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I am interested in detecting if an object (e.g. small box made of non-conductive material like plastic/acrylic/wood) is placed on top of a surface (e.g., a table), I don't need to identify the object, I just want to know if it is there.

There are various methods to doing this, but I wanted the easiest/cheapest way. I first thought of using a Mini Photocell ($1.50), which is a simple light sensor, however, this will not work if the room is initially dark.

Then I found the Optical Detector / Phototransistor - QRD1114 ($0.95).

  • I believe QRD1114 should work regardless of lighting conditions, since this uses IR?
  • I need to consider the approaching object (e.g., small box) should be able to reflect IR?
  • Any ideas on the IR beam?

The detecting sensor must be very small and cheap.

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  • \$\begingroup\$ If the box is going on a table, can you put a pressure sensor on the table instead? \$\endgroup\$ – Greg d'Eon Mar 16 '15 at 16:00
  • \$\begingroup\$ @Gregd'Eon: Interesting idea. The issue is the small box is not really that heavy and my guess would be that it wont exert enough force/pressure for the sensor to detect it? \$\endgroup\$ – lucidgold Mar 16 '15 at 16:01
  • \$\begingroup\$ Check out this answer for a couple of ideas: electronics.stackexchange.com/a/35314/49251 It looks like you can make it quite sensitive. \$\endgroup\$ – Greg d'Eon Mar 16 '15 at 16:02
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The QRD1114 will have a very short range, but yes with a IR LED pointed at the spot of the object, depending on the object properties, it will work.

The better thing might be a cheapo sonic sensor like you can find from HobbyKing "Ultrasonic Module HC-SR04" for about $3, which will give you a broader region to detect in, more range, and is also light-invariant.

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