# Picking the right sensor for a military style alarm

as a "squad engineer" of our airsoft team, I have been asked to create a device that will help us with our "capture the hill" tournaments.

As I mostly work with software, I have an hard time picking the right sensor to use. My first idea would be to build something similar to a trail camera, used by hunters to study the preys trails. In order to do so, I thought about getting a PIR sensor and link it to an Arduino Micro I have in my drawer from another project. Then, I could use a spare team radio to transmit alarm codes (morse tones).

My sensor specifications are the following:

• outdoor operation capable
• ~10m range
• ~60° angle
• at least 24h operation on battery (sharing the Arduino power supply)
• false positives resistant

Is PIR the right technology to pick in this case ? If so, I really need to know if there are some sensors with the aforementioned specifications; all the Arduino DIY projects I have seen so far employ short range sensors. I would prefer something already packaged (fresnel + diode) from an european dealer.

In the following picture, you can see what I thought.

I have evaluated those other solutions also:

• steel/nylon tripwire: I would prefer the device to stay undetected as long as possible (it is not a claymore mine, but a silent alarm)
• hacking an airsoft claymore mine (small PIR range, remote unit useless)
• laser tripwire: the reflector (or external laser) unit adds usage complexity, detection is only linear
• doppler radar: my understanding is that this kind of device is only good for linear detection of movement to/from the unit. I could be wrong, though.
• Unless you play at night, PIR is likely not the system you want. – Passerby Oct 29 '13 at 1:15
• @Passerby, I understand you are talking about direct sun exposition. What about if I install the sensor away from direct sun ? Does it change anything ? FYI, we play both at night and daytime. – ziu Oct 29 '13 at 1:21
• Indirect sunlight reflecting into the pir sensor's view will also trigger it. The wind blows and leaves or grass shaking will trigger it. – Passerby Oct 29 '13 at 1:26
• @Passerby, what you say is reasonable. Do you think I have other options than putting a tripwire ? – ziu Oct 29 '13 at 2:09
• IMHO, A set of laser trip wires would be your best bet. You could even plant battery lasers around a tree x distance away, and string some sensors around said tree for a wide area. If battery life is a problem, pulsing the laser in a constant pattern (think pwm) would decrease the current use, while still being fast enough to detect breaks. – Passerby Oct 29 '13 at 2:19

What about ultrasonic sensors? SparkFun carries a variety of sensors, ranging from ~$30-$100, and it looks like some of them have ranges that get close to your 10m requirement. The one issue is angle; you may have to use more than one to cover a 60° swath.
Your concern with radar would apply here too: it will only measure distance (or Δdistance) from the sensor. But to me this is almost a good thing; it detects "presense" instead of motion, so it could not be foiled by moving really slowly like a motion sensor is. You could program your Arduino to do collect say a hundred readings when powered on, and calculate thresholds to trip the alarm based on, say, the difference between the max and min reading (eg. int high_threshold=max+(max-min); int low_threshold=min-(max-min);)