Detect presence of solid object

Question

I have a microcontroller-based project (using an Atmega328) that needs to detect the presence of a solid object placed on the project box. What would be the best way to achieve this? I've thought about using a photosensitive resistor in a voltage divider configuration, but the issue I have with this is that if the project box is in a dark room it may not detect any light when the object is removed.

Any and all input on this would be appreciated as I'm not sure how reliable my voltage divider solution is. The objects placed on the project box will be solid and made of glass or plastic. And the only output needed from whatever sensor I use is a logical zero or one to signify if an object is there or not there. Note the objects may have concave bottoms, so the sensor would need to be able to identify it even if it's up to 1-2 cm away from the sensor.

Answer 1

A couple options come to mind.

Range finder

Use an ultrasonic range finder or, if you think the object would be opaque enough, an infrared proximity sensor. Put the sensor in the top of the box, pointing up and out of a hole you've cut there and poll it occasionally. When the measured distance decreases rapidly, and as a method of noise filtering stays low for a set number of samples, you've got an object in the way.

Properly placed load sensor

Another way is to put the load sensors on the bottom of the box. Assuming the object is actually placed on the box, the detected load will increase and can signify the presence of an object.

Strain gauge

Make the top of the box springy, that is, it's lifted slightly from a solid resting point. Now place a strain gauge in the gap. When an object is placed on the top it is depressed and bends the strain gauge. This signifies the presence of an object.

The above options don't necessarily give you a logical one/zero for presence, but neither would a optical sensor.

Momentary push buttons

If you're mechanically savvy, you could even use one or more push button switches. For instance, on the lid of your project box place three/four push button switches and attach (glue) a platform for the object on top of them. This method might not be terribly reliable though. I'd try to avoid too many moving parts.

Personally, I'd go with the optical range/proximity sensor.

Answer 2

You could implement a capacitive sensor on the top of the box. Plastic has a relative permittivity which is about 2.0 or greater and glass is greater than 4.0. So both should be easily distinguishable from air which is essentially 1.0.

The Arduino CapSense library uses a very simple GPIO pin based method for measuring capacitance. You could easily do this on your project. The most interesting part would be designing the sensor layout to maximize the reliability of detecting the object of interest without false readings e.g. when someone's hand is near the surface.

The sensor would work by the glass or plastic object causing the fringing field capacitance between the two sensor electrodes to increase when the object is present, compared to the capacitance when only air is present above the sensor surface. One electrode would be connected to ground and the other electrode to the MCU "receive pin" as in the following diagram:

Now I expect you'd want to use two electrodes and use the fringing field capacitance to do the sensing, rather than the "one-electrode" model used in the above figure, since the plastic or glass object won't be well coupled to ground.

You could use a spot sensor, like a circular sensor and a surrounding ground plane, or you could implement a wider-area sensor using interdigitated coplanar electrodes.

Answer 3

You could try a variation on a 'magic-eye' detector (beam interrupt).

With no object to block the beam the output from the phototransistor would be LOW. With an object in the way the output would go HIGH.

Answer 4

If the object contains any material metallic a very simple (and cheap) option is to use inductive sensing. You can make a coil of wire with a few loops and use this as an inductor in a LC oscillator. It's pretty straight forward to use an arduino (or any other controller de jour) to stimulate and measure the resonance frequency of the oscillator. When an object (in particular a metallic object) comes near or in the loop the inductance of the loop changes as does the oscillation frequency. Thus you can detect the presence of the object by observing oscillation frequency. This technique works well depending on your material. Theoretically almost any core material will change the inductors value, but the SNR might be questionable. I hacked up a sensor similar to this in about 30 mins a few months back in order to try to detect non-metallic materials. Though the frequency changes the SNR is low so you might need more complicated signal processing to differentiate the object. At that point you'd probably want to consider other sensors (ultrasonic, IR, load cell, etc) since the nice thing about this option is the simplicity/cost.

Incidentally this is the way that stop lights detect the presence of cars. If you've ever seen a loop cut into the pavement at a light this is actually where they embed coils of wire to make the L in the LC oscillator.