Ultrasonic Radar for a Home PC System


One of the fastest changing and most expensive fields, is that of
technology. Our computers, printers, modems, and much more is being
outdated faster than anything else in the world. Just as we buy a new
computer that does what we want, the industry comes out with a new option
on a smaller and better computer. There seems to be so much changing that
unless we invest our life savings into technology, we are considered
obsolete like our computers.

What used to fill an entire room, is so small now that it can be
swallowed with a glass of milk. A computer used to be a mechanical engine
that had many moving parts and was very slow. Now computers design
computers that are tenfold their own power and a tenth the size, with less
parts and using less power.

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 coffee-pot)
to spare. Ultrasonic radar is now a small component for your computer,
giving computer operators a chance to see low flying objects, household
furniture, and even themselves on their PC screen. Just to impress a
neighbour or friend is reason enough to build your own ultrasonic radar
station.

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
(Fig. 1) 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
(Fig. 2). 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 360o field of view. For entire assembly see Figure
3.

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
connections between the Experimenter, ranging board, and transducer are
shown in Figure 4.

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.

Figure 5 shows the timing diagram of the ranging boards's signals. 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 measurement.

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 conditions.

The stepper motor is used to rotate the radar so it can scan 360o
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