The electronic systems described here will deter intruders by various means. The projects include perimeter detectors, four kinds of alarms (buzzer, bell, intermittent, and siren), exit door timers, a fire detector, intruder deterrers, and a light-triggered siren. An infrared detector, a gas detector, a combination sentry, a touch alarm, and a power supply for the systems also are described. Instructions are simple enough for the average homeowner. Assembly procedures are illustrated with diagrams and supplemented with tables.