Laptop Alarm Circuit: Build Your Theft Protection Scheme

Description

Safeguard your valuable notebook against theft utilizing this compact alarm generator. Secured internally within the notebook case, it will emit a loud alert whenever an attempt is made to remove the device. This sensitive circuit employs a homemade tilt switch to activate the alarm through tilting of the notebook case. The circuit employs readily accessible components and can be assembled on a small Vero board or a general-purpose printed circuit board.

It is powered by a 12V miniature battery, commonly found in remote controls. Integrated Circuit TLO71 (IC1) serves as a voltage comparator, utilizing a potential divider formed by resistors R2 and R3 to provide a 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 within the tilt switch come into contact with water.

When the tilt switch is maintained in the horizontal position, the inverting input of IC1 experiences a higher voltage than its non-inverting input, and the output remains low. Integrated Circuit CD4538 (IC2) functions as a monostable with timing elements R5 and C1. With the specified values, the output of IC2 remains low for a duration of three minutes. CD4538 is a precise monostable multivibrator, characterized by its resistance to false triggering and enhanced reliability compared to the commonly utilized timer Integrated Circuit 555.

Integrated Circuit CD4538 (IC2) – Monostable Multivibrator

Integrated Circuit CD4538 is a versatile and reliable monostable multivibrator. This IC is frequently utilized for creating one-shot pulses, delivering a defined pulse duration that is essential in numerous electronic applications. Unlike the 555 timer IC, the CD4538 is renowned for its inherent resistance to false triggering, rendering it a dependable solution where precise timing is paramount. This IC is particularly useful in circuits needing a specific, isolated pulse rather than a continuously oscillating signal. Its robust performance makes it an excellent alternative when reliability is a key requirement.

Integrated Circuit TLO71 (IC1) – Voltage Comparator

Integrated Circuit TLO71 is a voltage comparator IC, designed for comparing two analog voltages. This IC actively seeks to minimize the difference between its two inputs. It operates as an amplifier for the difference, producing an output signal that varies depending on the magnitude of the voltage difference. The use of TLO71 provides efficient voltage monitoring, triggering alerts when voltage thresholds are breached. This IC is frequently found in applications requiring precise voltage detection and response, ensuring reliable operation and accurate monitoring of electrical signals.

Circuit diagram:

Its output becomes high when power is switched on and it becomes low when the trigger input (pin 5) receives a low-to-high transition pulse. The unit is fixed inside the notebook 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 attempts to remove the notebook case, the unit assumes the vertical position and the tilt switch breaks the electrical connection between the probes. Immediately the output of IC1 becomes high and monostable IC2 is triggered. The low output from IC2 activates the npn transistor (T1) and the buzzer initiates beeping. Assemble the circuit as compactly as possible so as to match the unit to a matchbox size.

Construct the tilt switch utilizing a small (2.5cm long and 1cm wide) plastic bottle containing two stainless pins as contacts. Fill two-thirds of the bottle with water such that the contacts never establish an electrical path when the tilt switch is in the vertical position. Ensure a leak-proof seal of the bottle with adhesive or wax. Fix the tilt switch inside the enclosure of the circuit in the horizontal position.

Integrate the unit within the notebook case in horizontal position using adhesive. Utilize a miniature buzzer and a micro switch (S1) to create a compact device. Maintain the notebook case in the horizontal position and activate the unit. Your notebook is now secured against theft.