Key Engineering Takeaways
- •LDR photoresistors drop in electrical resistance when photon energy excites electrons across the cadmium sulfide (CdS) bandgap.
- •NTC (Negative Temperature Coefficient) thermistors DROP in resistance as temperature RISES; standard value is 10kΩ at 25°C.
- •PTC (Positive Temperature Coefficient) thermistors INCREASE resistance as temperature rises; used as resettable PPTC fuses (Polyfuses).
- •The Steinhart-Hart and B-parameter equations convert measured NTC resistance into exact Celsius/Kelvin temperature.
- •Multi-turn trimpots (e.g. 25-turn Bourns 3296) allow sub-millivolt precision reference voltage tuning for comparator triggers.
Prerequisites
- • Voltage Dividers and Potentiometers
Required Hardware / Tools
- • GL5528 LDR Photoresistor
- • 10kΩ NTC Thermistor
- • 10kΩ Multi-Turn Trimpot
- • 10kΩ Resistors
- • Breadboard and Multimeter
Light Dependent Resistors (LDR / Photoresistors)
A **Light Dependent Resistor (LDR)** is a passive sensor whose resistance is inversely proportional to ambient light intensity (Lux).
### Physical Working Principle:
Constructed with a serpentine track of **Cadmium Sulfide (CdS)** semiconductor:
- **In Total Darkness**: Free charge carriers are scarce to High resistance (100 kΩ - 1 MΩ).
- **In Bright Sunlight**: Incident photons knock electrons into the conduction band to Low resistance (100 Ω - 1 kΩ).
By pairing an LDR with a 10 kΩ fixed resistor in a voltage divider, you obtain an analog voltage that tracks room ambient lighting.
Thermistors: NTC (Negative) vs PTC (Positive) Temperature Sensors
A **Thermistor** is a thermally sensitive ceramic semiconductor resistor:
### 1. NTC (Negative Temperature Coefficient):
- **Resistance drops sharply as temperature rises**.
- Standard rating: **10 kΩ at 25^circC (77^circF)**.
- Widely used in 3D printer hotends, lithium battery pack thermal monitors, and HVAC thermostats.
### 2. PTC (Positive Temperature Coefficient):
- **Resistance spikes dramatically above a threshold temperature**.
- Used as **Resettable Fuses (Polyfuses)**: If high motor current overheats the polyfuse, its resistance jumps to megaohms, cutting off current until the short is removed.
Steinhart-Hart & Beta Equation Temperature Math
To convert an NTC thermistor's measured resistance (R) into exact temperature in Kelvin (T), embedded firmware uses the **B-Parameter equation**:
**Formula:** (1 / T) = (1 / T_0) + (1 / B) ln((R / R_0))
Where:
- T_0 = 298.15 K (25^circC)
- R_0 = 10{,}000 Ω
- B = 3950 (Standard NTC Beta coefficient)
- Convert Kelvin to Celsius: T_^circC = T - 273.15
3D Printer ThermistorsThe standard 100kΩ NTC 3950 thermistor on 3D printer nozzles uses this exact formula in Marlin and Klipper firmware!
Frequently Asked Questions
Why are digital temperature sensors like DS18B20 or BME280 preferred over thermistors?
Digital sensors integrate the sensing element, ADC, calibration coefficients, and I2C/OneWire communication into a single chip, outputting factory-calibrated Celsius readings without complex analog math or reference resistor tolerances.
Tags:#LDR#Thermistor#NTC#Trimpots#Resistive Sensors#Analog Conditioning