Abstract: An alarm device for use with doors, windows, and the like for causing a loud gunshot-like signal upon unauthorized entry is disclosed. The alarm device includes a compact housing having a chamber therein containing a firing mechanism and a detonatable gas generator cartridge. The cartridge communicates with an opening in the housing whereby when the cartridge is detonated, the gases rush through the opening resulting in a loud gunshot-like report. The chamber of the housing is closed by a movable cover which is constructed to hold the firing mechanism against acting on the cartridge. The housing is shaped for mounting on a door or window surface with the cover engaging the frame of the door or another part of the window, such that when the door or window are opened the cover will move to release the firing mechanism to strike and detonate the cartridge. The alarm device is also provided with a lock to secure the cover from moving until it is desired to activate the device.