Volumetric Sensor Alarm

Detects small variations in air pressure

Can be battery powered (6/12V)

Circuit diagram:

Volumetric Sensor Alarm


R1,R2____________1K  1/4W Resistors
R3_______________3K3 1/4W Resistor
R4,R18_________470K  1/4W Resistors
R5,R6__________330K  1/4W Resistors
R7______________47K  1/4W Resistor
R8______________27K  1/4W Resistor
R9______________22K  1/4W Resistor
R10______________1M  1/4W Resistor
R11,R12_________10K  1/4W Resistors
R13_____________10K  1/2W Trimmer Carbon or Cermet
R14______________2M2 1/4W Resistor
R15,R16________100K  1/4W Resistors
R17____________500K  1/2W Trimmer Carbon or Cermet
R19____________680R  1/4W Resistor (See Notes)
R20______________1K  1/4W Resistor (Optional, see Notes)
R21,R22__________4K7 1/4W Resistors

C1,C3,C9-C12___100µF  16V or 25V Electrolytic Capacitors
C2,C4,C5,C7____100nF  63V Polyester Capacitors
C6______________10µF  25V Electrolytic Capacitor
C8______________33nF  63V Polyester Capacitor

D1___________1N4148   75V 150mA Diodes
D2________BZX79C5V1  5.1V 500mW Zener Diode
D3______________LED  (Any color and size) (Optional, see Notes)
D4___________1N4002  100V 1A Diode

IC1___________LM324  Low Power Quad Op-amp IC
IC2____________7555 or TS555CN or TLC555CP CMos Timer IC

Q1,Q2_________BC337   45V 800mA NPN Transistors

MIC1__________Miniature Electret Microphone Capsule

RL1___________Relay with SPDT 2A @ 230V switch
              Coil Voltage 12V. Coil resistance 100-300 Ohm (See Notes)

Circuit description:

This circuit is able to instantly detect the small variations in air pressure caused by the opening of doors and windows.
Unlike other anti-theft circuits or sensors that can not be used when someone is at home because the slightest movement would trigger them, this circuit allow you to move easily within the premises and will activate only when opening doors or windows.
This feature will allow, for example, its use in small shops, to signal the opening of the entrance door and the entry of a customer. Therefore, it can serve not only as an alarm but also for counting purposes.
No special purpose sensors are required: a common Electret Microphone capsule will do the job.

Circuit operation:

IC1A amplifies the signal picked-up by the microphone capsule MIC1, cutting heavily the pass band, in order to select only the very low part of the audio spectrum, given that the signals of interest for this circuit are in the 0.5-1.5Hz range.
IC1B produces further amplification and tailoring of the signal and IC1C acts as a trigger, whose sensitivity is adjusted by means of Trimmer R13.

IC1D, IC2 and Q1, Q2 form a set of timers, necessary to manage correctly the whole behavior of the circuit:
IC1D inhibits the output Relay through Q2 for some seconds (time constant set by means of R15 & C9) when the circuit is triggered by the opening of a door or window, in order to allow some time delay before the alarm starts.
IC2 has a double timing function. When the power is applied to the circuit it inhibits the output Relay for about 15-20 sec (time constant set by means of R18 & C11) in order to avoid false triggering and, more important, to allow the operator to exit the room and close the door before than the circuit starts operating.
The main timer of IC2 is used to set the duration of the alarm. This delay is variable by means of Trimmer R17.
Q1 and Q2 are wired in series and form an AND gate: the Relay will operate only when both transistors are in conduction.

The circuit is powered at 12V (battery or dc external power supply adaptor) but the two ICs use a 5V supply, derived by means of R19 and fixed by the Zener Diode D2. If you want to operate the whole circuit at 6V see Notes.