![]() Reset and voltage reference input that equals the 3.3V supply voltage Trigger inhibit control that cannot be changed Output pulse width control that defines the time duration during which the output pin (Vo) remains high after triggering and is therefore connected to the potentiometer for the time delay adjustment. Output pin and connected to the digital output of the HC-SR501 LOW: The HC-SR501 becomes non-re-triggerable. HIGH: the PIR motion sensor becomes re-triggerable. ![]() Trigger Selection and connected to the Trigger Selection Jumper. The minimum is 3 meters (fully clockwise) and the maximum is 7 meters (fully counter-clockwise). Meanwhile, the sensitivity pot adjusts how far the PIR sensor can detect. This means the interval where the DOUT pin stays low is around 2.4 s.Īs already mentioned, the time delay can be from 2.5 seconds (fully counter-clockwise) to 4 minutes (fully clockwise). , where R 9 is a 1M resistor and C 7 is a 100 nF capacitor. Meanwhile, the fixed interval where the DOUT pin goes low is: Hence, the time delay can be between 2.5 s to 4 minutes. , where R 10 is the series combination of a 10k resistor and 10M potentiometer and C 6 is 10 nF, according to the HC-SR501 datasheet. The BISS0001 datasheet, which is the controller on the HC-SR501 module, says that the time the DOUT pin stays high is: t9, t10, t11, which occurs after t8 triggers the sensor, extend the time delay since they occurred within the adjusted delay interval. t7 does not trigger the sensor because it is within the fixed delay interval. In repeat mode, t6 is the first motion detected. Consequently, t4 triggers the sensor and t5 does not. t2 and t3 are within the delay intervals and hence will not trigger the sensor. In the image above, non-repeat mode, t1 is the first motion detected. After this, the DOUT pin goes low for a fixed delay interval before being able to detect motion again. In the event that motion is no longer detected, the DOUT pin stays high for a time set through the time delay potentiometer. When in repeat mode, the DOUT pin goes high when motion is detected and stays high as long as the object is still moving. This means that any movement between the first trigger and the end of the fixed delay is undetected. After this time delay, the DOUT pin goes low for another fixed delay interval, regardless if motion is detected or not. When in non-repeat or single trigger mode, the DOUT pin goes high for a period of adjusted delay time set by the time delay potentiometer. The PIR sensor behaves differently depending on the jumper position. Meanwhile, if a thermistor is added, the reduced sensitivity brought by an increase in temperature is negated. With an LDR, the PIR sensor will trigger only when it’s dark. The one on top is for adding a CdS Cell or Light Dependent Resistor (LDR) while the one at the lower right is for adding a thermistor. There are two empty pads encircled in red in the picture above. Moreover, the DOUT pin’s voltage level will not go up beyond 3.3 V so it’s safe to use with a Pico. The DOUT pin goes high if motion is detected. While the module is supposed to run from 4.5 V to 20 V, it still works, surprisingly, from 3.3 V of the Raspberry Pi Pico. Power is provided to the module through the VCC and GND pins. The HC-SR501 module has three interface pins, a set of jumpers, and potentiometers. This is the reason why the PIR sensor has a conic reception area instead of a straight line. This lens focuses the received IR signal to the center of the sensor. The dome you see covering the sensor is called a Fresnel lens. This window filters all other types of radiation except infrared. The actual sensor is a metallic package with a window at the center. Detection RangeĪ PIR sensor detects IR at 100° to 130° and 3 to 7 meters from the sensor. This then triggers other circuit components to generate a pulse at a specific interval. When such a body is within range of a PIR sensor and then moves, the sensor treats this as a change in IR. Humans and other objects with temperatures above absolute zero emit invisible infrared radiation (IR). Want to learn more about how PIR sensors work? Then read ahead. If the 3 seconds in the original code is enough, turn the time delay knob fully counter-clockwise.
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