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Alarm Circuits
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Last Updated: June 02, 2021 01:44 PM


Circuits Designed by Dave Johnson, P.E.

Indicates when it is Time to Dust -  I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough.  I have not yet built and tested this circuit completely but in concept it should work. . . Circuit by Dave Johnson P.E.-June, 2000

It's Time to Dust Indicator -  I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough.  I have not yet built and tested this circuit completely but in concept it should work . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000


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

Keypad Controlled Switch No.2 -  Universal Keypad-Controlled Switches:  This is a simplified version of the 4-Digit Keypad Controlled Switch.  I have modified the design to reduce the complexity of the circuitand the number of components required.  As a resultthe code is somewhat less secure.  However, there should be lots of situations where it will still be adequate. __ Designed by Ron J.

Kitchen Circuits -  Do your kids leave the door to the refrigerator open? Or does your celery reach out to keep the door from closing properly when you turn your back? Here's a simple circuit that beeps whenever the door is open for an extended period of time (which will also put a little pressure on the fridge loiterers in the house) __ Contact: Charles Wenzel of Wenzel Associates, Inc.

Knock Alarm -  This circuit (Fig.1), used in conjunction with a thin piezoelectric plate,senses the vibration generated on knocking a surface (such as a door or a table) to activate the alarm.  It __ Electronics Projects for You

Knock Alarm -  This circuit uses a thin piezoelectric sensor to sense the vibrations generated by knocking on a surface; eg, a door or table.  Basically, it amplifies and processes the signal from the sensor and sounds an alarm for a preset period.  In operation, the piezoelectric sensor converts mechanical vibration into an electrical signal.  This sensor can be attached to a door, a cash box, cupboard, etc using adhesive. __ Raj. K. Gorkhali  c Silicon Chip Electronics

Knock Alarm with timer -  This is a circuit for a knock alarm that uses a piezoelectric plate to detects vibration due to knocking and activates an alarm for about ten seconds __   Electronics Projects for You

Knocking rings Doorbell -  In some apartment buildings and homes, not equipped with a door bell, it may be hard to hear someone knocking on the front door.  The circuit below provides a means to activate a loud beeping sound inside, whenever someone knocks on the door . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2009

Laser Alarm triggered by broken beam -  This circuit is a laser alarm system like the one we see in various movies.  It uses a laser pointer beam to secure your valuables and property.  Essentially, when the beam gets interrupted by a person, animal or object, the resistance of a photodiode will increase and an alarm will be activated.  The laser and the receiver can be fitted in same box, sharing a common power supply. __ Learning Electronics

Laser beam Alarm systE - M -  When I did the First Design of this project I used a 3 Terminal "2N5777".  Later I found some manufacturers changed this part to a Two Terminal Device, But the 2 Pin Version is the same, just without that adjustment.  Additionally some persons have had troubles in getting the Relay to work.  "The Circuit IS Correct".  In Any Case, the New Design below uses an SCR to Acomplish the same thing
as the Relay.  And its Cheaper. __ Designed by G.L. Chemelec

Laser Door Alarm -  This simple fake car alarm or alarm decoy circuit device will simulate the presence of a burglar alarm in automobiles or homes.  It has a few components, uses LM3909 and is easy to build.  Mount R1 where daylight can fall on it.  You may change the capacitor value for different flashing rates.  During darkness, LED1 flashes, __ Designed by Popescu Marian

Laser-Based Intruder Alarm -  You can use this laser-based intruder alarm conveniently at the entrance of a hall or any other similar location to protect the same from unauthorised access as and when desired.  Its __ Electronics Projects for You

LDR & LED Smoke Detector -  Smoke detectors are generally used in advanced alarm systems.  Most of this professional devices use gas-detectors, ionization rooms or radioactive elements as sensors.  In this circuit we are not using any of this complicated components.  Instead of these, we are using two LDRs and one LED.  LM1801 special-purpose IC , which is designed especially for smoke detectors, provides us building this circuit by using the minimum number of components.  It includes one internal zener supply, two reference voltage outputs, one voltage comperator, fixing diodes and one 500mA output transistor. 

LED 7 segment Count Down Alarm -  Here I have added two fets for the time-out switch, it is like an electronic relay, just much more reliable it can be used to sound a big horn, or what ever you like. __ Designed by Thomas Scherrer OZ2CPU

LED Drives Flash-and-Beep -  06/05/97 EDN-Design Ideas Sometimes, you need an alarm that is both audible and visible in an application that's stingy with space, power, and budget.  The circuit in Figure 1a is compact, efficient, and inexpensive.  The F336HD flashing LED (part no.276-036 at Radio Shack) is a useful device.  It operates directly from 5V and produces a consistent pulse of light at approximately 1 Hz without a time-constant capacitor. Design by Dennis Eichenberg, Parma Heights, OH

Light Alarm with LDR -  This musical light alarm circuit is very simple, uses only 7 components, a LDR and a 3.6 V battery or 3 x 1.2 volts rechargeable batteries.  The well-known UM66 is used as the sound generator and will give a pleasent wake up alarm.  As you probably know the LDR is a light dependent resistor.  Normally the resistance of an LDR is __ Designed by Popescu Marian

Light Fence Security Beeper -  General purpose hobby circuit of a simple light fence security beeper is presented here.  This circuit can be used as a door alarm, gate alarm, pathway alarm, etc.  Any 12 Volt dc power supply can power the whole circuit.  Working of this circuit is straight forward.  In standby mode photo transistor T1 receives light from the __ Designed by T.K. Hareendran

Light/Dark Activated Motion Detector -  This circuit can sound an alarm when the shadow of a hand or arm moves over  two small photo diodes. . . Circuit by David Johnson P.E.-August, 2005

Light-Sensitive Alarm -  The circuit detects a sudden shadow falling on a light-sensor and sounds a bleeper for a short time (adjustable).  Normal lighting can be used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor.  Breaking this beam will then cause the bleeper to sound __ Designed by John Hewes

Light-Sensitive Fire Alarm -  This light-sensitive fire alarm circuit is extremely sensitive, low-cost and easy to assemble even by a beginner.  It uses infrared photo transistor as fire sensor.  Circuit and working Circuit diagram of the light-sensitive __ Electronics Projects for You

Linear Thermometer -  T1 has a built in pair of thermistors with linearizing resistors custom scaled for a specific temperature range.  , 0C to 100C in this example.  The flying capacitor LTC1043 eliminates any dc loading on the precision network to maintain linearity over the entire temperature range.  Amplifier gain is scaled to provide an output __ Linear Technology/Analog Devices App Note, Jun 21st 2011

Linearized Platinum Resistance Thermometer with 0.025C Accuracy Over 0 to 100C -  Platinum RTD's are typically driven with a constant current so that their resistance value directly translates to a temperature dependant voltage.  Here the LT1001 and the LM129 create a constant current through the input 10K resistor which all flows through the sense resistor.  A second amplifier inverts the signal to provide __ Linear Technology/Analog Devices App Note, Jun 21st 2011

Liquid Level Alarm -  Here is a simple circuit for liquid level alarm.  It is built around two BC547 transistors (T1 and T2) and two timer 555 ICs (IC1 and IC2).  Both IC1 and IC2 __ Electronics Projects for You

Liquid-Level Alarm -  In water-level controllers for tanks, a DC current is passed through the metallic probes fitted in the water tank to sense the water level.  This causes electrolysis and corrosion of probes, __ Electronics Projects for You

Liquid-Level Indicator -  The LTC2990 is used to monitor system temperatures, voltages and currents.  Through the I2C serial interface, the device can be configured to measure many combinations of internal temperature, remote temperature, remote voltage, remote current and internal VCC.  The internal 10ppm/C reference minimizes the number of supporting __ Linear Technology/Analog Devices App Note, Mar 16th 2011

Little Door Guard -  If some intruder tries to open the door of your house, this circuit sounds an alarm to alert you against the attempted intrusion.  The circuit (Fig.1) uses readily available, low-cost components. __ Electronics Projects for You

Locker Alarm -  The locker alarm circuit or cashbox watcher can be used to protect a cashbox/locker from unauthorised access.  This tried and tested design forms a fool-proof, remotely operated alarm/electromagnetic relay driver that receives its control signal from a standard reed switch.  The circuit works off a 12V dc power supply.  After __ Designed by T.K. Hareendran

Locker-security alarm -  Protect your valuables from burglary using this simple circuit.  It generates warning beeps when someone attempts to open the locked safe.  The warning alarm sounds at an interval of a few __ Electronics Projects for You

Long-Range Burglar Alarm using Laser Torch -  Laser torch-based burglar alarms normally work in darkness only.  But this long-range photoelectric alarm can work reliably in daytime also to warn you against intruders in your big compounds, etc.  The __ Electronics Projects for You

Loop Sensor -  Many security systems use a closed loop of wires and switches arranged so that whenever a door or window is opened, the loop will be broken and the alarm will sound.  An obvious problem is that someone can tamper with the system, short out the loop, and later on, come back and burglarize the premises

Low Cost Burglar Alarm for Boats -  This low-cost burglar alarm employs a 12V strobe light and a truck reversing horn as the visible and audible alarm outputs while the alarm itself is a 12V horn relay and some pressure mat switches.  This simple approach has the benefit that the alarm continues to operate even if the entry point is immediately closed and it draws no current while in the armed condition __ Learning Electronics

Low Cost Door Sensor -  This compact and affordable door sensor mechanism uses a Hall-effect sensor fixed at the corner of the door frame and a standard bar magnet fixed on the door panel close to the Hall-effect sensor.  When the door is opened, the magnet moves away from the sensor to generate a wake-up signal to the rest of the electronics.  This __ Designed by T.K. Hareendran

Low Power RFID 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

Low Voltage 120vAC Power Line Alarm -  This circuit will turn on a beeper when the line voltage drops below 100 volt AC.. . . Circuit by David A. Johnson P.E.-July, 2006

Low Voltage Alarm -  NE555 Low Voltage Battery Disconnect Circuit __ Designed by Aaron Cake

LPG leakage Alarm using SEN-1327 gas sensor Module (detects 4 other dangerous gases) -  Alarms and security related electronic circuit diagrams / circuit schematics

LTC Design Note: Electrolytic capacitor-based data backup power solution for a 12V system with 5V to 36V input -  EDN-Design Ideas 11/30/2016    This module provIdeas hold-up power in e.   g., embedded systems, and can also be combined with a buck-boost module for a robust power system suitable Design 

LTC2057 Differential Thermocouple Amplifier -  The LTC2057 is a high voltage, low noise, zero-drift operational amplifier that offers precision DC performance over a wide supply range of 4.75V to 36V or 4.75V to 60V for the LTC2057HV.  Offset voltage and 1/f noise are suppressed, allowing this amplifier to achieve a maximum offset voltage of 4μV and a DC to 10Hz input __ Linear Technology/Analog Devices App Note, Jun 19th 2013

LTC2990-Motor Protection/Regulation -  The LTC2990 is used to monitor system temperatures, voltages and currents. Through the I2C serial interface, the device can be configured to measure many combinations of internal temperature, remote temperature, remote voltage, remote current and internal VCC. __ Linear Technology/Analog Devices App Note, Oct 26th 2010

Luggage Security System based on UM3561 melody generator IC -  While traveling by a train or bus, we generally lock our luggage using a chain-and-lock arrangement.  But, still we are under tension, apprehending that somebody may cut the chain and steal our luggage.  Here is a simple circuit to alarm you when somebody tries to cut the chain.  Transistor T1 enables supply to the sound generator chip when the base current starts flowing through it. __ Learning Electronics

Luggage, Bike Security Alarm -  This luggage or bike alarm can be used while travelling by trains or bus and we generally lock our luggage using a chain-and-lock arrangement.  But, still we are under tension, apprehending that somebody may cut the chain and steal our luggage.  Here is a simple circuit to alarm you when somebody tries to cut the chain.  T1 __ Designed by Popescu Marian


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

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