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Ultrasonic Radar For A Home PC System


An airport or an army base used to have huge structures
that could send out signals to find out if any aircraft
were approaching. This technology is now offered to people
who have a computer with Microsoft's quick basic, or a
Macintosh, and space (equivalent to that of a coffeepot) to
spare. Ultrasonic radar is now a small component for a
computer, giving computer operators a chance to see low
flying objects, household furniture, and even themselves on
their PC screen. 

Similar to that of a Polaroid, ultrasonic transducers are
used in this type of radar. A rangefinder emits a brief
pulse of high frequency sound that produces an echo when it
hits an object. This echo returns to the emitter where the
time delay is measured and thus the result is displayed.
The Polaroid rangefinder is composed of two different
parts. The transducer acts as a microphone and a speaker.
It emits an ultrasonic pulse then waits for the echo to
return. The ranging board is the second part This board
provides the high voltages required for the transducer,
sensitive amplifiers, and control logic. Since R1 is
variable it controls the sensitivity of the echo detector.
A stepper motor rotates the transducer to get a 360 degree
field of view. 

An Experimenter is hooked up to the ranging board to
control the ranging board and to measure the round trip
time of pulses. It also controls the stepper motor and
communicates with the control computer. 

The ranging board's power requirements are usually under a
100 mA, but at peak transmission the circuit can draw up to
2 Amps of current. Power passes from GND (pin 1) and V+
(pin 9). To avoid malfunction a 300mF or greater should be
connected between pin 1 and pin 9 (or alternately pin 16
and pin 5). Another 300mF resistor should be added to the
Experimenter end of the cable.
It takes about 360 microseconds to transmit the pulses. The
transmitter waits 1 millisecond for the pulse transmission
and transducer to complete it's task. Then the experimenter
waits for the pulse echo to return. If a pulse is detected
the board sets ECHO at high. The Experimenter times the
difference between BINH going high to ECHO going high. The
experimenter sets INIT to low, waits 0.5 seconds for the
echo, if no echo is heard the experimenter cancels the
The measured time is sent to the computer which then
calculates, at thousands of calculations per second, the
distance based on the speed of sound (1100 feet per
second). With a program called DISTANCE.BAS the exact speed
of sound can be calculated according to the local weather
The stepper motor is used to rotate the radar so it can
scan 360 degrees around the room. An ordinary DC motor
would not do for such a project. The rotation must coincide
with the emissions and the receptions of the echoes. In a
DC motor the armature rotates and the brushes connect
successive commuter bars to windings to provide the torque.
The speed of this motor depends heavily on how much load
there is and how much voltage is applied.
A stepper motor has different wires to each winding. By
energizing a winding the armature rotates slightly, usually
a few degrees. By sequentially charging one winding after
another the armature can rotate completely around. By
controlling the windings energized, the operator (in this
case the Experimenter board) can control exactly how many
degrees the motor turns and at a precisely controlled speed.
In this case a stepper motor is used because it gives a
precise motor-shaft location for the Experimenter board to
follow. In a DC motor the board wouldn't know shaft
position and it would not be possible for the computer to
take the distance readings at evenly spaced intervals. With
the control of the stepper we can control the number of
steps and the step rate required between each transmission.
The Experimenter will control all this.
There are many types of stepper motors available. These
motors have either two coils, three coils, two coils with
center taps, or four separate coils. These are low-cost,
light- duty motors that the Experimenter can drive. The
Experimenter board can control any stepper motor with drive
voltages from 4.5 - 36 volts and currents up to one Amp.
The Experimenter has different hookups for different

While all the stepper motors will operate the radar system,
it is imperative that the different advantages and
disadvantages of each be considered. The motor's power
consumption, torque, and resolution are all factors that
must be considered when choosing the appropriate motor. A
unipolar stepper motor with its common leads connected to
the positive power supply can be driven in modes 7, 9, 11.
In mode 7 (also called the one-phase drive) the stepper
motor minimizes power consumption, because only one coil is
activated at any one time. This mode has very little
torque. Mode 9 (also called the two-phase drive) runs two
coils at the same time. This provides maximum torque,
although the power consumption is doubled. Mode 11 (called
the half-step drive) uses one coil, then two coils,
alternating between modes 7 and 9. This doubles the number
of steps per revolution.
If a stepper motor of twelve volts or less (indicated on
the motor, along with maximum current, coil resistance and
step angle) is used it is possible to run both the stepper
and the Experimenter from the same power supply. It may be
more economical to use a rechargeable power supply as an
alternative to a small power supply.
If the ranging board, Experimenter, and stepper are run off
the same power supply, it is necessary to know that the
boards use about 100 mA each. If a 9 v, 500 mA supply is
used the two boards would use about 200 mA combined. The
motor thus has 300 mA for it's own power consumption.
Depending on the stepper it must be calculated how much
current is available per coil. If we were to use a two coil
stepper that would be 150 mA per coil. At this low current
the voltage drop would be about 0.7 volts per coil for a
total drop of 1.4 volts.
The new resistance can be calculated and installed in the
wiring grid on the Experimenter. In this hypothetical case
the resistance value would be 48 ohms. To be sure of the
power rating on the circuit, the equation P = I2R should be
used and the proper wattage value should be placed on the
resistors. On the Experimenter power can be drawn from +A
drive on driver A.
In building this unit two electrical contacts must be
maintained as the transducer is turning. This is done using
a brass tube three inches long which will provide the
ground connection between the ranging board and the
ultrasonic transducer. One end must be insulted with
electrical tape and covered with a larger 0.5 inch long
brass tube (so the two tubes don't touch). A hole drilled
in the upper (longer) tube provides a space where a wire
can be fed through the tube and used as one of the leads
for the transducer. The other end of this wire must be
soldered to the small brass ring over the insulation. The
other lead of the transducer may be connected onto the top
of the longer brass tube. The outer ring will be the
positive (+) lead and the inner will be the negative (-)
lead of the transducer (which can be connected immediately).
The longer tube can be glued to the shaft of the motor. A
plastic cap has been placed on the back of the transducer
for appearance. Automotive alternator brushes can be used
as contact leads for the brass tubes. The negative lead
(from E2 on the ranging board) must be connected with the
brush to the upper (inner) brass tube. The positive lead
(from E1 on the ranging board) must be connected to the
lower (outer) tube. This assembly can be mounted with the
aid of two non-conducting blocks (ie. wood or rubber).
To operate this device one company has taken the initiative
to create software programs for the PC despite there being
no ready made radar kit on the market today. "Fascinating
Electronics" has written a radar control program to work
with the Experimenter board. The programs are written in
Quickbasic called EXPER1.EXE, to operate the radar and
DISTANCE.BAS to measure distances and the speed of sound.
If these programs were not available, any computer hacker
with the knowledge of the Experimenter board would be able
to write a simple version of such a program in several
The DISTANCE.BAS program pulses the rangefinder several
times per second to measure within 0.01-inch resolution
over a range of 6 inches to thirty five feet. To calibrate
the radar system a flat unit like a box can be placed at a
measured distance and picked up on the radar. When the
program is run, it will report the distance of the box it
has measured. If this measurement is wrong the program can
be calibrated for the weather conditions. The program
assumes the speed of sound is 1100 feet per second. This
can be calibrated by pressing "4" to increase the speed by
10 feet per second, "3" to increase the speed by 1 foot per
second, "2" to decrease the speed by 10 feet per second,
and "1" to decrease the speed by 1 foot per second. This
new speed of sound will be incorporated into your results
by a RADAR.DAT file.
To achieve color graphical results the computer must have
EGA, or VGA displays. If the computer only has CGA the
results will be in black or white 

Each rangefinder distance is plotted in real-time. This
provides scale information with bars of different colors to
and lengths drawn along the axes. Tens of feet are marked
by long green bars; five foot marks are red; the one foot
marks are shorter green bars; half- foot markers are black
bars; and green dots indicate quarter-foot measurements. To
the left of the display, the program reports the range
values and the number of scanning points in each rotation
of the transducer. The distance and bearing are updated
with each revolution. By pressing "L" the displayed range
increases (up to 35 feet). By pressing "S" the displayed
range decreases (down to 5 feet). Pressing "M" will result
in scan more points per revolution (up to the resolution of
the stepper motor used). "F" is used to decrease the points
scanned per revolution.
With any text file RADAR.DAT can be altered to change the
parameters. This screen can be printed in the monochrome
mode (CGA). 

This radar assembly is a very interesting project. It can
be costly, but for the enjoyment and learning experiences
it can be an asset. It will one day come in a package at
one tenth the cost of the parts (about $250.00 today).
Although its range is restricted, the transducer can be
changed and amplified to increase to range. This radar
assembly can open the gates as monitoring equipment and
perhaps one day as a property monitoring alarm system on
your own PC at very little cost. This radar assembly has a
great potential. 



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