Dual PIR photo trigger by crossing the streams

Motion sensing can be quite effective when taking photographs of wildlife. But how can one be sure that the motion was at the center of the frame? A PIR sensor picks up motion in its entire viewing range. It’s not really something that can be aimed. But if you use two PIR sensors you can monitor a focused area for motion.

The trick is to use a logic circuit. By building an AND gate you can trigger based on motion in the area which is overlapped by both of the sensors. In this case the AND gate is built from a voltage divider. The outputs of the PIR sensors are connected above and below the divider’s connection to the photo trigger. Both have a protection diode, and the divider is tuned so that both PIR outputs must come one in order to raise the trigger input above the voltage threshold. So much for never crossing the streams.

[via TriggerTrap]

Comments

  1. Uky says:

    Nice work! I’ve made something extremely similar to this, but had trouble with the PIR modules response duration. The modules, once triggered, maintain a signal for a certain time. The time can be reduced by a small pot on the chip, but even by bridging the connection for no resistance the signal persists for some time, causing a delay.

    I had problems with this delay causing false signals when the first sensor will read out motion even if there is none in its angle.

    Anyone know if it would be possible to remove this extended signal from the chip or if there are modules without it?

  2. John says:

    Put the camera perpindicular to the (single) beam. Done

  3. Leo says:

    You can put the PIR sensor in a bigger box and make a hole in it such that only the light from the area of interest is seen by the sensor. Wouldn’t that be easier ? (There should be some distance between the hole and the sensor so that the hole does not act as a iris but as a mask).

    • mh says:

      Why box? wouldnt a tube or a cone (depending on how exact you want to point) over the sensor work just fine? you could even make it adjustable in length for larger/smaller “hotspot”

  4. Nuno.S.Almeida says:

    Or just ditch the fresnel lens (the white plastic in front) and use a simple convex…

  5. echodelta says:

    There is a pulse stretcher on the PIR board most likely. Get rid of it’s cap.

  6. anon says:

    “both PIR outputs must come one”

  7. ApexLogic says:

    I second the use of a tube. The cardboard tube at the center of a roll of paper towels works quite well.

  8. macegr says:

    It’s also possible to just tape off the sensor so that only a slit in the correct direction is uncovered.

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