Alarms of all kind, page 1

Alarm Circuits Page 1
Alarms: #-C D-G H-O P-R S T-U V-Z
Last Updated on: Monday, October 03, 2011 06:07 PM

Circuits Designed by Dave Johnson, P.E.

Bad Light Bulb Indicator
This request came from a Discover Circuits visitor. He wanted to turn on a LED indicator light whenever a circuit detected a burned out tail/brake light on a car or truck. The circuit shown below uses a transistor switch, to detect current flowing through a low value resistor. If current is flowing to a light bulb, the circuit shunts current around a LED light, keeping the LED turned off. If there is no current detected, the LED turns on. The circuit should operate from 6v to 24v....

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,...

Cut Phone Line Alarm
Burglars, rapists and other criminals will often cut a phone line before they break into a home. The cut phone line will prevent many alarm systems from alerting the police. The battery powered circuit below will sound an alarm when the unit no longer detects the voltage present on an active phone line....

Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:

12 Volt Alarm Power Supply - A 12 Volt power supplied designed for Ron's Modular Burglar Alarm. However, being a popular supply voltage this circuit will have many other uses as well….(circuit / schematic design added 6/06)

120 VAC Low Voltage Alarm - This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac. [Designed by David A. Johnson]

2 Zone Intruder Alarm - This is a two-zone alarm - with automatic exit, entry and siren cut-off timers. It can be triggered by the usual types of normally-closed input devices - such as magnetic reed contacts - foil tape - PIRs etc. I've used a 12-volt supply in the diagram - but the circuit will work at anything from 9 to 15-volts. All you need do is select a siren, buzzer and relay to suit the voltage you want to use.... [Circuit designed by Ron J.]

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....(electronic circuit added 05/06/07)

3 Zone Duress Alarm - This is a 3 zone alarm for use in high risk areas or possible duress situations. Typical examples being banking or betting booths. The alarm consists of 3 normally open push button switches which can be hidden or in plain view. Once pressed the alarm will latch and a LED will light showing which zone has pressed the alarm... [Circuit by Andy Collison]

4 Digit Keypad - (diagram added 6/03)

4 in 1 Burglar Alarm - In this circuit, the alarm will be switched on under the following four different conditions: 1. When light falls on LDR1 (at the entry to the premises). 2. When light falling on LDR2 is obstructed. 3. When door switches are opened or a wire is broken. 4. When a handle is touched. The light dependent resistor LDR1 should be placed in darkness near the door lock or handle etc. If an intruder flashes his torch.... [Radioland.nt.au]

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. (Circuit design added 08/08/08)

5 Zone Alarm Circuit - (electronic schematic added 8/02)

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. (Circuit design added 08/08/08)

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. (Circuit design added 08/08/08)

6 Zone Alarm with 7 Segment Display - This alarm system has 6 independent zones, 1 timed entry/exit zone, a 7 segment LED display and a test or walkthrough facility. Suitable for a small office or home environment, it can also be adapted to use a combination lock or keypad to set and reset the alarm.... [Andy Collison, designer]

A 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…. [Designer Ron J.]

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....(electronic schematic added 05/09) [Designer Ron J.]

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. (Circuit design added 08/08/08) [Circuit Designed by Ron J.]

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. (Circuit design added 08/08/08) [Circuit Designed by Ron J.]

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.... [Ron J.'s design]

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.... [Design by Ron J.]

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.… [Design by Ron J.]

A CMOS Based Single Zone 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, see.... [Design by Ron J.]

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..... [Circuit designer is Ron J.]

A One-Time-Only Burglar Alarm - This alarm is designed to sound its Siren only once. That is - when the alarm is activated - the Siren will sound for a preset length of time. Then it will switch off and remain off. The alarm will not re-activate. [Circuit designer is Ron J.]

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.... [Circuit by Ron J.]

A simple electronic buzzer - This very simple circuit just uses a couple of resistors, a capacitor and the easily available 555 timer IC. The 555 is setup as an astable multivibrator operating at a frequency of about 1kHz that produces a shrill noise when switched on. The frequency can be changed by varying the 10K resistor.... [Radioland.nt.au]

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.... [Circuit by Ron J.]

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…. [Design by Ron J.]

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"... [Design by Ron J.]

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... [Design by Ron J.]

A Transistor Based Burglar Alarm - This is a simple transistor-based burglar alarm circuit. Its features include automatic Exit and Entry delays - together with a timed Bell cut-off and Reset. It's designed to be used with the usual types of normally-closed input devices such as - magnetic-reed contacts - micro switches - foil tape - and PIRs....(circuit added 01/2010) [Circuit Designed by Ron J.]

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"... [Circuit Designed by Ron J.]

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.... [Designed by David A. Johnson]

Alarm Circuit - Surround your Entire Yard with this Laser Alarm ….(Circuit design added 12/08)

Alarm control keypad - The IC is a quad 2 input "AND" gate, a CMOS 4081. These gates only produce a HIGH output, when BOTH the inputs are HIGH. When the key wired to 'E' is pressed, current through R1 and D1 switchs Q5 on.The relay energises; and Q5 is 'latched on' by R8. Thus, the Alarm is set by pressing a single key,say one of the two non-numeric symbols.... [Ron J.'s circuit

Alarm monitors rotational speed of DC-mmotor - 2-Oct-08 Issue of EDN This Design Idea monitors the rotating speed of a dc fan motor, sounding an alarm and powering down if the motor stalls.... [Design Idea by Peter Demchenko, Vilnius, Lithuania]

Alarm Power Supply - ….(Circuit design added 03/08)

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....(Circuit design added 08/08/08)

Alarm PSU - A 12 Volt power suppiled designed for Ron's Modular Burglar Alarm. However, being a popular supply voltage this circuit will have many other uses as well….(circuit / schematic design added 6/06)

Alarm System - (design added 8/03)

Alarm tells you to close the refrigerator door - 25-Jun-09 Issue of EDN A simple gadget that you place inside the refrigerator alerts you by beeping if the door stays open for 20 seconds or longer.... [Design Idea by Boris Khaykin, TRW Automotive, Livonia, MI]

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....(electronic design added 08/08/08)

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.... [Circuit by Ron J.]

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... [Circuit designed by Ron J.]

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.... [Ron J.'s circuit]

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.... [Ron J.'s circuit]

An SCR Based Burglar Alarm - (electronic design added 6/07)

Anit Theft Motorcycle Alarm - (schematic added 9/02)

Audio Security Monitor - A remote listening circuit. The area to be monitored is connected via a cable and allows remote audio listening….. [designer Andy Collison]

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 - Circuit designed by Ron J (electronic design added 6/07)

Automatic Night Lamp with Morning Alarm - (electronic Circuit / Schematic added 10/04)

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…. [G. Forrest Cook]

Battery Powered One Time Alarm Only (Ron J) - This is a single zone alarm - with independently adjustable Exit, Entry and Siren Cut-Off timers. It will accommodate the usual types of normally-closed input devices - such as magnetic-reed contacts, foil tape and PIRs…. [Ron J.'s design]

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 - [Radioland.nt.au]

Beeper bug transmitter for fox hunt & vehicle tracking - (diagram added 05/07)

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.... (added 10/05) [Radioland.nt.au]

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.... [Radioland.nt.au]

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....(electronic design added 08/08/08)

Broken Charger-Connection Alert - Detects if a device is not properly connected to its supply. Suitable for battery chargers, portable appliance supplies etc. (added 3/050

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....(electronic design added 7/08)

Car Alarm - (application / block diagram added 6/07)

Car Alarm and Immobilizer - This circuit features automatic exit and entry delays - an optional instant alarm zone - an optional intermittent siren output - and an automatic reset. By adding external relays you can immobilize the vehicle and flash the lights…. [Design by Ron J.]

Car Alarm Sound Booster - For car alarms, emphasis should be put on hearing the audible alert and identifying it as belonging to your ‘wheels’. Unfortunately, modern car alarm systems seem to have the more or less the same alarm sound — especially if they are from the same brand. Also….must register on this site... [Published in Elecktor July/August 2010]

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. Receiver is.... [Radioland.nt.au]

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. (Tony van Roon's electronic design added 6/06) [Tony van Roon's circuit]

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....(electronic design added 6/07)

Car indicator alarm - Schematic only, no circuit description

Car-Police Siren - This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail. (Circuit design added 08/08)

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,.... [Designed by David A. Johnson]

Cheap Bicycle Alarm - The author wanted a very cheap and simple alarm for some of his possessions, such as his electrically assisted bicycle. This alarm is based on a cheap window alarm, which has a time-switch added to it with a 1-minute time-out. The output pulse of the 555 replaces the reed switch in the window alarm.….must register on this site... [Published in Elecktor July/August 2010]

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.... [Circuit design by Ron J.]

CMOS 4060 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…. [Circuit design by Ron J.]

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....(Circuit design added 06/07/08)

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....[Ron J.'s circuit]

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….(Circuit design added 06/07/08)

CMOS Single Zone Alarm - (diagram added 6/03)

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.... [Andy Collison's circuit]

Combination Digital Lock - A multiple input combination loack using CMOS counter IC's. Flexibility and code change is allowed by changing output connections…. [Circuit designed by Andy Collison]

Combination Lock PIC16F84 - (diagram added 6/03)

Controller provides multiple alarm-driver formats - 15-Jan-98 Issue of EDN Using a piezoelectric element for alarm applications offers low cost, low power, and flexibility. By coupling this element with a 12C508 programmable controller (Microchip Technology, Chandler, AZ], you can implement an eight-pin alarm generator. This approach provides multiple driver formats with a minimum of additional cost and footprin.... [Design Idea by William Grill, Riverhead Systems, Littleton, CO]

Countersurveillance Monitor - This simple-to-build, yet effective circuit can help you search out and remove electronic eavesdropping devices…. [Tony van Roon's circuit]

Cut Phone Line Detector - 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... [Arron Cake]

Cut Phone Line Detector - 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.... [Tony van Roon's circuit]

Alarms: #-C D-G H-O P-R S T-U V-Z