Displacement Alarm

Warns if an object is moved

Small gadget - 9V Battery supply

Circuit diagram:

Displacement Alarm


R1_____________220R  1/4W Resistor
R2,R3____________1M  1/4W Resistors

C1______________22µF 25V Electrolytic Capacitor
C2_____________100nF 63V Ceramic or Polyester Capacitor

IC1_____________7555 or TS555CN CMos Timer IC

BZ1____________Piezo sounder (incorporating 3KHz oscillator)

SW1_____________SPST Rolling-ball Tilt Sensor Switch

SW2_____________SPST Toggle or slide switch (Optional, see Improvements)

B1________________9V PP3 Battery (See Improvements)

Clip for PP3 Battery


It should be clear that this small alarm device was not designed to protect against thieves, but in order to defend, even against simple displacements, personal items that you do not want to be touched, nor removed. So this is a small and simple defense against any tampering, also inadvertent, e.g. at home.
A typical use of this device is to be placed inside a drawer to signal when it is pulled.

Circuit operation:

IC1, a CMos 555 timer, is wired as an astable multivibrator: this allows the Piezo sounder, driven by pin #3 of IC1, to beep at a 4-5 times per second rate.
SW1 is a Rolling-ball Tilt Sensor Switch usually available in the form of a small cylinder with two outer leads. If this sensor is placed on the circuit board horizontally, or slightly inclined downward at the side opposite to that of the outer leads, the rolling-ball contained in its interior, rests automatically to the opposite side of the internal contacts and the circuit remains idle.
As soon as the drawer containing this gadget is pulled, the rolling-ball moves and touches the internal contacts, connecting, even for a moment, pin #4 of IC1 to the positive rail through R1: the IC is therefore enabled and the beeper sounds.
At the same time, C1 is charged and maintains IC1 enabled even if the internal switch of SW1 is no more closed. This capacitor discharges slowly through R2. With the values shown in the Parts List for C1 and R2, the beeper will sound intermittently for about 50 seconds.