Door activated burglar alarm utilizing time delay

Claim 1

In a burglar alarm system comprising a door switch electrically interconnected for activating a noise-emitting device and an operating circuit arranged to connect in series a power supply means having a positive terminal and a negative terminal, an on-off switch and the noise-emitting device, the improvement comprising: the door switch having terminals arranged to be in open and closed positions in response to the door being closed and opened, respectively; a controlled rectifier being interposed into the operating circuit so as to have an anode connected to the on-off switch for receiving a positive potential from the power supply means and a cathode interconnected to the noise-emitting device; charging circuit for a capacitor comprising said capacitor being electrically coupled to the negative terminal of the power supply means and, through a first resistor, to a circuit point on the operating circuit between the on-off switch and the controlled rectifier; a discharge circuit and exit time delay network comprising a second resistor being connected from a circuit point on the charging circuit between the first resistor and the capacitor, through the door switch, to the gate of the controlled rectifier, and further comprising a third resistor connected from the cathode of the controlled rectifier to the negative terminal of the power supply means, and further comprising a fourth resistor connected between the gate and the cathode of the controlled rectifier, whereby the capacitor will not be sufficiently charged to provide an operative triggering current pulse to the controlled rectifier until the on-off switch is closed and the door switch is opened by closing the door, and whereby the charging of the capacitor creates a predetermined exit time delay in arming the noise-emitting device after the door is closed.