Displacement Alarm

Posted in Alarms and security, on 2015-10-15

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.


  • The sensitivity of this circuit can be largely improved by using four Rolling-ball Tilt Sensor Switches, all wired in parallel, and cross-assembled on the circuit board so as to form four right angles: in this way, the circuit becomes sensible to movement in any direction. In practice, you can vary the sensitivity and the directivity of this circuit by using more or less Rolling-ball Tilt Sensor Switches wired in parallel, and arranged spatially in the way you want.
  • When the circuit is idle, current consumption is negligible: therefore, SW2 can be omitted.
  • The beeping rate can be varied by changing the values of R3 and/or C2; the duration, by changing the values of R2 and/or C1
  • Using a suitable Piezo sounder, the circuit will work fine with battery voltages from 9 to 3V, but the intensity of the beep will decrease accordingly.

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