Alarm Circuits

12 Volt Alarm Power Supply

2-input alarm - This article describes a 2-Input alarm developed on the PIC LICK-1 Module using a Microchip PIC16F628-04.  The program uses the internal 4MHz oscillator and if any other frequency is used, the timer values will need to be changed.

4 Digit Keypad

4-Digit Alarm Control Keypad - This Keypad is suitable for the Modular Burglar Alarm. However, it has other applications. Pressing a single key will energize the relay. Entering the Four-Digit code of your choice will de-energize it.

5 Zone Alarm Circuit

5 Zone alarm circuit MIT

555-Timer Based Motorcycle Alarm - This circuit features an intermittent siren output and automatic reset. It can be operated manually using a key-switch or a hidden switch; but it can also be wired to set itself automatically when you turn-off the ignition. By adding external relays you can immobilize the bike, flash the lights etc.

5-Digit Alarm Control Keypad - This Keypad is suitable for the Modular Burglar Alarm. However, it has other applications. Entering the First Four Digits of your chosen Five-Digit code, will energize the relay. Entering the Full Five-Digit code, will de-energize it.

A 4-Digit Keypad Controlled Switch - This is a Universal version of the Four-Digit Alarm Control Keypad. I have modified the design to free up the relay contacts. This allows the circuit to operate as a general-purpose switch. I've used a SPCO/SPDT relay, but you can use a multi-pole relay if you wish.

A 555-Timer Based Motorcycle Alarm - This circuit features an intermittent siren output and automatic reset. It can be operated manually using a key-switch or a hidden switch; but it can also be wired to set itself automatically when you turn-off the ignition. By adding external relays you can immobilize the bike, flash the lights etc.

A 5-Digit Alarm Control Keypad - This Keypad is suitable for the Modular Burglar Alarm. However, it has other applications. Entering the First Four Digits of your chosen Five-Digit code, will energize the relay. Entering the Full Five-Digit code, will de-energize it.

A Battery-Powered One-Time-Only Burglar Alarm - This is a single zone alarm, with independently adjustable Exit, Entry and Siren Cut-Off timers. When the alarm is activated its Siren will sound once, for up to 20-minutes. Then it will switch off and remain off. If you wish, you can use a mains power supply. But the extremely low standby current makes battery power a realistic option.

A CMOS 4060 Burglar Alarm - This is a single zone alarm, with automatic exit, entry and siren cut-off timers. It will accommodate all the usual types of normally-closed input devices, such as magnetic reed contacts, foil tape, PIRs etc. But it's easy to add a normally-open trigger.

A CMOS Based Motorcycle Alarm  - This circuit features an intermittent siren output and automatic reset. It can be operated manually using a key-switch or a hidden switch; but it can also be wired to set itself automatically when you turn-off the ignition.

A CMOS Based Single Zone Alarm - This circuit features automatic Exit/Entry delays, timed Bell Cut-off and System Reset. It has provision for normally-open and normally-closed switches and will accommodate the usual input devices (Pressure Mats, Magnetic Reed contacts, Foil Tape, PIRs and Inertia Sensors).

A CMOS Based Vehicle Anti-Theft Alarm - This car alarm circuit features Exit and Entry delays, an instant alarm zone, an intermittent siren output and automatic Reset. By adding external relays you can immobilize the vehicle and flash the lights.

A CMOS Single Zone Intruder Alarm - This circuit features automatic Exit and Entry delays, timed bell cut-off, and system reset. It has provision for normally-open and normally-closed switches, and will suit all of the usual input devices (Pressure Mats, Magnetic Reed contacts, Foil Tape, PIRs and Inertia-Sensors

A Dozen Small CMOS Alarm Circuits - This is a selection of small self-contained alarm circuits. They have a very low standby current; and are suitable for battery operation. Some are triggered by normally-open and normally-closed switches. Some react to changes in light or temperature. The result is a variety of output times and patterns.

A One-Time-Only Burglar Alarm - When this alarm is activated its Siren will sound once, for up to 20-minutes. Then it will switch off and remain off. The basic circuit has a single zone with independently adjustable Exit and Entry delays. This will be adequate in many situations. However, there's also range of Expansion Modules. These will allow you to add any number of Instant Zones, Tamper Zones and Personal Attack Zones to your system.

A Shed/ Garage Alarm - This is a basic single-zone burglar alarm circuit. Its features include automatic Exit and Entry delays. It has an extremely small standby current. This makes it ideal for battery-powered operation. Use it in your caravan, mobile home, lock-up, or anywhere mains power is not available.

A Simple Fire-Door Alarm  - This circuit will let you know when something that should be kept closed, has in fact been left open. Attach it to a Fire-Door. It will allow you to pass through the door as usual. However, if it's left open for more than 30-seconds or so, the buzzer will sound.

A Simple Shop-Door Alarm  - Attach this circuit to the door of an unattended shop, or reception area, and the sound of the buzzer will tell you when you have a customer. Add a siren, and it becomes a simple intruder alarm.

A Simple Transistor Based Motorcycle Alarm - This is an even simpler transistor based motorcycle alarm. The circuit features a timed output and automatic reset. It can be operated manually using a key-switch or a hidden switch. By adding an external relay, it will set itself automatically, and/or immobilize the machine, every time you turn-off the ignition. It's easily adapted for a 6-volt system, so it will protect your "Classic Bike"

A Simpler CMOS Single Zone Alarm - This circuit features automatic Exit/Entry delays, timed Bell Cut-off and System Reset. It will accommodate the usual normally-closed input devices (Magnetic Reed contacts, Micro Switches, Foil Tape and PIRs). And, with a simple modification, a normally-open trigger may be added.

A Transistor Based Motorcycle Alarm - This circuit features a timed output and automatic reset. It can be operated manually using a key-switch or a hidden switch. By adding an external relay, it will set itself automatically, and/or immobilize the machine, every time you turn-off the ignition. It's easily adapted for a 6-volt system, so it will protect your "Classic Bike".

AC Line Under/Over Voltage Alarm - Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment.  The circuit below sounds an alarm whenever the line voltage is higher or lower than normal.  I set the alarm limits at about +-15% from standard levels.  The circuit rectifies and filters the power line signal.  I set the resistor values, so the DC voltage produced is close to 1% of the RMS value of the line.  Thus, a 120vac line would yield about 1.2v DC.  That voltage is fed to a pair of voltage comparators.  The outputs of the comparators decide if the sampled voltage is above, below or acceptable.

Alarm Circuit - Surround your Entire Yard with this Laser Alarm

Alarm control keypad

Alarm monitors rotational speed of DC motor - EDN-Design Ideas: This Design Idea monitors the rotating speed of a dc fan motor, sounding an alarm and powering down if the motor stalls.

Alarm Power Supply

Alarm Power Supply With Battery Back-up - This Power Supply is suitable for the Modular Burglar Alarm. However, it has other applications. It is designed to provide an output of 12-volts, with a current of up to 1-amp. In the event of mains failure, the back-up battery takes over automatically.

Alarm System

Alarm tells you to close the refrigerator door - EDN- Design Ideas,  A simple gadget that you place inside the refrigerator alerts you by beeping if the door stays open for 20 seconds or longer.

American Police Car Siren (linscott) - Scroll down to locate this circuit. The 555 on the right is wired as an alarm tone generator and the second 555 timer on the left is wired as a low frequency astable timer which generates a ramp waveform of about 6 seconds that is buffered by the transistor and again used to frequency modulate the tone generator. The transistor is used to help strengthen the signal to the speaker.

An Enhanced Shed/ Garage Alarm  - This is an enhanced version of the simple Garage/Shed Alarm. The Entry and Exit delays have been increased to about 30-seconds, and I've added a timed Siren cut-off and automatic Reset. I've also replaced the LED with an entry Buzzer. These enhancements mean that the new version will have a much wider application.

An Expandable Multi-Zone Modular Burglar Alarm - This circuit features automatic Exit and Entry delays and a timed Bell Cut-off. It has provision for both normally-closed and normally-open contacts, and a 24-hour Personal Attack/Tamper zone. By using the Expansion Modules, you can add as many zones as you require; some or all of which may be the inertia (shock) sensor type. 

An Expandable SCR Based Burglar Alarm - This is a simple SCR based burglar alarm circuit. Its features include automatic Exit and Entry delays, together with a timed Bell Cut-off and Reset. The basic alarm has a single zone. This will be adequate in many situations. However, the modular design means that you can add as many zones as you like to the system. Details of a Four-Zone expansion module are provided.

An Expandable Transistor Based Burglar Alarm - This transistor based alarm features automatic Exit and Entry delays, together with a timed Bell Cut-off and system Reset. Along with the Exit/Entry zone, the basic alarm board has one Instant Zone. This will be adequate in many situations. However, the modular design means that it's easy to add as many zones as you like to the system. Details of a Four-Zone expansion module are provided.

An SCR Based Burglar Alarm

Anit Theft Motorcycle Alarm

Auto Burglar Alarm (Randy Linscott) - This alarm circuit is based on two 555 timers. The alarm will sound your car horn if anyone opens the car door while the circuit is armed. The timers will allow you to leave the car without sounding the horn. To turn the circuit on S1 must be closed.

Automatic Intruder Alarm

Automatic Night Lamp with Morning Alarm

Battery Low Voltage Beeper - This circuit provides an audible and visual low voltage warning for 12V battery powered devices. Idle current: 6ma Low Voltage Warning current: 15ma.

Battery-Powered One-Time-Only Burglar Alarm - This is a single zone alarm, with independently adjustable Exit, Entry and Siren Cut-Off timers. When the alarm is activated its Siren will sound once, for up to 20-minutes. Then it will switch off and remain off. If you wish, you can use a mains power supply. But the extremely low standby current makes battery power a realistic option.

Beeper bug transmitter for fox hunt & vehicle tracking

Big Ben Sound - This circuit produces  famous Big Ben sound. It produces  "ding dong" sound when switched ON. Basically  circuit alternates between two frequencies which are adjustable. This produces  "ding-dong" sound. The first C (left) oscillates at about 1Hz. The second IC's tone is modulated by  changing voltage at  output of  first IC.

Brakelight Flasher - This is basically a flasher circuit modified to turn on and off a bulb instead of a LED. It uses a 555 timer IC working as an astable multivibrator. The flashing rate can be varied from very fast to a maximum of once in 1.5 sec by varying  preset VR1.  The ON time of  circuit is given by: TON= 0.69xC1x(R1 + VR1) second and  OFF time is: TOFF= 0.69xC1xVR1 second.

British Police Car Siren - The first circuit simulates the siren of a British police car. It uses two 555 timers in the circuit. The 555 on the right is wired as an alarm tone generator and the second 555 timer on the left is a 1 Hz astable multivibrater.

Broken Charger-Connection Alert - Detects if a device is not properly connected to its supply.  Suitable for battery chargers, portable appliance supplies etc.

Burglar Alarm - This is a single zone alarm, with automatic exit, entry and siren cut-off timers. It will accommodate all the usual types of normally-closed input devices, such as magnetic reed contacts, foil tape, PIRs etc.

Car Alarm

Car Alarm & Immobilizer - This circuit features exit and entry delays, an instant alarm zone, an intermittent siren output and automatic reset. By adding external relays you can immobilize  vehicle and flash  lights.

Car Anti-Theft Wireless Alarm - This FM radio-controlled antift alarm can be used with any vehicle having 6to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in  vehicle at night when it is parked in  car porch or car park. The receiver unit with CXA1019, a single IC-based FM radio module, which is freely available in  market at reasonable rate, is kept inside.

Car Back-up Alarm - The brake lights of  automobile trigger this circuit on and off.  This save  annoyance of  alarm when it is not needed.  This is an older circuit which was published in Popular Electronics Magazine, but still a good circuit today.

Car HiJack Alarm - Before fitting this or any other engine cut-out to your vehicle, carefully consider both the safety implications of its possible failure, and the legal consequences of installing a device that could cause an accident. If you decide to proceed, you will need to use the highest standards of materials and workmanship.

Car-Police Siren - This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail.

Chain Link Fence Alarm - Long stretches of steel fence can be monitored using the simple circuit below. An insulated wire is woven through the links in the fence.  Where the fence begins, the circuit is connected to an earth ground and the security wire.  At the end of the fence, the wire is connected to one of the steel posts, held in the ground.  The typical earth ground resistance will be in the 1000s of ohms range, which is low enough to keep the n-channel FET turned off in the circuit below.  If the wire is cut or broken

Cmos 4017 Sequential Timer - This circuit uses a Cmos 4017 decade counter to create a sequence of four separate events. The number of events in the sequence may be increased to nine or ten. And the length of each event is controlled independently. D13 causes the sequence to repeat continuously. If you leave out D13, the sequence will run only once. I've used a 12-volt supply in the diagram, but the circuit will work at anything from 5 to 15-volts.

CMOS 4060 Burglar Alarm - This is a single zone alarm, with automatic exit, entry and siren cut-off timers. It will accommodate all the usual types of normally-closed input devices, such as magnetic reed contacts, foil tape, PIRs etc. But it's easy to add a normally-open trigger.

CMOS Based Motorcycle Alarm - This circuit features an intermittent siren output and automatic reset. It can be operated manually using a key-switch or a hidden switch; but it can also be wired to set itself automatically when you turn-off the ignition.

CMOS Based Vehicle Anti-Theft Alarm - This car alarm circuit features Exit and Entry delays, an instant alarm zone, an intermittent siren output and automatic Reset. By adding external relays you can immobilize the vehicle and flash the lights.

CMOS Single Zone Alarm

Cold Activated Switch - The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees celsius. A suitable bead type thermistor is found in the Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range of temperatures. A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents relay chatter when temperature is near the switching threshold of this circuit.

Cold Activated Switch (Andy Collinson) - A simple thermistor triggered cold switch with adjustable threshold.

Combination Digital Lock - A multiple input combination lock using CMOS counter IC's. Flexibility and code change is allowed by changing output connections.

Combination Lock PIC16F84

Combination Lock-Digital

Controller Provides Multiple Alarm Driver Formats - EDN-Design Ideas: DC/DC converters for use inside the telephone handset require operation from the high-source-impedance phone line. Additionally, the CCITT specifications call for maximum on-hook power consumption of 25 mA. The dc/dc converter in Figure 1 is 70%-efficient at an input power of 25 mA, providing 5V at 3.4 mA. Controlled, low-peak switch current ensures that the ­48V input line experiences no excessive voltage drops during switching.

Countersurveillance Monitor - This simple-to-build, yet effective circuit can help you search out and remove electronic eavesdropping devices.

Cut Phone Line Detector (Aaron Cake) - A while ago I got an email asking for the schematic of a circuit to detect cut phone lines. It didn't take me long to find this circuit in Electronics Now. When the circuit detects that a phone line has been cut, it activates a MOSFET which can be used to drive a relay, motor, etc. It can also be connected to a security system.

Cut Phone Line Detector (Tony van Roon) - This circuit speaks for itself.  When the phone line is okay, Q1 acts as a short with a very high ohms value via R1, R2, and R3 for a total of about 66 MegOhm so very high. When the phone line is cut (Tony van Roon's open), it activates the MosFet (Tony van Roon's Q3) via transistor Q2 to drive the load.  Substitutes are fine, none of the components are critical..