PIR sensors do not 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.<\/p>\n
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.<\/p>\n
Typical values:<\/strong> 10-100 ms for fast sensors, up to 1 second for slow sensors.<\/p>\n A short thermal time constant allows detection of fast events (e.g., a running person). A long thermal time constant makes the sensor more sluggish but may reduce sensitivity to rapid fluctuations (noise).<\/p>\n The electrical time constant is set by the sensor’s internal capacitance and the external load resistance: \u03c4_elec = R_load \u00d7 C_sensor<\/strong>.<\/p>\n Typical C_sensor = 20-50 pF. With R_load = 47 k\u03a9, \u03c4_elec = 1-2 \u00b5s \u2013 negligible compared to thermal effects. However, if R_load is very large (e.g., >1 M\u03a9), \u03c4_elec can become significant (microseconds to milliseconds).<\/p>\n 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.<\/p>\n For fast events like running or falling, you need a sensor with short thermal time constant (fast response). Some sensors are optimized for this purpose.<\/p>\n Slow events like creeping or seated movements 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n Look for:<\/p>\n 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.<\/p>\n 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.<\/p>\n Response time is a critical but often overlooked parameter. Matching sensor response to your application’s motion speed ensures reliable detection.<\/p>\n","protected":false},"excerpt":{"rendered":" Introduction PIR sensors do not 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 The thermal time constant describes how quickly the pyroelectric element heats […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[],"class_list":["post-4050","post","type-post","status-publish","format-standard","hentry","category-reference"],"blocksy_meta":[],"yoast_head":"\r\nElectrical Time Constant<\/h2>\n
Overall Frequency Response<\/h2>\n
Impact on Applications<\/h2>\n
Fast Motion Detection<\/h3>\n
Slow Motion Detection<\/h3>\n
Presence Detection<\/h3>\n
Measuring Response Time<\/h2>\n
Datasheet Specifications<\/h2>\n
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Example: Murata IRA-S200ST01<\/h2>\n
Circuit Design Considerations<\/h2>\n
Conclusion<\/h2>\n