Any break in the invisible beam between these low-cost kit units sounds a continuous alarm. Setting up a foolproof electronic burglar alarm is easy with two inexpensive kits. The heart of the system is an ultra-sensitive photoelectronic relay which costs only $12.95 in kit form. This “eye” is positioned to intercept an imperceptible beam of light which crosses the only path a burglar might take and it triggers the alarm the instant he breaks the beam. Stepping out of the beam avails him nothing—the alarm continues to sound.
Actually, any light beam maybe used with the photocell but the sealed beam light source is handy and inexpensive at just $7.75. The inclusion of a transformer also allows the lamp to operate at five volts, which extends its useful life. To cut down the light so that it is almost invisible at close range,. a red filter is supplied. Yet the photocell will still operate at distances up to 150 feet away. For completely invisible light, a Wratten No. 70 filter which is sold at most photo supply houses may be employed. This will allow operation up to 50 feet distant.
An interesting application is to use mirrors to bend the light beam around corners. In this way, a door and one or more windows can be protected by a single installation. The setup, of course, may also be used to open garage doors, turn on lights or to announce visitors. It may even be employed to count assembly-line production in a factory.
Complete instructions for assembly of the photoelectronic relay and the light source are supplied with the kits and you should be able to complete the job in an evening or two. For an alarm you can hook up a loud bell or a chime which is also available for $4.70. Then sleep in peace