8/6/2019 81 Security System Switcher

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circuit

ideas

140 • January 2010 •   electronics for you w w w . e f y m a g . c o m

A audi sigal ca be used as

a frm f iput t ctrl asecurit sstem. Fr example,

a autmatic securit camera ca be

congured to respond to a knock on

the dr. The circuit described here

allws the securit sstem t autmati-

call switch whe a master switch

is i state. It uses a trasducer t

detect itruders ad a 5V regulated

DC pwer suppl prvides pwer t

the circuit.

As shw i Fig. 1, a cdesermicrphe is cected t the iput

f small sigal preamplifier built

arud trasistr T1. Biasig resistr

R1 determies t a large extet the

micrphe sesitivit. A micrpheusuall has a iteral FET which

requires a bias vltage t perate. The

sound picked up by the microphone

is amplied and fed to input pin 2 of

IC1 (LMC555) wired i mstable

conguration.

IC2 (CD4538B) is a dual,

precisi mstable mul-

tivibratr with idepedet

trigger ad reset ctrls. The

utput f IC1 is cected t

the rst trigger input pin 4 of

IC2(A) through switch S1. If

an intruder opens or breaks

the dr, IC1 is triggered b

sud sigals; the timer ut-

put pin 3 of IC1 goes high and

enables rst monostable multi

vibrator IC2(A). IC2(A) pro-

vides a time perid f arud

5 to 125 seconds, which is adjusted

with preset VR1.

Ather mstable multivibratr

IC2(B) also provides a time period of

around 25 to 600 seconds, which is

adjusted with preset VR2. The outputof IC2(B) is used to energise relay RL1.

T.K. Hareendran

SecuriTy SySTem SwiTcHer

 s.c.  d w i v e d i

Fig. 1: Security system switcher 

Fig. 2: Proposed cabinet 

+5V ADAPTOR

FOR POWER

SUPPLY

CONNECTOR

FOR

SECURITY

GADGET

Idicatr LED1 is prvided t displa

the rela activit. A AC/DC per-

ated securit gadget is activated r

deactivated thrugh a securit switch.

Thus, the securit switch f the gadget

is cected i the n/o ctacts f

the rela. yu ca als perate high-

pwer beacs, sires r hters i

place f the securit switch fr a

AC/DC perated securit gadget.

Assemble the circuit a ge-

eral-purpse PCB ad eclse it i

a cabinet as shown in Fig. 2 along

with 5V adaptr fr pwerig the

circuit. Cect the securit switch

accrdig t the circuit diagram ad

use apprpriate AC/DC pwer sup-

pl required t perate the securit

gadget.

 Warning! All relevat electrical

safety precautions should be taken

whe cectig mais pwer suppl

t the rela ctacts. With the help

of single pole double throw (SPDT)

switch S1, internal or external trig-

ger iput (active high sigal) ca beselected.