Introduction
PIR sensors don’t respond instantly. Their response time is governed by two factors: the thermal time constant of the pyroelectric element and the electrical time constant of the readout circuit. This article explains these parameters and their practical implications.
Thermal Time Constant (τ_th)
The thermal time constant describes how quickly the pyroelectric element heats up in response to incident IR. It is determined by the element’s heat capacity and thermal conductivity to the substrate.
Typical values: 10-100 ms for fast sensors, up to 1 second for slow sensors.
A short τ_th allows detection of fast events (e.g., a running person). A long τ_th makes the sensor more sluggish but may reduce sensitivity to rapid fluctuations (noise).
Electrical Time Constant (τ_elec)
The electrical time constant is set by the sensor’s internal capacitance and the external load resistance: τ_elec = R_load × C_sensor.
Typical C_sensor = 20-50 pF. With R_load = 47 kΩ, τ_elec = 1-2 µs – negligible compared to thermal effects. However, if R_load is very large (e.g., >1 MΩ), τ_elec can become significant (microseconds to milliseconds).
Overall Frequency Response
The combination of thermal and electrical time constants creates a bandpass response. The sensor is most sensitive to frequencies where both effects are minimal. Typical passband: 0.1 Hz to 10 Hz.
Measuring Response Time
To measure response time, use a pulsed IR source (e.g., an IR LED modulated at various frequencies). Observe the output amplitude vs. frequency to determine the 3dB bandwidth.
Impact on Applications
Fast Motion Detection (e.g., running, falling)
For fast events, you need a sensor with short thermal time constant (fast response). Some sensors are optimized for this.
Slow Motion Detection (e.g., creeping, sitting movements)
Slow events produce low-frequency signals. The sensor’s response down to 0.1 Hz is important. The electrical time constant and high-pass filtering in the amplifier affect this.
Presence Detection (very slow)
For detecting subtle movements like breathing, you need a sensor with response extending to very low frequencies (<0.1 Hz). This may require special sensor selection and circuit design.
Sensor Datasheet Specifications
Look for:
- Responsivity vs. frequency: A graph showing output vs. modulation frequency.
- Thermal time constant: Sometimes specified directly.
- Frequency range: Typical operating range.
Example: Murata IRA-S200ST01
According to its datasheet, the IRA-S200ST01 has a thermal time constant of approximately 100 ms, giving it a usable frequency range from about 0.1 Hz to 10 Hz.
Circuit Design Considerations
The amplifier’s high-pass filter (for DC blocking) also affects low-frequency response. Choose coupling capacitors and feedback networks to ensure the desired lower cutoff frequency.
Conclusion
Response time is a critical but often overlooked parameter. Matching sensor response to your application’s motion speed ensures reliable detection.