Small Personal Alarm

Small, portable, anti-bag-snatching unit. Also suitable for doors and windows control

This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound.

The device can be reverse connected, i.e. the box can be placed in a pocket and the cord connected to the bag. This device can be very useful in signalling the opening of a door or window: place the box on the frame and the magnet on the movable part in a way that magnet and reed switch are very close when the door or window is closed.

Personal Alarm Circuit Diagram :

Personal Alarm Circuit Diagram

Circuit operation:

A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part count and 3V battery supply allow a very compact construction.

Parts:

R1____________330K 1/4W Resistor
R2____________100R 1/4W Resistor

C1_____________10nF 63V Polyester or Ceramic Capacitor
C2____________100µF 25V Electrolytic Capacitor

Q1____________BC547 45V 100mA NPN Transistor
Q2____________BC327 45V 800mA PNP Transistor

SW1____________Reed Switch and small magnet (See Notes)

SPKR___________8 Ohm Loudspeaker (See Notes)

B1_____________3V Battery (two A or AA cells wired in series etc.)

 Notes:

The loudspeaker can be any type, its dimensions are limited only by the box that will enclose it.
An on-off switch is unnecessary because the stand-by current drawing is less than 20µA.
Current consumption when the alarm is sounding is about 100mA.
If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3.5mm mono Jack socket and the magnet by a 3.5mm. mono Jack plug having its internal leads shorted. The Jack plug will be connected to the tiny cord etc.
Do not supply this circuit at voltages exceeding 4.5V: it will not work and Q2 could be damaged. In any case a 3V supply is the best compromise.

Source : www.redcircuits.com

Transceiver Homebrew QRP SSB 80M Band

Radio communication transceiver is a radio transmitter at the same time the plane doubles as a radio receiver used for communication purposes. It consists of the transmitter and the receiver are assembled in an integrated way. In mulamula generation, the transmitter or receiver or transmitter and receiver sections are assembled separately and is part of a stand sendirisendiri and can work well sendirisendiri Currently employed both parts are integrated in turn. Aircraft simple transmitter consists of an oscillator generating radio vibration and this vibration after vibration boarded with our voice, in a technique called dimodulir radio, then by the antenna is converted into radio waves and transmitted. As we know that the sound waves we can not reach long distances, although its power is quite large, while the radio waves with a relatively small force can reach a distance of thousands of kilometers. In order for our voice can reach a far distance, then our voice superimposed on radio w...

Mutchus

Search This Blog