Ultrasonic radar. (rangefinder design)

IT'S DIFFICULT TO IMPRESS PEOPLE with a home computer. Even if the computer has a 486 processor running faster than blazes, from the outside it just looks like a tan box. There's not much you can bring up on the screen that will impress them either--they've seen it all before on TV. However, this month's project puts a live-action radar-like display in high-resolution color graphics on your home computer screen--and it's guaranteed to impress. People will be amazed as a blip moves about on the computer's "radar" display, following their movements about the room.

The system works as follows: An ultrasonic rangefinder measures the distances to surrounding objects. (Note that because ultrasonic energy rather than radio frequency is used, this project is more like sonar than radar.) A stepper motor rotates the transducer, giving full 360[degrees] coverage. The ultrasonic rangefinder and stepper motor are controlled and interfaced to your home computer by the Experimenter interface presented last month. The software, written in Microsoft's QuickBASIC for PC-compatible computers, is available on the Electronics Now BBS (516-293-2283), or on disk from the source mentioned in the Parts List. A Macintosh version is also available.

The rangefinder

Our project uses the same ultrasonic transducer that is used in Polaroid cameras. Note that we will not be building the transducer unit because it is not available as a kit--only as a preassembled surplus unit from the source mentioned in the Parts List. An ultrasonic rangefinder emits a brief pulse of high-frequency sound. Any object hit by the sound produces an echo. The distance to the object is determined by measuring the time delay between the transmission of the original pulse and the return of the echo, which is displayed.

The Polaroid rangefinder is made up of two parts: a transducer and a ranging board. The transducer, shown in Fig. 1, acts as both a speaker and a microphone. It emits the ultrasonic pulse and "listens" for the echo. The ranging board (Fig. 2) provides the high voltages required to run the transducer, sensitive amplifiers for echo detection, and control logic. The only adjustment on the ranging board, VRI, changes the sensitivity of the echo detector. The transducer is mounted on top of the output shaft of a stepper motor, which gives the transducer a 360[degrees] field of view. The ranging board mounts beside the stepper motor. Figure 3 shows the entire radar assembly.

The Experimenter controls he ranging board, measures the round--trip time of the pulses, controls the stepper motor, and communicates with your computer. Figure 4 shows the connections between the transducer, ranging board, and the Experimenter. The ranging board uses a nine-conductor flexible cable. The connector for this cable can be installed in the XI connector mounting area on the Experimenter. The cable has a black stripe on it to indicate pin 1.

The ranging board's power requirements are normally under 100 milliamps, but peak at about 2 …