).

• Choose a suitable sensor component (NTC thermistor or LDR).

• Determine the supply voltage appropriate for the circuit.

• Select a fixed resistor that balances sensitivity and stability.

• Connect the components in the required potential divider configuration.

• Measure the output voltage at different conditions.

• Adjust resistor values for the desired voltage-response profile.

• Include protective components (fuses or current-limiting resistors) where appropriate.

Circuit Design Considerations and Limitations

While potential divider circuits are simple and effective, practical designs must consider limitations and real-world influences:

• Loading effects:
  When a voltmeter or external circuit is connected, it effectively adds parallel resistance, reducing measured voltage.

• Temperature drift:
  Ambient temperature variations can affect both sensor and resistor characteristics.

• Power dissipation:
  Excessive current through resistors may alter readings or damage components.

• Response time:
  Some sensors, particularly thermistors, respond slowly to environmental changes.

• Noise and interference:
  Electrical noise may cause output fluctuations; shielding and filtering may be necessary.

By anticipating these effects, designers can ensure their sensing systems remain accurate, responsive, and safe under various operating conditions.

Application Examples in Practical Systems

Potential divider-based sensing systems are integral to modern automation and measurement technologies. Examples include:

• Automatic lighting controls responding to daylight levels.

• Climate-control systems adjusting heating based on ambient temperature.

• Safety alarms that trigger under high-temperature or darkness conditions.

• Industrial monitoring circuits providing analogue feedback to control systems.

Each application relies on the same fundamental principle: converting physical variations into corresponding electrical signals through effective use of the potential divider.