Basic Electronics/Electrical Projects & Kits

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Anti-Theft Alarm for Vehicles Save Rs. 100.00
Rs. 400.00
Rs. 300.00

Anti-Theft Alarm for Vehicles

Anti-Theft Alarm for Vehicles

by KNS Pvt Ltd
Save Rs. 100.00
Rs. 400.00
Rs. 300.00

This anti theft alarm for vehicles is a security system for vehicles that generates a loud and continuous alarm in case any unauthorized person tries to gain access to your vehicle. As soon as a thief tries to start the ignition, the vehicle alarm is activated, leaving no option other than to flee as the alarm can be reset and turned OFF by the owner alone. Build this kit to prevent theft of your vehicle.

Learning from this project

  • Application of music generator IC UM3561
  • Application of NPN transistors for switching purposes
  • Application of SCRs for sensing and switching
  • Application of capacitors for temporary storage

Product description

The anti theft alarm for vehicles simulates a police siren whenever someone attempts to commit theft of your vehicle. The alarm sounds continuously for a few seconds even when the intruder switches off the ignition key.

Circuit working

The circuit consists of an SCR based trigger circuit and an audio alarm circuit. When the ignition key of the vehicle is switched off, the base voltage of the NPN transistor SL100 (T1) is low and it remains off. When the ignition key is switched on for starting the vehicle, a positive voltage is applied to the base of transistor T1 through diode D1, toggle switch (S2) and a resistor (R1) which slowly charges the 100 microfarad capacitor (C1). As a result, the base voltage of T1 rises.

As soon as the biasing voltage crosses cut-in voltage, T1 turns on and the SCR fires, giving 12V DC voltage to the alarm circuit. The alarm circuit is built around the music generator UM3561 (IC1) which has an inbuilt oscillator. The output from IC1 is fed to a transistor amplifier built around transistors BC548 (T2) and SL100 (T3). The circuit gives sufficient time delay to switch ON the alarm and leave the vehicle. The alarm once triggered, sounds until the toggle switch S1 is pressed to switch off the power supply.

A 220 microfarad capacitor (C2) is provided to sound the alarm even when the intruder switches off the ignition key. When the ignition key is switched off immediately, C2 discharges through a resistor R4 and keeps the alarm activated for half a minute. Reset switch (S3) is used to reset the alarm if needed.

Click To View Circuit Diagram

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Musical Hold On For Telephone Save Rs. 50.00
Rs. 200.00
Rs. 150.00

Musical Hold On For Telephone

Musical Hold On For Telephone

by KNS Pvt Ltd
Save Rs. 50.00
Rs. 200.00
Rs. 150.00


The Musical Hold On for Telephone is an add-on external circuit that can be attached to the telephone to generate a melodious tune on the telephone line. The circuit allows the person on the other end to enjoy this melodious tune during the hold-on interval. To resume talking, just a simple switch needs to be flicked into the OFF position. A Red LED included in the circuit indicates that phone line is on-hold and it automatically turns OFF when the music stops. No. of ICs used: 1


Things which you can learn:
*Application of the melody generator IC UM66.
*Application of diodes for rectification and voltage regulation.
*Application of transistors for amplification and switching purposes.

Product Description

Build a cool Musical Hold on a device using simple components and fascinate your friends when you put them on hold.The heart of the Musical Hold-On for Telephone is the melody generator IC UM66. This IC generates a melodious musical tone when continuous power is supplied to it.


The circuit utilizes the voltage supply present on the telephone line, which is further applied to the bridge rectifier formed by four diodes (D1-D4). 

In a normal state with a switch (S1) kept in OFF position, no voltage is applied to the base of the PNP transistor MPSA92 (T1) and it remains reverse biased. Thus, the red indicator LED (D5) and the melody generator IC UM66 (IC1) are OFF and the phone line is free for normal usage. However, while talking if the switch (S1) is closed, then a positive DC voltage (available across the telephone line) gets connected to the base of transistor T1.

This forward biases T1 which acts as an amplifier and an electro-switch. A regulator circuit (comprising of resistors, a capacitor, and a zener diode) regulates the voltage available at the collector of transistor T1. The red indicator LED glows and IC1 also begins generating music as voltage gets applied to its Pin2. The low-value output from Pin1 is amplified by the NPN transistor BC547 (T2). This is coupled to the telephone line using a capacitor and thus the musical tune is heard on both phone.