Electronics Puzzler Solution: Artifical Vegitation Detector

By: Forrest M. Mims III

Is It Fake or Real?

Some digital cameras can tell the difference between living and artificial vegetation. Do you know how? Can you think of a simple non-contact electronic method to determine if what appears to be real vegetation really is?

This Puzzler's hint is:
The colors of light are those we can see.
Those we cannot are the key.

The human eye can see wavelengths of light having a wavelength of about 400 nm in the violet to about 700 nm in the red. We cannot see near-infrared (near-IR) wavelengths above 700 nm unless the light is extremely intense.

The chlorophyll in living, green leaves of plants strongly absorbs the blue and red wavelengths used to stimulate photosynthesis. Most leaves reflect near-IR much better than most natural things.

These facts provide two important clues for solving this Puzzler. What we need is a way to compare the red and near-IR reflection of a questionable green leaf. If the near-IR signal exceeds the red light signal, then the leaf is probably real. Otherwise it's probably fake.

Several electronic solutions can exploit these facts to solve this Puzzler, and we'll look at one in detail. Recall from previous Puzzlers that LEDs can be used to sense wavelengths of light close to the wavelengths emitted by the LED. An AlGaAs red LED will detect red light, which is well absorbed but poorly reflected by living leaves. An AlGaAs near-IR LED will detect near-IR that is reflected very well by living leaves. Therefore, a pair of these LEDs can form a living leaf detector when connected in a circuit such as the one in Fig. 1.
Vegitation Detector
Fig. 1

How to Make a Test Circuit

You can assemble a test circuit rather quickly. While mine worked properly the first time, your results will depend on the way you mount the two sensor LEDs and how you illuminate the green leaf and a white card used as a calibration target.

For best results, follow the instructions carefully.

Parts List:

LED1 - AlGaAs super bright red LED

LED2 - AlGaAs 880-nm near IR LED

LED3 - Green LED

R1 - 1MΩ potentiometer

R2 - 100K resistor

R3 - 1K resistor

TLC272 (2) or similar single-supply op amp

Miscellaneous - solderless breadboard, jumper wires, 3/4-inch length of 1/2-inch black heat shrinkable tubing, 5- to 9-volt battery or power supply, white card (calibration target), fresh green leaf and small flashlight (must use an incandescent lamp, not a white LED).

Preparing the Two Sensor LEDs

Begin by preparing the two LEDs, which should both be encapsulated in water-clear epoxy that forms a narrow-beam lens. Since the LEDs will appear nearly identical to one another, use a red ink pen marking pen to color the base of the red LED. Then use the pen to mark the center section of the anode lead of each LED with red ink. Tape the LEDs together so they face the same direction. Then fold their leads down so they can be inserted into the holes of a plastic solderless breadboard. Insert the pair of LEDs into a short length of black heat shrink tubing to form a collimator that blocks most external light. If you plan to spend some quality time evaluating this circuit, you can gently heat the LED end of the assembly to secure the LEDs in place while leaving an opening at the opposite end.

Assembling the Circuit

Build the circuit on a solderless breadboard. Be sure to observe the polarity of the LED leads and connect them exactly as shown in Fig. 2. Be sure the bare leads do not touch one another.

It's best to connect potentiometer R1 to the board by means of hookup wires soldered to its center and one outer terminal. Clip leads can be used, but their length might induce oscillation in the circuit.

Connect all the ground leads to a common row of connection holes in the board. Be sure to connect the two inputs (pins 5 and 6) of the unused opamp to ground.

After the circuit is built, carefully check all the connections and correct any errors or missing wires before applying power. Then connect a 6- to 9-volt battery or power supply to the circuit.

Calibrating the Circuit

The following steps require a small flashlight that uses an incandescent bulb. An LED flashlight does not emit infrared and will not work.

Use tape or a clothespin to mount a white card a few inches away from and perpendicular with the business end of the two sensor LEDs.

Illuminate the center of the card with the beam from the flashlight. For best results, the flashlight beam must not illuminate the back end of the LEDs. It should remain fairly still during the calibration.

Adjust R1 until the green LED begins to glow. Then back off on R1 until the green LED darkens. Next, place a green leaf in the flashlight beam in front of the white card. The green LED should glow brightly. Figure 2 is an animated gif that shows the prototype circuit detecting a freshly cut iris leaf.

Detector Circuit
Fig. 2

Troubleshooting the Circuit

If the green LED fails to glow when the leaf is illuminated, check the following:

Are the red and IR LEDs properly connected to the TLC272 op amp?

Does the flashlight use an incandescent lamp? (A white LED flashlight will not work.)

Is light from flashlight entering the back side of the LEDs?

Did the flashlight move during the test?

Going Further

This circuit is specifically designed to work with an incandescent lamp. White LEDs and fluorescent lamps will not work. Sunlight might work if the circuit is calibrated for it.

There are other ways to design the leaf detector circuit. For example, I have described a similar circuit that used four NAND logic gates to indicate the status of a pair of op amps connected to a red LED and a near IR LED. See The Forrest Mims Circuit Scrapbook, Vol. 1, LLH Publishing, pp. 24-26 (2000).

The circuit will also indicate the presence of a green leaf between the flashlight beam and the sensor LEDs. This is possible since a leaf transmits much more near IR than red light.

Near-IR sensitive photographic film was long used to detect camouflaged military weapons and fortresses. It was also very popular for surveying agricultural fields and woodlands to determine the presence of healthy plants. Today handheld near-IR digital cameras and satellite systems have virtually replaced near-IR photographic film. You can learn much more about this topic from various web sites, including this pdf report: Click here to review the PDF document

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