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Alarm Circuits
Alarms:  #'s       A - C       D - G       H - L       M - Q       R - T      
U - Z

 

Last Updated: November 30, 2017 07:38 AM

 

Circuits Designed by Dave Johnson, P.E.

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 . . . Hobby Circuit designed by David A. Johnson P.E.-March 29, 2009

AC Power Loss Alarm   -  i’ve been having a problem with my instant hot water system lately.  This system circulates electrically heated water throughout my house using a small pump.  The pump is wired into a ground fault interrupter (GFI) module.  Every now and then, the GFI trips, leaving me without hot water . . . Circuit by Dave Johnson P.E.-March 29, 2009

Alarm Beeps when the “Check Engine” Light Turns On   -  When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on . . . Circuit by Dave Johnson P.E.-July 09, 2006

Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor   -  This schematic is the alarm discriminator plus the battery monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . Hobby Circuit designed by Dave Johnson P.E.-March 26, 2002

Alarm for a Chain Link Fence   -  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,  . . . Hobby Circuit designed by Dave Johnson P.E.-February 22, 2009

Alarm Sounds At Freezing Temperatures   -  This electronic hobby circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F).  A 9v battery powers the hobby circuit.  The average 9ua current is so low that the battery should last for many years . . . Hobby Circuit designed by David Johnson P.E.-July 11, 2006

Alarm Sounds when Phone Line is Cut   -  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 . . . Hobby Circuit designed by David Johnson P.E.-October 16, 2005

Alarm sounds when Power Cord is Unplugged   -  Many medical devices, such a portable X-ray and ultrasound equipment, carry their own batteries.  However, the frantic pace of some hospital emergency rooms might cause the device to be shoved off into a corner without being plugged back into an AC power source to recharge their batteries.  So, I designed this circuit to emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time . . . Hobby Circuit designed by David A. Johnson P.E.-July 08, 2000

Alarm sounds when Water is Detected   -  This simple circuit sounds a beeper when its electrodes detect water.  It is powered by a single 1.5v N cell.  A small 1.5v button battery will also work . . . Circuit by David Johnson P.E.-June 11, 2000

Alarm-Piezo Device Detects Motion   -  Using a piezo device, this circuit will activate a beeper whenever the circuit is moved.  It could be used as an earthquake alarm . . . Circuit by Dave Johnson P.E.-July 09, 2006

Another Very Loud piezo Alarm Beeper   -  This is yet another beeper circuit that really draws attention.  It sweeps the drive frequency slightly to produce a very annoying sound.  It uses a transformer to increase the drive voltage across the piezo device to more than 200 volts peak to peak . . . Circuit by David A. Johnson P.E.-June 27, 2000

Appliance Unplugged Alarm   -  I received a call from an administrator at a large prison.  He told me that he was having problems with inmates who keep unplugging refrigerators, vending machines and microwave ovens in a commons area.  He wanted a battery-powered device, which could be mounted to the outside of the power cord of an appliance.   . . . Hobby Circuit designed by Dave Johnson P.E.-April 25, 2010

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 . . . Circuit by David Johnson P.E.-December 17, 2006

Basement Doorbell Beeper   -  if you can't hear your doorbell when you are in your basement try this circuit.  This circuit takes advantage of the 24vac power source located near the furnace.  Using a simple current transformer technique, the circuit sounds a beeper whenever the . . . Circuit by David Johnson P.E.-June 04, 2000

Basement Remote Doorbell   -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac . . . Hobby Circuit designed by David A. Johnson P.E.-July 11, 2006

Battery-Powered Freezer Alarm   -  A typical freezer can contain hundreds of dollars with of food.  If the device becomes unplugged or if the door is left open too long, the food inside can quickly become spoiled.  The hobby circuit below is a battery powered alarm system, which is attached . . . Hobby Circuit designed by David Johnson P.E.-July 20, 2006

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,  . . . Circuit by Dave Johnson P.E.-February 22, 2009

Circuit Monitors Power Loss   -  For some medical equipment it is important for an operator to know when power is lost to the machine.  The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts . . . Hobby Circuit designed by David A. Johnson P.E.-March 26, 1999

Clip-on AC Current Indicator Circuit   -  Several times over the last few years I have received calls from people looking for a simple circuit that would tell them when a sump pump or water well pump motor was running.  These people told me that the electrical circuits that turn on and off . . . Hobby Circuit designed by David A. Johnson P.E.-November 18, 2004

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 . . . Circuit by David A. Johnson P.E.-October 16, 2005

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

A 4-Digit Alarm Control KeypA - D -  Alarm Control Keypads:  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. __ Designed by Ron J.

A 555 -Timer Based Motorcycle Alarm -  Motorcycle Alarm Circuits:  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. Now With Free SiMetrix Circuit Simulation. __ Designed by Ron J.

A 5-Digit Alarm Control KeypA - D -  Alarm Control Keypads:  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. __ Designed by Ron J.

A Battery-Powered One-Time-Only Burglar Alarm -  Battery-Powered Alarm Circuits:  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. __ Designed by Ron J.

A Cheap 'n' Easy Motorbike Alarm -  It's simple to build, can be fitted to almost any bike and is very easy to operate.  It's armed and disarmed with the ignition key. __ SiliconChip

A CMOS 4060 Burglar Alarm -  Single-Zone Burglar Alarm Circuits:  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.  When the alarm is activated the siren will sound for a fixed length of time.  Then it will switch offand remain off.  The alarm will not reactivate.  The Circuit Description offers a good introduction to both the CMOS 4060and the SCR. __ Designed by Ron J.

A CMOS Based Motorcycle Alarm -  Motorcycle Alarm Circuits:  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.  It's easily adapted for a 6-volt systemso it will protect your "Classic Bike". - Now With Free SiMetrix Circuit Simulation. __ Designed by Ron J.

A CMOS Based Single Zone Alarm -  Single-Zone Burglar Alarm Circuits:  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). __ Designed by Ron J.

A CMOS Based Vehicle Anti-Theft Alarm -  Automobile Alarm Circuits:  This car alarm circuit features Exit and Entry delaysan instant alarm zonean optional intermittent siren output and an automatic Reset.  By adding external relays you can immobilize the vehicle and flash the lights. __ Designed by Ron J.

A Current Limiting Bench Power Supply -  Power Supply Circuits:  This is a 1-amp variable-voltage PSU.  It adjusts from about 3v to 24v: and has the added feature that you can limit the maximum output current.  This is invaluable when   (for example) you power-up a project for the first time or soak-test a piece of equipment. __ Designed by Ron J.

A Dozen Small CMOS Alarm Circuits -  Battery-Powered Alarm Circuits:  This is a selection of small self-contained alarm circuits.  They have a very low standby current and are ideal for battery operation.  Some are triggered by normally-open and normally-closed switches.  And some react to changes in light or temperature.  Included with each is a free SiMetrix circuit simulation. __ Designed by Ron J.

A Fridge That's Alert for Midnight Snacks -  22-Oct-07 -Design News:   When you've popped in your fridge alarm, getting peckish in the middle of the night and forgetting to close the door along with escalating electricity bills will be a thing of the past.  Italian inventor Flavio Dellepiane has designed a 3V battery-powered fridge alarm that beeps if you leave the __   Gadget Freak-Case #110

A handy 2-Input Alarm using the PIC Lick-1 Module -  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 __ Designed by Collin Mitchell

A Low Power RF ID Transponder -  This is the Grand Prize winner in the design category of the 1993 RF Design Awards Contest.  This entry exhibited both innovative use of RF technology and an elegant implementation of that technology __ Designed by Raymond T. Page, Reprint courtesy of RF Design Magazine

A One-Time-Only Burglar Alarm -  Multi-Zone Burglar Alarm Circuits:  When this alarm is activated its Siren will sound oncefor 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 a range of Expansion Modules.  These will allow you to add any number of Instant Zones, Tamper Zones and Personal Attack Zones to your system. __ Designed by Ron J.

A PC-Controlled Burglar Alarm SystE - M -  Fancy a full-featured alarm control panel with dialler capabilities? This one is PC-programmed and controlled and can handle up to eight zones.  The PC only needs to be powered up for arming and disarming, or you can use an optional keypad. __ SiliconChip

A Robust Glass-Breakage Detector using the MSP430 -  Glass Break Detector can be designed with 3 types of implementations in mind: Standalone, wireless connectivity and Fixed/Wired Connectivity.  The first two implementations are battery operated.  The functional block diagram of the glass break detector is divided into 3 sub-blocks: the sensing block, the processor and the communication interface blocks.  When implementing a basic standalone type of glass break detector, the processor and the communication interface are not needed __ Designed by Texas Instruments App Notes, 5-Feb-08

A 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.  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 __ Designed by Ron J

A Shed / Garage Alarm -  Battery-Powered Alarm Circuits:  This is a basic single-zone burglar alarm circuit.  Its features include automatic Exit and Entry delays and an optional Siren Cut-Off timer.  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. __ Designed by Ron J.

A Simple Circuit 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 __ Designed by Radioland.nt.au

A Simple Circuit Fire-Door Alarm -  Battery-Powered Alarm Circuits:  This circuit will let you know when something that should be kept closed has in fact been left open.  If you attach it to say a Fire-Door it will allow you to pass back and forth freely.  It's only when the door is open for more than about 30-secondsthat the buzzer will sound the alarm.  The Page includes a free SiMetrix circuit simulation. __ Designed by Ron J.

A Simple Circuit Glass Breakage Detector using the MSP430 -  Glass Break Detector can be designed with 3 types of implementations in mind: Standalone, wireless connectivity and Fixed/Wired Connectivity.  The first two implementations are battery operated.  The functional block diagram of the glass break detector is divided into 3 sub-blocks: the sensing block, the processor and the communication interface blocks.  When implementing a basic standalone type of glass break detector, the processor and the communication interface are not needed __ Designed by Texas Instruments App Notes, 27-Apr-07

A Simple Circuit Shop-Door Alarm -  Battery-Powered Alarm Circuits:  Attach this circuit to the door of an unattended shop or reception areaand the sound of the buzzer will tell you when you have a customer.  Add a sirenand it becomes a simple intruder alarm.  The Page includes a free SiMetrix circuit simulation. __ Designed by Ron J.

A Simple Circuit Transistor Based Motorcycle Alarm -  Motorcycle Alarm Circuits:  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 machineevery time you turn-off the ignition.  It's easily adapted for a 6-volt system so it will protect your "Classic Bike". - Now With Free SiMetrix Circuit Simulation. __ Designed by Ron J.

A Simpler CMOS Single Zone Alarm -  Single-Zone Burglar Alarm Circuits:  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. __ Designed by Ron J.

A Solar-Powered Intruder Alarm for Sheds & Boats -  Most blokes have got a shed or wish they had! While many people have alarms for their home and cars, a lot of valuable stuff is unprotected in garages and sheds.  It needs protecting and now you can do it with this simple alarm based on a PIR sensor.  It's solar-powered so no mains supply is needed.  And let's not forget boats on moorings they need protecting too. __ SiliconChip

A Speed Alarm for Cars; Pt 1 -  Is your license looking a bit a bit dodgey? This compact speed alarm will help you stick to the speed limits. __ SiliconChip

A Temperature Sensor Control -  A basic circuit using Diodes to detect temperature differences __ Designed by G.L. Chemelec

A Thermostat Control for Electric Heaters -  One Advantage of these two units is the Sensor can be placed Away from the heater, therefore sensing the Actual Room temperature, not the direct heat from the heater.  These can also be used in a confined space with a Heat Lamp, to create a Drying Oven __ Designed by G.L. Chemelec

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. __ Designed by Ron J.

A Transistor Based Motorcycle Alarm -  Motorcycle Alarm Circuits:  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 systems it will protect your "Classic Bike". - Now With Free SiMetrix Circuit Simulation. __ Designed by Ron J.

A Transistor Based Single Zone Alarm -  Single-Zone Burglar Alarm Circuits:  The 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 the usual input devices   (Pressure Mats, Magnetic Reed contacts, Foil Tape, PIRs and Inertia Sensors). __ Designed by Ron J.

A-4 Small Transistor Alarm Circuits -  Battery-Powered Alarm Circuits:  This is a selection of simple self-contained transistor based alarm circuits complete with photographs.  They are designed around the Complementary Latch.  All may be triggered by both normally-open and normally-closed switches.  They have a very low standby current and are ideal for battery operation. __ 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 . . . Hobby Circuit designed by David A. Johnson P.E.-March, 2009

AC power line box heat Monitor -  This simple circuit monitors the mains distribution box constantly and sounds an alarm when it senses a high temperature due to overheating, helping to prevent disasters caused by any sparking in the mains box due to short circuits.  It also automatically switches on a bright white LED when the power fails.  The LED gives ample light to check the mains box wiring or fuses in darkness.  The circuit beeps once when power fails and again when power resumes. __ Designed by D. Mohan Kumar

AC power line failure/resumption alarm -  This mains indicator sounds an alarm whenever AC mains fails or resumes.  It is very useful in industrial installations, cinema halls, hospitals, etc. __ Designed by V. David

AC power line supply failure backup light -  In the event of a sudden blackout at night, this circuit switches on automatically to provide sufficient light for around 30 seconds   (extendible), which is enough to switch on an emergency lamp or light up a candle. __ Designed by Arthur Louis

AC Power Loss Alarm -  I’ve been having a problem with my instant hot water system lately.  This system circulates electrically heated water throughout my house using a small pump.  The pump is wired into a ground fault interrupter   (GFI) module.  Every now and then, the GFI trips, leaving me without hot water. . . Circuit by Dave Johnson P.E.-March, 2009

Acoustic Thermometry Measures Temperature to 0.1 Degree -  Start Clock Launches Acoustic Pulse into Measurement Path, Sets Width Decode Flip-Flop High.  Acoustic Paths Return Pulse, Amplified by Receiver, Trips Trigger, Resetting Flip-Flop.  Resultant Q Width Output, Representing Path’s Temperature Dependent Transit Time, is Converted to Temperature Reading by Microprocessor. __ Linear Technology/Analog Devices App Note, Apr 29th 2011

Alarm Beeps when the “Check Engine” Light Turns On -  When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on. . . Circuit by Dave Johnson P.E.-July, 2006

Alarm control keypA - D -  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 switches Q5 on.  The relay energizes; 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 __ Designed by Ron J.

Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor -  This schematic is the alarm discriminator plus the battery monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . Hobby Circuit designed by Dave Johnson P.E.-March, 2002

Alarm for a Chain Link Fence -  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, . . . Hobby Circuit designed by Dave Johnson P.E.-October, 2005

Alarm Monitors Rotational Speed Of DC Motor -  2-Oct-08 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 __ Circuit Design by Peter Demchenko, Vilnius, Lithuania

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Alarms:  #'s       A - C       D - G       H - L       M -Q       R - T       U - Z


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