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gate. It then waits for the gate control to
indicate that the gate is indeed open.
After receiving a gate-open signal,
the command center tells the truck to
proceed forward. After the truck passes the gate, it triggers another ground
sensor, which is monitored by the
command center. This indicates that
the vehicle is now safely inside the
gate. The command center then tells
the truck to stop and it informs the gate
control to close the gate. The experiment is over after the control closes the
gate and informs the command center.
Figure 2 shows the apparatus and connections. Figure 3a is a schematic of the
truck controller with a single additional MOSFET for turning the motor on and
off. Figure 3b is a diagram of the command center with ground optical sensors.
The gate controller has a stepper motor
interface and optical sensors for opening and closing the gate (see Figure 3c).
a)
b)
COMPONENTS
BASIC Stamp 2 microcontrollers
control the three electronic modules in
the design: the command center, the
truck, and the gate controller. There’s
a degree of overkill here. Less powerful
controllers can be substituted in, but
for quick and easy development, I used
an interpreter rather than a compiler.
The modules are equipped with a
Ming Microsystem wireless transmitter (TX-99) and a receiver (RE-99) pair,
which are distributed by Reynolds
Electronics. Operating at 300 MHz
(AM), the transceiver setup is adequate
for the short distances of the experiment. The transmitter requires a short
(9.36″) wire antenna, whereas the
receiver has its own built-in loop antenna. Both modules operate from 5 VDC,
drawing a meager 1.6 mA each.
The master microcontroller in the
command center handles communication and monitors two ground sensors,
S1 and S2, which are reflective optical
sensors for detecting the truck position. The microcontroller in the gate
control monitors two slotted optical
sensors to detect gate open/gate close
positions. It also controls the stepper
motor that operates the gate.
The microcontroller in the truck
receives command center signals and
sends back status data. It also controls
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Issue 177
April 2005
c)
Figure 3—All the modules have RS-232C connections to a PC for programming. The truck controller uses a power
MOSFET to control a small motor (a). The command center interfaces to two reflective optical sensors (b). The gate
controller uses a stepper motor IC to simplify the process of driving the motor (c).
the small siren circuit that comes
with the truck. The virtues of the
BASIC Stamp 2 are well known.
The stepper motor operating the gate,
as well as the optical sensors for position
direction, is from an old floppy drive.
Although an SAA1027 stepper motor
driver IC generates the control pulses to
operate the motor, you can experiment
with other methods. The SAA1027
requires 3 bits to operate its functions:
step, direction, and reset. Photo 1 shows
the layout and main components.
CIRCUIT CELLAR®
CIRCUITRY
The setup is mounted on a 6′ × 1.5′ ×
0.75″ sheet of wood. The circuit
boards are screwed into the base with
0.5″ plastic spacers.
The command center consists of a
BASIC Stamp 2 board, a Ming RX-99
receiver and TX-99 transmitter, and optical sensors S1 and S2. Bit 0 connects to
the TX-99’s data input from the BASIC
Stamp 2 board. Bit 15 receives data from
the RE-99, while bits 8 and 9 receive
input status from the optical senwww.circuitcellar.com