Hello this is Scott from MaxBotix Inc.
I saw that you stated: “I have thought about using the MaxSonar sensor (http://www.adafruit.com/blog/2009/06/23/getting-started-with-the-maxbotix-sonar-sensor-quick-start-guide/), but I don't know if it will be precise enough. The sensors will be placed at 3 different locations at a spiral staircase, and should be activated when people are present in it's area."
You may want to consider using one of our ProxSonar sensors. We have them available in either a USB or Pin Out interface depending on the interface you want to use. Both sensors have a pin that can be monitored that gives a digital high/low signal if a person is in the detection zone of the sensor. The interface with the pin out configure is able to operate within 2.5V to 5.5V and the USB only operates from the voltage supplied by the USB port. Our HRLV-MaxSonar-EZ may be a good rangefinding sensor for your application also. There is some multi-sensor operation built into the sensor. We are currently testing these parameters yet at our last trade show, we had 4 sensors (3 ProxSonars and 1 HRLV-MaxSonar) operating at our booth independently without interference in the reported range.
The LV-ProxSonar-EZ ultrasonic proximity sensor can be seen here http://maxbotix.com/Ultrasonic_Sensors/Ultrasonic_Proximity_Sensor.htm.
The USB-ProxSonar-EZ ultrasonic USB proximity sensor can be seen here http://maxbotix.com/Ultrasonic_Sensors/USB_Proximity_Sensor.htm.
If you are looking for the range to the person in the stair well you may want to test the HRLV-MaxSonar-EZ. The sensor can be connected to Arduino or other control device that is programmed to ignore distances after a certain amount. The HRLV-MaxSonar-EZ can be seen here http://maxbotix.com/Ultrasonic_Sensors/High_Resolution_Sensors.htm.
Please let me know if you have any questions by emailing me at firstname.lastname@example.org. I hope this information help.
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