Assume we have a security camera in front door attached to door itself and students trying block it by any possible object (solid/liquid/small/big). we want a proximity-sensor to Alert manager if any particle reach its distance limit area (for example we set 10cm distance limit and if anything try pass through this border, sensor must Alert). What is the cheapest sensor which for example use radiowave/infrared to detect any type of coming particles with any size and how much is false positive ratio ?
Electronic – distance detection and prevention methods
proximity-sensor
Related Solutions
Russel has some good ideas, but not sure about feasability. I would suggest a safe way to treat bats is to tell them to keep out of the 50 m radius with an ultrasonic distress signal known to bats. the they will avoid any collisions and you don't need any fancy remote monitors. Or just jam the airwaves on the Bat's comm channel with piezo tweeters with pulses filtered from 25 to 100Khz at a variable rate from 100 to 1KHz and then. go test it near a cave of bats at night and consult with experts and get testimonials.
you can use FSR(force sensitive resistor) to detect whether a person is sitting on a seat or stands up. The working of FSR is similar to LDR (Light dependent resistor the resistance of LDR would decrease if light falls much),in FSR if the applied pressure/weight increased the the resistance would get decreased,you can calibrate this resistance change in terms of weight/pressure.
fig(1) weight Vs Resistance and conductance
fig(2) circuit diagram- connection of FSR
fig(3) Output voltage Vs wight(response of circuit diagram fig(2))
For more details http://www.tekscan.com/flexiforce/force-sensors-for-design
http://learn.adafruit.com/force-sensitive-resistor-fsr
Best Answer
Consider using a Passive Infrared sensor for the purpose:
Images from eBay listings.
You can find a prebuilt module like the one above on sites like eBay, for as little as US$1.50 including international shipping.
The module linked above has sensitivity adjustment, so it can be tuned to react to motion within a specific range. 10 cm may be too small a distance, beyond the lowest sensitivity setting, as per an experiment I just did. The minimum sensitivity setting seems to represent about 90 cm.
False positives are very low at the low sensitivity you would operate this module at.
Using such PIR modules is very simple: They need a DC supply of (typically) 3.3 to 5 Volts, very low current required, and the signal line goes high when motion is detected, falling back to low after an adjustable period (the second preset on the module).
Coding for it is simple too: There are several open source examples for using these modules with Arduino, MSP430 Launchpad, and various PIC, ATmega and ATtiny microcontrollers .