2007. Plant Management Network. This article is in the public domain.
Accepted for publication 2 February 2007. Published 15 May 2007.
Development of a Trigger-On Indicator for a
eed Sensing Spray Unit
Mark C. Siemens and Donald E. Hulick, USDA-ARS, Columbia
Plateau Conservation Research Center, Pendleton, OR 97801; and
Bill Jepsen, Grower, Heppner, OR 97836-0188
Corresponding author: Mark C. Siemens. email@example.com
Siemens, M. C., Hulick, D. E., and Jepsen, B. 2007. Development of a trigger-on indicator
for a weed sensing spray unit. Online. Crop Management doi:10.1094/CM-2007-0515-01-
Intermittent spray systems that automatically spray only when weeds are
present have been commercially available since 1992 and can significantly
reduce herbicide usage by 50 to 90% as compared to broadcast applications (2).
The Weedseeker (NTech Industries, Ukiah, Calif.) is one such system that
utilizes optics to detect the presence of weeds. The system works by exploiting
the fact that chlorophyll selectively absorbs red wavebands of light and reflects
near-infrared light (1). Each sensor unit has its own built-in light source and a
sensor that detects the light reflected back to the unit over a 12-inch-wide field
of view. When the sensor detects an increase in the ratio of near-infrared to red
light above the base threshold level, a solenoid valve is activated to spray the
Although the sensor unit reliably detects the presence of green plants (2), a
major drawback with the current design is that it doesn't provide any feedback
to the user when one of the spray units is actively spraying. This is problematic,
particularly on wide-boom, multi-sensor-unit sprayers since each sensing unit is
calibrated independently in the field prior to spraying from the reflectance of the
ground surface beneath it whenever the calibration button is pushed. If one of
the sensing units is calibrated when the sensor is over a weed, the unit will not
detect weeds of similar size or smaller when spraying. Also, because the sensing
unit is sensitive to background reflectance, if one of the sensors is calibrated
hen it is positioned over a spot of ground that is not characteristic of the rest of
the field, the unit will "ghost fire" and spray when weeds are not present. Either
scenario is undesirable, but likely to occur since recalibration is necessary
several times during the day due to changes in ambient light and field
conditions. To overcome this problem, a
trigger-on indicating device was
developed to help the operator
determine that the sensor units are
operating correctly. The Weedseeker
unit has a small LED on the back of the
unit that illuminates whenever the unit
is spraying. The device developed
utilizes a phototransistor to detect
when the LED is illuminated and
activate a super-bright LED that is
visible from the tractor cab (Fig. 1). The
circuit designed includes a resistor and
a NPN transistor that function to
prevent low levels of light from
activating the super-bright LED (Fig.
2). All electronic components are encased in a waterproof BUD box and attached
to a Weedseeker sensor via Velcro and zip ties (Figs. 1 and 3). An opaque, non-
translucent flexible rubber washer is used to seal the phototransistor from
ambient light and dust (Fig. 1). Power is supplied by two 1.5-V, AA batteries.
Fig. 1. Trigger-on indicator assembly
for Weedseeker sensor unit.
ssuming a 10% on time, battery life was calculated to be 420 h. Table 1
provides a listing and cost of the principal components needed to construct the
device. Construction and installation time was estimated to be about 2 h.
Fig. 2. Trigger-on indicator circuit diagram.
Fig. 3. Trigger-on indicators mounted
on commercial farm spray boom. Fig. 4. Trigger-on indicators
operating while spraying
weeds in standing stubble.
Table 1. Trigger-on indicator part list and cost.
x RSR Electronics Inc. part number. Industry part number was not available.
y Mouser Electronics part number. Industry part number was not available.
z Industrial Webbing Corp. part number. Industry part number was not available.
To evaluate the performance of the device, 40 units were installed on a 40-ft
spray boom equipped with Weedseeker spray sensor units. Boom spacing and
height were 12 and 22 inches, respectively. The sprayer was used to control
weeds on 3000 acres of commercial farmland near Heppner, OR during 2006
(Fig. 4). Travel speed was 5 mph. The units performed reliably over the 150-h
use period. The indicator lights were readily visible by the operator, even during
intense daylight. During the trial, four 2N2222A NPN transistors failed, possibly
due to moisture contamination. Although a 10% failure rate is marginally
acceptable, research on the use of desicant and different transistors is underway
to address this problem. In summary, a simple, low cost trigger-on indicator for
Weedseeker spray units that was developed and field tested. The device, which
requires no modification to the Weedseeker unit, performed reliably and was an
aide to the operator. A circuit diagram and parts list is provided so operators
interested in this technology can make the device (Fig. 2).
Reference to a product or company is for specific information only and does
not endorse or recommend that product or company to the exclusion of others
that may be suitable.
1. Felton, W. L., Alston, C. L., Haigh, B. M., Nash, P. G., Wicks, G. A., and Hanson, G.
E. Using reflectance sensors in agronomy and weed science. Weed Tech. 16:520-
2. Hanks, J. E., and Beck, J. L. 1998. Sensor-controlled hooded sprayer for row crops.
Weed Tech. 12:308-314.
Component Industry part number Cost per assembly
Phototransistor LT189X-82-0125 $0.55
Super bright LED 08LCHR5x$0.20
NPN transistor N2222A $0.25
2.2 K ohm, ¼ watt resistor 130052.2Kx$0.06
BUD box AN-1312 $9.00
BPS PC board (cut in half) PR425X320 $3.00
LED mount 351-100y$1.10
Velcro, 4"x2" Spectrum 0172z$0.49
AA battery holder 12BH321Py$0.67
AA battery (2 required) $1.70
Flexible lead wire, 1 foot $1.00
Zip ties (3 required) $0.21