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A bare pyroelectric PIR element does not produce a ready-to-use logic signal. In the ST reference design, human motion creates roughly a 1 mV peak-to-peak AC waveform riding on a variable sensor-dependent bias. A practical front end must preserve the bias, reject its unwanted DC variation, filter noise, amplify the motion band, and then feed either an ADC or a threshold detector. The example below uses a 0.5–5 Hz motion band, about 69 dB of total gain, and a window comparator, but those numbers are application-specific rather than universal requirements.
This architecture is described in the All About Circuits Industry Article (July 15, 2016), based on ST application note AN4368 (November 2013). Treat the circuit as a design method and reference topology; verify every component against current datasheets and availability.
What a bare PIR element actually detects
PIR means passive infrared: the element does not transmit energy. It responds to changes in incident infrared radiation. A common dual-element device produces a differential output. With both elements seeing the same, steady thermal scene, their outputs largely cancel. When a warm body crosses from one lens zone to the other, the differential signal changes polarity.
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Thus the sensor detects a changing thermal pattern, not motion as an abstract quantity. A stationary person can fade from the AC output after the thermal scene settles. Lens geometry is part of the detector: a Fresnel lens segments the field of view and can increase effective coverage and range, but performance depends on focal geometry, mounting and the thermal environment. The source discussion of this operating principle is at All About Circuits.
#1 Best Overall
- WWZMDiB 5 Pcs PIR Sensor: When a human body enters the sensing range, the temperature difference between the body and the background causes a voltage change in the pyroelectric device. After amplification and comparison, the voltage signal is output.
- Voltage：DC 4.5-20V
- Detection Angle: <110 ° cone angle Lens size
- Detection range: 3-7 meters (10-23 feet)(adjustable)
- Two triggering modes: H: The output signal is maintained as long as a person is present. L: Triggered once with each change.
Why signal conditioning is unavoidable
- The useful signal may be only about 1 mVpp in the cited design.
- A much larger, changing DC level accompanies it.
- Switching supplies, mains pickup, mechanical thermal changes and wideband noise can overwhelm the motion waveform.
- Applying high gain before controlling the DC component can drive an amplifier into saturation.
The central rule is to avoid amplifying the unwanted DC component along with the motion signal. Establish a controlled bias, AC-couple or high-pass the signal, and distribute gain across stages with adequate headroom.
Reference signal chain
The ST architecture has three functional sections: two gain/filter stages followed by a window detector. The original circuit is documented in AN4368.
Stage 1: bias control, high-pass filtering and initial gain
The first stage removes the slowly varying component while amplifying the AC signal. The reference material gives a gain of approximately 53.3 (about 35 dB), a low-frequency corner near 0.6 Hz and a high-frequency limit near 5 Hz for the human-motion example. The input and output common-mode ranges must remain valid around the chosen bias; otherwise even a tiny motion signal can be clipped by the sensor baseline.
Stage 2: additional gain and band limiting
A second moderate-gain stage contributes approximately 34 dB, producing about 69 dB total. Two stages are easier to stabilize and debug than one extreme-gain amplifier. Filtering can be placed where it has the greatest benefit, each stage has more manageable output swing, and an oscilloscope can reveal where overload or noise begins.
Rank #2
- Detects human motion up to 7 meters away with 110° coverage using a built-in Fresnel lens for enhanced accuracy and range
- Adjustable sensitivity and delay time via onboard potentiometers—customize response for indoor lighting, security alarms, or automated systems
- Low-power design consumes under 65µA in standby mode, perfect for battery-operated IoT devices and energy-efficient installations
- Compatible with Arduino, Raspberry Pi, and 5V logic systems—directly connects to digital pins with no external circuitry required
- Robust green PCB with stable output and wide operating voltage (3.6V–30V DC), suitable for both prototyping and permanent installations
Stage 3: window comparator
Motion can drive the conditioned waveform above or below its nominal level, so a window detector responds to both polarities. The example uses an upper reference of 0.84 × VCC (about 2.77 V with a 3.3 V supply) and a lower reference of approximately 0.53 V. The comparator outputs go low when the corresponding threshold is crossed. This creates a direct interrupt signal without an ADC.
In the reference circuit, a TSU104 op amp is used as a slow threshold detector. At these very low frequencies that can work, but an op amp is not automatically equivalent to a dedicated comparator. A comparator is preferable when propagation delay, overdrive recovery, defined logic behavior, hysteresis or an open-drain output must be specified.
Choosing the passband
For a first-order RC network, the corner frequency is:
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The approximately 0.6–5 Hz passband is a human-motion example, not a law of PIR operation. A lower corner that is too high suppresses slow crossings; a high corner that is too low admits more interference and can attenuate useful transitions. Filters also add phase shift and settling time. Select corners from the target’s distance, temperature contrast, crossing speed, lens zones and required false-alarm rate, then verify them with the assembled sensor and enclosure.
Rank #3
- Operating voltage range: DC 4.5-20V
- Quiescent Current: <50uA Trigger: L can not be repeated trigger/H can be repeated trigger(Default repeated trigger)
- Delay time: 5-200S(adjustable) the range is (0.xx second to tens of second)
- Board Dimensions: 32mm*24mm
- Angle Sensor: <100 ° cone angle Lens size sensor:Diameter:23mm(Default)
Gain, headroom and measurement workflow
Voltage gain in decibels is:
GdB = 20 log10(G)
Approximately 69 dB corresponds to about 2,800× voltage gain. It is the gain of the cited implementation, not a universal recommendation. Required gain depends on element sensitivity, lens and target geometry, ambient temperature, ADC range, thresholds and allowable false alarms.
- Probe the raw element under the actual lens, mounting and thermal conditions.
- Measure the smallest motion waveform that must be detected and the largest thermal transient expected.
- Allocate gain across two or more stages and calculate each stage’s output swing.
- Check common-mode range, output swing, noise gain and overload recovery at the minimum and maximum supply.
- Leave tolerance and temperature margin, then set comparator thresholds from measured distributions rather than a nominal waveform alone.
Op-amp and comparator selection
The original design emphasizes low-current ST TSU101, TSU102 and TSU104 devices. The article reports about 1.2 µA for a TSU102 example, 19 µA for the PIR element, 2.4 µA in the divider and approximately 24 µA for the complete TSU104 analog chain. These are source-reported implementation figures, not guarantees for a redesign. Check current specifications and lifecycle status in the ST low-power op-amp documentation.
Evaluate:
- Quiescent current and supply range.
- Input common-mode and rail-to-rail input/output behavior.
- Gain-bandwidth product, stability at the selected gain and phase margin.
- Voltage/current noise, offset and offset drift.
- Input bias current with the chosen resistor values.
- Startup, overload recovery, output drive and temperature qualification.
The reference calculation calls for gain-bandwidth product above roughly 2.7 kHz (5 Hz × 53 × tenfold margin). That is a minimum calculation for the example, not a complete stability or noise analysis. ST’s current TSZ precision-op-amp material lists families such as TSZ12, TSZ15 and TSZ18 for low-offset applications. Zero-drift parts may improve baseline accuracy but can bring higher current, cost or switching artifacts; select from the individual datasheet.
Analog output or digital output?
| Requirement | Window comparator | ADC |
|---|---|---|
| Simple motion interrupt | Strong fit; no ADC required | Usually unnecessary |
| Adaptive thresholds | Limited | Strong fit |
| Direction or polarity | Use two outputs | Directly available in firmware |
| Diagnostics and signal strength | Poor | Strong |
| Firmware complexity | Low | Higher |
| Calibration flexibility | Low to moderate | High |
An ADC supports digital filtering, confidence scoring, direction and adaptive thresholds, but sampling and reference noise add design work. A digital output simplifies a sleeping MCU wake-up path, yet still depends on analog gain, filtering, bias, hysteresis and startup control.
Rank #4
- Working voltage: DC 2.7-12V.
- AM312 Human Sensing Module: Based on passive body infrared technology digital intelligent automatic control products, high sensitivity, reliability, widely used in various types of automatic induction electrical equipment.
- Low power consumption and small size for easy embedded installation.
- Sensing range: ≤100 degree cone angle, 3-5 meters; (depending on the specific lens)
Startup, blanking and threshold behavior
Detection is not necessarily valid immediately after power is applied. PIR warm-up and charging of coupling and filter capacitors can produce large transients. Scope the raw sensor and every amplifier output during power-up, measure when the baseline settles, and ignore comparator transitions until that point. Repeat the check after battery replacement, brownout and intermittent power; the source does not establish one universal blanking time.
Add hysteresis or firmware pulse qualification if noise causes threshold chatter. A narrow threshold window without hysteresis can turn a small baseline ripple into repeated interrupts.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Leakage, layout and current trade-offs
Large divider resistors reduce quiescent current but make sensitive nodes vulnerable to dust, humidity, capacitor leakage and PCB contamination. Keep high-impedance traces short and clean, route them away from clocks, antennas, DC/DC inductors and switching nodes, and verify op-amp bias current over temperature. Do not choose resistor values from the current calculation alone. Lower values may be the better production choice if the battery budget allows.
Budget every always-on path: PIR element, amplifier channels, reference and divider networks, comparator, MCU wake-ups and startup current. Measure standby, startup and detection states on the assembled board.
Best Value
- Using Potentiometer 105, output timing is from 0.5S to 200S
- Widely used in:Security Products,human body sensors toys,human body sensor lighting industrial automation and control, etc
- NOTE: On this retrigger jumper is a solder jumper, and you need solder it by yourself
- Pls note that there is no IR emitter in this module, the principle of PIR sensor is to detect the infrared radiation emitted by the human body, it only have a IR sensor (cell)
- Package Included: 5 X HC-SR501 PIR Infared Sensor
Lens, placement and environmental limits
Electrical filtering cannot correct an unsuitable optical or thermal installation. Test for direct sunlight and hot reflections, HVAC airflow, radiators and machinery, moving curtains or foliage, enclosure heat conduction, pet height and sensor orientation. Motion crossing alternating lens zones generally produces a stronger differential response than motion directly toward the sensor. Detection can also weaken when ambient temperature approaches body temperature.
Separate electrical false alarms from thermal/optical false alarms. A quieter amplifier will not fix a lens aimed at an unstable heat source.
Validation checklist
- No-target operation with the final enclosure and lens.
- Slow and fast crossings from several directions and distances.
- Different target temperatures and body sizes.
- HVAC, sunlight, heaters and moving background objects.
- Power-up, brownout, battery-voltage range and recovery.
- Temperature and humidity extremes, including contaminated-board scenarios.
- Sensor-to-sensor variation and threshold tolerance.
Bare element or integrated PIR module?
A bare element plus discrete conditioning is appropriate when lowest current, a custom lens, raw-signal access or adjustable detection behavior matters. It demands more characterization and is sensitive to layout, leakage, drift and startup. An integrated module is faster to prototype when a fixed digital event, timeout and detection profile are acceptable, but its ASIC behavior, current, lens and raw-signal access may not suit a custom product. The ST reference design addresses the bare-sensor problem, not a generic ready-made module.
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