Silent Sentry

NASA developed personal security system, commercialized by Sentry Products, Inc. enables resident to summon instant help in case of medical emergency or threatened violence. Its principal elements are a pen-shaped signaling device, silent" because its ultrasonic alert signal is inaudible to the human ear and a system of receivers interconnected with a constantly-monitored aster console. SCAN pen weighs only two ounces and is worn on necklace or attached by a clip to the user's clothing. The person in trouble simply presses a clasp releasing a hammer inside the pen which strikes an aluminum bar. The impact causes the bar to resonate like a tuning fork and emit the ultrasonic signal which is detected by the nearest receiver."

Full article: http://hdl.handle.net/hdl:2060/20030004784