Alarm Circuits designed by Dave Johnson:     Page 2

• DOORKNOB ALARM
  Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms. A metal chain attached to a box holding the electronics is placed around the inside doorknob of a wood door. Anyone grabbing the knob from the outside is detected by the electrical capacitance change that occurs from the human hand contact between the knob and the box. Almost all of the commercial devices sold use a more expensive and power consuming radio frequency circuit approach to detect the capacitance change. But, a very inexpensive and micro power technique can also work. This circuit schematic should dramatically reduce the cost of the device and allows it to operate for many years from one set of batteries.
  Also see Completed Project  by Matthew C. Ruschmann

• Earthquake Alarm Circuit
  Geophones are dandy motion sensors.  They are metal cans containing a powerful magnet, suspended with springs inside a coil of wire.  The magnet functions as a floating mass.  The slightest vertical motion of the device causes the coil to move relative to the magnet.  This induces a small voltage in the wire coil...

• ELECTRIC FIELD DISTURBANCE MONITOR
  This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at Electric Field Disturbance Monitor.  The system can detect human and animal motion by the electric fields they disturb.

• ELECTRIC FIELD DISTURBANCE MONITOR-2
  This schematic is the motion discriminator alarm and battery monitor sections of the field monitor that is discussed in more detail a Electric Field Disturbance Monitor. (this link is off-site)

• Fake Car Alarm Light
  Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system. (added 7/06)

• Freezer Over-Temperature Alarm
  This electronic hobby circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F).  A 9v battery powers the hobby circuit.  The average 9ua current is so low that the battery should last for many years....

• Freezing Temperature Alarm
  This circuit turns on a beeper whenever the outside air temperature is less than zero degrees Centigrade.  The circuit draws only a few microamps from a 9 volt battery.  It uses a glass bead thermistor accurate to 1 degree C.

• Human Traffic Footstep Alarm
  This circuit processes the amplified signals from the footstep detector circuit and activates a microprocessor when the circuit detects footsteps.  At has an adjustable sensitivity.

• Low Voltage 120vac Power Line Alarm
  This circuit will turn on a beeper when the line voltage drops below 100 volt AC.

• Simple Security Wire Loop Alarm
  A wire loop is used to protect valuable objects in this simple alarm circuit.  The electronic hobby circuit is powered by a 9v battery.  The alarm beeper is activated if the wire loop is severed.  The standby current is so low that the 9v battery should last for many years.

• Wire Security Loop Alarm
  A wire loop is used to protect valuable objects in this alarm circuit.  The circuit is powered by a 9v battery.  (added 7/06)