Perimeter Monitor:    Using a single cable such as speaker wire or doorbell cable, this circuit can be remotely positioned, for example, at the bottom of a garden or garage, and used to detect all sound in that area. The cable can be buried in a hosepipe or duct and is concealed out of sight. The mic is an ordinary dynamic mic insert and should be housed in a waterproof enclosure with the rest of the circuit. (added 9/02)

Personal Alarm :    This one is particularly loud even though it runs on only two AA batteries. It uses a Push Pull transformer output   (added 2/05)

Personal Alarm redcircuits_:    Small portable, anti-bag-snatching unit.  Also suitable for Doors and windows control (added 9/04)

Personal Alarm:    (project / circuit added 10/06)

PIR Motion Detector:    (diagram added 6/03)

Police Siren:    This circuit produces a sound similar to the police siren. It makes use of two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC. The first IC (left) is wired to work around 1Hz. The 47uF capacitor is charged and discharged periodically and the voltage across it gradually increases and decreases periodically. This varying voltage modulates the frequency of the 2nd IC. This process repeats and what you hear is the sound remarkably similar to the police siren. Two presets VR1 and VR2 are provided to vary the siren period of repetition and the tone of the siren. By varying VR1 you can set how fast the siren changes from high freq. to low freq. VR2 sets the siren frequency. Adjust VR1 and VR2 to suit your taste.  (added 10/05)

Power line outage flashes red alert:    06/13/02  EDN Design Ideas / (added 6/06)    This Design Idea expands on a circuit in a previous one to configure a power-outage detector with a flashing alarm (Figure 1, Reference 1). The circuit plugs into a mains outlet and uses trickle-charged nickel-cadmium batteries. The green-LED monitors the presence of line voltage. The BZV55-C4V3 zener diode, D1 protects the batteries against overvoltage..

Power Supply Failure Alarm:    Most of the power supply failure indicator circuits need a separate power supply for themselves. But the alarm circuit presented here needs no additional supply source. It employs an electrolytic capacitor to store adequate charge, to feed power to the alarm circuit which sounds an alarm for a reasonable duration when the supply fails. This circuit.... (added 10/05)

Power-Supply Failure Alarm:    (electronic Circuit / Schematic added 10/04)

Proximity Alarm #1:    (electronic Circuit diagram added 03/03)

Proximity Alarm #3:    (electronic circuit added 4/05)

R/C Emergency Locator Alarm:    (electronic circuit added 4/05)

Radio Shack Special:    Electronic circuit (added 02/05)

Radio Wave Alarm zen22142:    (diagram added 6/03)

Radio Wave Alarm:    (electronic design added 6/07)

Radio Wave Alarm:    This simple circuit is sure to have the police beating a path to your door - however, it has the added advantage of alerting you to their presence even before their footsteps fall on the...(added 05/07)

Rain Alarm:    This circuit gives out an alarm when its sensor is wetted by water. A 555 astable multivibrator is used here which gives a tone of about 1kHz upon detecting water. The sensor when wetted by water completes the circuit and makes the 555 oscillate at about 1kHz.  The sensor is also shown in the circuit diagram.  It has to placed making an angle of about 30 45 degrees to the ground. This ....  (added 10/05)

Refrigerator Door Alarm:    (electronic circuit added 7/03)

Refrigerator Pig Schematic:    Upon opening your fridge door the photocell is exposed to light and the piggy oinks. (added 2/05)

RF Alarm:    (diagram added 6/03)