I have a PIR sensor module based on BIS00001 IC. I am using the 3.3 VDC output signal of the sensor module to trigger an LED matrix using a low side NPN transistor and a current limiting resistor.


simulate this circuit – Schematic created using CircuitLab

However, the problem is that after the preset time of the sensor, the LED resets, turning off, instead of continuing to remain ON if the human is present in a steady position. It remains ON if the human is in constant motion.

I have come across an article with some details on the PIR light controller which uses 555 timer as a missing pulse detector. However, I am unable to comprehend if it can keep the light source ON even when the IR source is not in motion.


Is there a way keep the LED ON, even when the human is in steady position and not in motion?

  • \$\begingroup\$ The missing pulse detector is designed to turn the light off if no pulse is detected. The reason they have a 12 - 15s timeout is because no pulses will be detected if the subject isn't moving. In their system, if the person stands still, the light will turn off after 15s. \$\endgroup\$ – Transistor Nov 21 '15 at 10:44
  • \$\begingroup\$ Paragraph 2 - "the LED restarts" doesn't make sense. Do you mean "resets"? Edit required? \$\endgroup\$ – Transistor Nov 21 '15 at 10:45
  • \$\begingroup\$ @transistor resets, not restarts. \$\endgroup\$ – Passerby Nov 21 '15 at 10:46

As Passerby is saying the PIR sensors are designed to trigger when the rate of change of IR signal it receives is higher than the threshold set by the sensitivity adjustment. To aid this most have a dimpled lens or multi-faced reflector so that as a person walks across the field of view the detector gets a series of pulses. This in turn makes them more sensitive to traffic across the field of view than traffic walking towards the sensor.

One of the advantages of this method is that it self-compensates for slowly changing background IR radiation. In your case you seem to need to trigger and hold when the IR exceeds some absolute (fixed) level of radiation. If your sensor can see material that warms or cools - say due to sunlight - then you will probably have trouble with false triggering when it gets warm or no triggering when the background level reduces the total IR, even with a target present, below the threshold.

If you want to try threshold triggering you would need to see if you can get the analog signal from the sensor inside the PIR unit and process that somehow. A simple voltage comparitor circuit may do the trick.

http://learn.adafruit.com/downloads/pdf/pir-passive-infrared-proximity-motion-sensor.pdf has some very good explanation and diagrams.


The circuit you have is active high, i.e. it only turns on if there is activity on the PIR sensor. A PIR sensor senses changes in IR states, not IR intensities. Someone standing still in front of an PIR sensor will not trigger it. It will only trigger if the field it monitors changes Rapidly. Slow changes do not trigger it. There is no way to turn an average PIR sensor into what you want.

  • \$\begingroup\$ Thanks. Are there any other sensors which could achieve this function? \$\endgroup\$ – Shaun Nov 21 '15 at 9:50

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