Sensor Circuits (Thermistors & LDRs)

• As temperature rises, thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). (NTC) resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). falls.
• Smaller R_2 means smaller share of V_in across R_2.
• So V_out across the thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). decreases as temperature increases.
• A variable resistor as R_1 sets the switching threshold.

• As light intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. increases, LDRLight-dependent resistor. A component whose resistance decreases as light intensity increases. resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω). falls.
• Smaller R_2 means V_out across the LDRLight-dependent resistor. A component whose resistance decreases as light intensity increases. decreases.
• A variable resistor as R_1 sets the light level threshold.
• These circuits can trigger alarms, fans, or lighting systems.

If you want V_out to increase with temperature, swap the positions: put the thermistorA component whose resistance decreases significantly as temperature increases (negative temperature coefficient). as R_1 (top) and the fixed resistor as R_2 (bottom). Then as R_1 falls, a larger fraction of V_in appears across R_2.