Laptop Protector

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Protect your valuable
laptop against theft using this miniature alarm generator. Fixed in-side
the laptop case, it will sound a loud alarm when someone tries to take
the laptop. This highly sensitive circuit uses a homemade tilt switch to
activate the alarm through tilting of the laptop case. The circuit uses
readily available components and can be assembled on a small piece of
Vero board or a general-purpose PCB.

It is powered by a 12V miniature battery used in remote control
devices. IC TLO71 (IC1) is used as a voltage comparator with a potential
divider comprising R2 and R3 providing half supply voltage at the
non-inverting input (pin 3) of IC1. The inverting input receives a
higher voltage through a water-activated tilt switch only when the
probes in the tilt switch make contact with water.

When the tilt switch is kept in the horizontal position, the
inverting input of IC1 gets a higher voltage than its non-inverting
input and the output remains low. IC CD4538 (IC2) is used as a
monostable with timing elements R5 and C1. With the shown values, the
output of IC2 remains low for a period of three minutes. CD4538 is a
precision monostable multivibrator free from false triggering and is
more reliable than the popular timer IC 555.

Circuit diagram:

Laptop Protector Circuit

Laptop Protector Circuit Diagram

Its output becomes high when power is switched on and it becomes low
when the trigger input (pin 5) gets a low-to-high transition pulse. The
unit is fixed inside the laptop case in horizontal position. In this
position, water inside the tilt switch effectively shorts the contacts,
so the output of IC1 remains low. The alarm generator remains silent in
the standby mode as trigger pin 5 of IC2 is low.

When someone tries to take the laptop case, the unit takes the
vertical position and the tilt switch breaks the electrical contact
between the probes Immediately the output of IC1 becomes high and
monostable IC2 is triggered. The low output from IC2 triggers the pnp
transistor (T1) and the buzzer starts beeping. Assemble the circuit as
compactly as possible so as to make the unit matchbox size.

Make the tilt switch using a small (2.5cm long and 1cm wide) plastic
bottle with two stainless pins as contacts. Fill two-third of the
bottle with water such that the contacts never make electrical path when
the tilt switch is in vertical position. Make the bottle leak-proof
with adhesive or wax. Fix the tilt switch inside the enclosure of the
circuit in horizontal position.

Fit the unit inside the laptop case in horizontal position using
adhesive. Use a miniature buzzer and a micro switch (S1) to make the
gadget compact. Keep the laptop case in horizontal position and switch
on the unit. Your laptop is now protected.

Source: EFY Mag