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

 

Last Updated: October 24, 2017 02:54 PM

 

Circuits Designed by Dave Johnson, P.E.

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

Shadow Activated Motion Detector   -  The hobby circuit below works quite well in typical indoor room lighting.  I would not recommend its use in direct sunlight.  Two small PIN photodiodes positioned about one inch apart form a shadow detector.  With no shadow cast on the devices, both device . . . Hobby Circuit designed by David A. Johnson P.E.-August 21, 2005

Simple Circuit Wire Security Loop Alarm   -  A wire loop is used to protect valuable objects in this alarm circuit.  The circuit is powered by a 9v battery.   . . . Circuit by David A. Johnson P.E.-July 09, 2006

Snow Depth Alarm   -  The design of this system alerts a snowplow driver when the snow depth has exceeded four inches.  A visible red LED packaged in a 10mm package is pulsed at a rate of about 50Hz with 10uS pulses.  Twenty four inches away a phototransistor detects the light from the LED.  The LED and the phototransistor are housed in a section of ½ inch PVC pipe, forming a waterproof seal.   . . . Hobby Circuit designed by Dave Johnson P.E.-December 20, 2013

Star Trek Doorbell   -  in the Star Trek “Next Generation” TV series, the doorbell outside the private quarters of a crew member makes a particular “beep-boop” sound.  The 3v battery powered circuit below tries to simulate this sound.  The circuit uses one 74HCT74 dual D flip/flop . . . Circuit by Dave Johnson P.E.-October 12, 2001

Still 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 . . . Hobby Circuit designed by Dave Johnson P.E.-June 27, 2000

Sweeping Siren Alarm-3 volts   -  A while back I was challenged by a visitor to the website.  He needed a very loud sweeping siren type audio sound generator powered by 3v.  He tried some of the commercial sirens but they were not very loud when powered by 3v.  He also said that those . . . Hobby Circuit designed by Dave Johnson P.E.-June 24, 2008

Time to Dust   -  I have used the circuit below for many different applications, which require ultra low power drain.  It uses a single CA4013 flip/flop.  One section forms a pulse generator.  The generator produces 100uS pulses at a rate of about 1 per second . . . Hobby Circuit designed by David A. Johnson P.E.-May 07, 2006

Touch Alarm for Door Knob   -  I designed this hobby circuit many years ago and it seems be a very popular construction project.  Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms.  A metal chain attached to a box holding the electronics is place . . . Hobby Circuit designed by David A. Johnson P.E.-May 29, 2000

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

Receiver Battery Low Voltage Alarm -  Here is another equally cool low voltage alarm circuit for your glider receiver battery that I've shamelessly stolen from George Steiner's book "A to Z--Radio Control Electronic Journal" (see below).  I've modified it to use with small battery packs in R/C gliders.  This design has a trigger voltage at about 4.3 volts, and it draws 1mA or less when quiet and about 4mA when buzzing.  This can be constructed from parts fromt Radio Shack, though you may need to order a few through them. __ Contact: Rob Crockett

Red Alert Alarm, #1 -  The circuit above provides a 'pulse-tone', like the one you know from the old Star Trek.    The output stage contains a power-booster with Q1 (2N3055) , providing several watts to the loudspeaker SPKR1.  Clamping diodes are used to limit the speaker's inductive kickback to a safe level.    To prevent the output signal from feeding back into the 555's via the supply line, the supply lines are filtered by a capacitor and an isolating diode (C5 and D2) __ Designed by Tony van Roon  VA3AVR

Red Alert Alarm, #2 -  "Red Alert" 555 Siren; under construction __ Designed by Tony van Roon  VA3AVR

Refrigerator Alarm -  Use this refrigerator alarm circuit to prevent leaving the fridge door open by sending out an alarm signal every time the door is left open for some time.  The sensor of the alarm is an LDR.  After a short delay, it triggers an alarm signal and keep the signal on until the door is shut close.  When the light falls on the LDR, it __ Designed by Popescu Marian

Refrigerator Door Alarm -  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.

Relay Based Motorcycle Alarms -  Motorcycle Alarm Circuits:  These are twoeasy to buildrelay-based alarms.  You can use them to protect your motorcyclebut they have many more applications.  If you use relays with 6-volt coilsthey'll protect your "Classic Bike".  Both alarms are very small.  The completed boards occupy about half a cubic-inch8 cc.  The standby current is zeroso they won't drain your battery. __ Designed by Ron J.

Remote Alarm for Smoke Detector -  This alarm circuit was designed to monitor a mains-powered smoke detector located in a shed (which is used to house dog kennels).  It provides complete isolation from the mains so that low-voltage (12V) cabling could be run to the alarm circuit which is located inside the house.  In operation, the alarm signal (I) from the smoke detector is rectified using bridge rectifier BR1 and then fed to optoiso­lator OPTO1 via resistor R3.  This in turn drives the gate of SCR1 which turns on and activates a piezo siren with inbuilt oscillator. __ Learning Electronics

Remote Controlled Alarm -  A very interesting remote controlled alarm circuit using TSOP1736.  Routing of an electric cable to attach a calling bell switch near the bed of an old age/patient is not a good idea.  Here is an ultra simple battery operated remote controlled alarm to help old age members/patients in bed, especially at home/hospital.  Any __ Designed by T.K. Hareendran

Remote CPU/ASIC/FPGA Temperature Sensor -  The LTC2997 is a high-accuracy analog output temperature sensor.  It converts the temperature of an external sensor or its own temperature to an analog voltage output.  A built-in algorithm eliminates errors due to series resistance between the LTC2997 and the sensor diode. __ Linear Technology/Analog Devices App Note, Jul 7th 2011

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

Remote Emergency Alarm for Unmanned Lifts -  In unmanned lifts or elevators, sudden power failure cannot be detected from the remote operating room, and this can prove dangerous for the lift users.  Here is a simple circuit that __ Electronics Projects for You

Remote-Alert -  Remotely-operates a Beeper and/or a LED via mains supply line __ Contact: IQ Technologies

Reverse warning Device -  Schematic only, no circuit description included __ Designed by Seiichi Inoue

RF Alarm -  This simple circuit is sure to have the police beating a path to your door, however, it has the added advantage of alerting you to their presence even before their footsteps fall on the doormat.  The circuit transmits on Medium Wave (this is the small problem with the police).  IC1a, together with a sensor (try a 20cm x 20cm sheet of tin foil) oscillates __ Designed by Rev. Thomas Scarborough

RFID Transponder-Low Power -  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 Page

RoomGuard: a Low Cost Intruder Alarm -  Simple design interfaces to a battery-powered smoke detector. __ SiliconChip

Save Property & Life with this -  05/13/10 EDN-Design Ideas The circuit in Figure 1 sounds an alarm whenever the perimeter- protection line—a thin copper wire of appropriate length—breaks.  You can get the thin wire from a used relay coil.  Place the wire around a tent, a camping area, or a __ Circuit Design by Vladimir Oleynik, Moscow, Russia

School Zone Speed Alert -  Have you been booked for driving through a school zone at well above the 40km/h limit? It's easy to do and can be a very expensive mistake.  Here's a project to help you stay out of trouble.  It keeps track of the time and flashes a warning LED during the morning or afternoon 40km/h time periods. __ SiliconChip

SCR Based Burglar Alarm -  This is a simple SCR based burglar alarm circuit.  Its features include automatic Exit and Entry delaystogether with a timed Bell cut-off and Reset.  It's designed to be used with the usual types of normally-closed input devices such asmagnetic-reed contactsmicro switchesfoil tapeand PIRs __ Designed by Ron J

Security Alarm -  Thwart any attempt of burglary in your house using this alarm circuit.  When someone opens the door of your room, it sounds an alarm intermittently and flashes light as well.  The __ Electronics Projects for You

Seismic Alarm -  My original "Deer Repellent/ Seismic Sensor" recommended using a speaker with a weight glued to the cone as a vibration sensor (see bottom of page) but it recently occurred to me that the speaker could supply the mass itself.  By gluing __ Contact: Charles Wenzel of Wenzel Associates, Inc.

Sensitive Freezer Alarm with Buzzer -  Here is a simple freezer alarm circuit, but ultra sensitive with a buzzer.  The circuit raises an audio alert when the temperature inside the freezer box goes up above a preset level.  The circuit requires 9V dc supply and can power from a standard 9V alkaline battery.  A negative temperature co-efficient type (NTC type) small __ Designed by T.K. Hareendran

Sensitive Optical Burglar Alarm -  This optical burglar alarm uses two 555 timer ICs.  Both the ICs are wired as astable multivibrators.  The first astable multivibrator built around IC1 produces low frequencies, while the second astable __ Electronics Projects for You

Sensitive Optical Burglar Alarm -  This optical burglar alarm uses two 555 timer ICs.  Both the ICs are wired as astable multivibrators.  The first astable multivibrator built around IC1 produces low frequencies, while the second astable multivibrator built around IC2 produces audio frequencies. 

Shadow Activated Motion Detector -  The hobby circuit below works quite well in typical indoor room lighting.  I would not recommend its use in direct sunlight.  Two small PIN photodiodes positioned about one inch apart form a shadow detector.  With no shadow cast on the devices, both device . . . Hobby Circuit designed by David A. Johnson P.E.-August, 2005

ShadoW Alarm -  This shadow alarm circuit can sense a moving shadow in a confined area.  It can be used to protect things from theft.  When somebody approaches the unit, it will give a loud alarm to abort the attempt of theft.  The circuit uses the light sensing property of the Photo diode.  The circuit uses the light sensing property of the PIN __ Designed by D Mohankumar

ShadoW Sensor Alarm -  Shadow sensors are widely used to detect the movement of a person in a confined area.  Many circuits published earlier have a serious drawback that only one light dependent resistor (LDR) is used for shadow detection.  Needless to say, this is not a perfect solution! Shadow Sensor Circuit Schematic Described here is a simple but __ Designed by T.K. Hareendran

Shed / Garage Alarm -  This is a simple single-zone burglar alarm circuit.  Its features include automatic Exit and Entry delays.  It's designed to be used with the usual types of normally-closed input devices such asmagnetic-reed contactsmicro switchesfoil tapeand PIRs. __ Designed by Ron J

Shed / Garage Alarm -  Battery-Powered Alarm Circuits:  This is a basic single-zone burglar alarm circuit.  Its features include automatic Exit and Entry delaysand 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.

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/alarms.htm Page 16

Simple  Smoke Detector -  This simple smoke detector is highly sensitive but inexpensive.  It uses a Darlington-pair amplifier employing two npn transistors and an infrared photo-interrupter module as the sensor.  The circuit gives audio-visual alarm __ Electronics Projects for You

Simple  Suitcase-Lift Alarm -  This suitcase-lift alarm circuit uses vibration sensor HDX-2NC instead of a commercially available sensor.  A home-made mercury switch can also be used instead.  Circuit and working Circuit diagram of the simple suitcase-lift alarm __ Electronics Projects for You

Simple Alert Timer -  This timer is a useful device for people who often forget to turn off devices in time.  Suppose you want to boil water in your electric kettle or cook something on __ Electronics Projects for You

Simple Circuit 6-Input Alarm -  This simple alarm circuit was designed for use in a combined garage and rumpus room.  It can be assembled on Veroboard and uses just one IC plus a handful of cheap components.  The circuit is based on a straightforward 555 timer circuit (IC1).  This is wired as a monostable and sets the siren period which is adjustable up to about three minutes using potentiometer VR1. __ Learning Electronics

Simple Circuit Alarm with telephone dialler interface -  This is the lowest cost dialing alarm on the market and shows what can be done with an 8-pin microcontroller.  The complete circuit is shown below.  You cannot see all the features of this project by looking at the circuitmost of them are contained in the program __ Designed by Colin Mitchell, Talking Electronics

Simple Circuit Alarms -  To use the circuit shown below (left), the diode should be permanently illuminated by a beam of light.  When the light beam is cut, by an intruder, the conductivity of the diode decreases and the alarm sounds.  The alarm will continue to sound, even when the light returns, until the reset switch is pressed.  The value of resistor R is found by experiment but will probably be between 100k and 560k (start with 180k.)  If you want an alarm which sounds when the diode is exposed to light (for example, in order to protect a bag, drawer, suitcase etc) you will need to use the modifications to the circuit shown in the diagram on the right. __ Designed by © David Hoult

Simple Circuit Blown-Fuse Indicator Sounds an Alarm -  10-Jul-08 EDN-Design Ideas A simple blown-fuse-indicator circuit sounds a buzzer and flashes an LED when a fuse blows __ Circuit Design by Vladimir Oleynik, Moscow, Russia

Simple Circuit Darkness Activated Alarm -  Most darkness activated alarms employ opamps and some logic ICs.  Here, a less expensive approach is shown based on the eternal 555, this time in monostable multivibrator mode.  Components R2 and C1 represent a one-second network.  When the LDR (light dependent resistor) is in the dark, its resistance is high, pulling pin 2 of the 555 to ground. __ Learning Electronics

Simple Circuit Door Alarm -  This is a simple, easy to build, alarm circuit.  For power, I used a small 9-volt battery.  But the circuit itself will work from 5 to 15-volts, just choose a buzzer that's suitable for the voltage

Simple Circuit Fire-Door Alarm -  Battery-Powered Alarm Circuits:  This circuit will let you know when something that should be kept closedhas in fact been left open.  If you attach it to say a Fire-Doorit 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 SiMetrixcircuit simulation. __ Designed by Ron J.

Simple Circuit 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

Simple Circuit laser Alarm that gets triggered when beam from a laser pointer is broken -  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

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.

Simple Circuit Touch Alarm -  Touch the sensor of the alarm with your finger and it starts beeping, goes on for some time and then stops.  Touching it again, and it goes again! This little and flexible circuit consists of a touch sensor and a directly coupled transistor amplifier with a small loudspeaker as the output load.  Two sensor strips of metal are __ Designed by T.K. Hareendran

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 automaticallyand/or immobilize the machineevery time you turn-off the ignition.  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.

Simple Circuit Wire Security Loop Alarm -  A wire loop is used to protect valuable objects in this alarm circuit.  The circuit is powered by a 9v battery.. . . Circuit by David A. Johnson P.E.-December, 2004

Simple Digital Security SystE - M -  You can use this simple and reliable security system as a watch dog by installing the sensing loops around your building.  You have to stretch the loop wires two feet above __ Electronics Projects for You

Simple, gated, pulsed alarm -  Sometimes the need arises for a simple, gated, pulsed alarm.  The circuit shown here employs just four components and a piezo sounder and is unlikely to be out-done for simplicity.  While it does not offer the most powerful output, it is likely to be adequate for many applications.  A dual CMOS timer IC type 7556 is used for the purpose, __ Learning Electronics

Single Supply Type K Thermocouple Signal Conditioning Amplifier with Cold Junction Compensation -  The LTC2050 is a zero-drift precision chopper stabilized operational amplifier available in the 5- or 6-lead SOT-23 and SO-8 packages.  It minimizes board area while providing uncompromising DC performance, including 3mV (max) DC offset and 30nV/°C (max) DC offset drift.  It operates from a 2.7V supply while still supporting __ Linear Technology/Analog Devices App Note, Apr 9th 2010

Single Zone Alarm -  A single zone alarm circuit with entry and exit delay and other facilities. __ Designed by Andy Collison

Single Zone CMOS Alarm -  This is a single zone alarmwith automatic exit, entry and siren cut-off timers.  It will accommodate all the usual types of normally-closed input devicessuch as magnetic-reed contactsfoil tapePIRs etc. __ Designed by Andy Collison

Single-button lock provides high security -  03/04/99 EDN-Design Ideas (contains several designs, scroll to find this one)   Figure 1 is the block diagram of an easily programmed, single-button combination lock.  You operate the lock by using a series of short and long pulses from a momentary switch that masquerades as a doorbell button.  The circuit uses inexpensive CMOS logic.  The retriggerable timer, T1, locks out entries made after the T1 code-entry window, thereby greatly enhancing security.  The circuit in Figure 2 operates as follows: The Schmitt-trigger quad NAND gate, IC1, debounces the code-entry switch and, with the aid of simple analog circuitry, produces separate outputs for activation times of less than and more than 0.3 sec.  These outputs connect to the select gate, IC5 __ Circuit Design by Maxwell Strange, Fulton,MD

Siren Simulator -  This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail. __ Contact: Charles Wenzel of Wenzel Associates, Inc.

Six-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 __ Designed by Andy Collison

Sleep Switch Cum Wake Up Timer -  Here is a sleep-switch circuit that can be easily converted into a wake-up timer.  A dual-mode time setting makes the system versatile.  The circuit is low-cost and can function as a __ Electronics Projects for You

Small CMOS Alarm Circuits -  Battery-Powered Alarm Circuits:  This is a selection of small self-contained alarm circuits.  They have a very low standby currentand 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.

Smart Loop Burglar Alarm -  Simple loop burglar alarms sound whenever the loop breaks.  What if a clever thief comes to know of the working of this alarm? He may simply short the loop by using __ Electronics Projects for You

Smart Smoke Alarm -  At the heart of this ultra simple smoke alarm is H21A1, which is a photo-interrupter module consists of a gallium-arsenide infrared LED coupled to a silicon phototransistor in a plastic housing.  The slot between the infrared sender(LED) and receiver (phototransistor) allows interruption of the signal with smoke, switching the __ Designed by T.K. Hareendran

Smoke Alarm -  The original smoke alarm circuit http://www.  electroschematics.  com/5450/smart-smoke-sensor-alarm/ has some problems and has left some hobbyists and students disappointed.  Rather than to attempt to solve its many problems, I have created an update that uses the venerable LM741 op amp.  It is simpler, more conventional, easier to __ Designed by Jim Keith

Smoke Alarm Battery Life Extender -  While smoke alarms are quite cheap devices, the cost of 9V batteries quickly exceeds their purchase price.  Added to that is the irritation of random beeps from the alarm as the battery reaches the end of its useful life.  This circuit allows typical smoke alarms to be powered from the 12V supply in a burglar alarm while still keeping the standard 9V batteries in place __ Learning Electronics

Smoke Detector -  Smoke Detectors are designed for 3 basic types of system implementations; Standalone, Wireless Connectivity, and Fixed/Wired Connectivity, the first two being battery powered implementations.  Smoke Detectors have 3 main functional blocks; The Sensor and Signal Chain, Processor, and the Communication Interface.  Where the processor and communications interface typically are not needed in your very basic standalone detectors

Smoke Detector -  This home smoke detector circuit warns the user against fire accidents.  It relies on the smoke that is produced in the event of a fire and passes between a bulb and an LDR, the amount of light falling on the LDR decreases.  This type of circuit is called optical smoke detector.  Do not use it as a home smoke detector it’s __ Designed by Popescu Marian

Smoke, Alcohol & LPG-Detection Alarm -  Presented here is a circuit for raising an alarm on detecting smoke or LPG cooking gas leakage, or even alcohol vapours in breath.  This is achieved by using a basic unit __ Electronics Projects for You

A/alarms.htm Page 17

Snore Alarm -  The idea behind this snore alarm, is just to rouse the snorer, not the entire household.  To wake the sleeper, vibration is used, not an audible alert.  The vibration is provided by a small motor housed in a small 35mm film case, which can be placed under the sleepers mattress or pillow.  This circuit has a level control and peak display indicator, a variable trigger __ Designed by Andy Collison

Snore Alarm Electronic Device -  Here I describe a little electronic device that detects snoring.  The principle though very simple is efficient and well protected against false triggerings.  Upon detection, it generates a noisy and painfull alarm.  Just what's necessary to wake up everybody except the snorer. __ Designed by chabin laurent

Snow Depth Alarm -  The design of this system alerts a snowplow driver when the snow depth has exceeded four inches.  A visible red LED packaged in a 10mm package is pulsed at a rate of about 50Hz with 10uS pulses.  Twenty four inches away a phototransistor detects the light from the LED.  The LED and the phototransistor are housed in a section of ½ inch PVC pipe, forming a waterproof seal. . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

Solar-Powered Intruder Alarm for Sheds & Boats -  Most blokes have got a shedor 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 mooringsthey need protecting too. __ SiliconChip

Sound operated music bell using 555 timer -  Sometimes ringing the bell manually becomes difficult, especially when you are holding something in your hands. The sound-operated music bell takes care of the problem

Sound Sensor Alarm -  This 6V battery-operated circuit triggers an acoustic piezobuzzer when a sound is detected.  It can also be used as a cheap acoustic-type glass break detector and/or ambient sound level monitor.  The circuit __ Electronics Projects for You

Sound-Operated Intruder Alarm -  When this burglar alarm detects any sound, such as that created by opening of a door or inserting a key into the lock, it starts flashing a light as well as __ Electronics Projects for You

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

Speed Limit Alarm -  Wireless portable unit; Adaptable with most internal combustion engine vehicles __ Contact: Flavio Dellepiane, fladello @ tin.it

Star Trek Doorbell -  In the Star Trek “Next Generation” TV series, the doorbell outside the private quarters of a crew member makes a particular “beep-boop” sound.  The 3v battery powered circuit below tries to simulate this sound.  The circuit uses one 74HCT74 dual D flip/flop. . . Circuit by Dave Johnson P.E.-October, 2001

Step Alarm -  This Circuit can monitor the door steps or Staircase.  When a person crosses the steps, the alarm sounds indicating the entry.  The circuit is too sensitive and operates in day light.  The circuit uses an NPN Darlington phototransistor L14F1.  It senses the intensity of light through its exposed base and passes current in the __ Designed by D Mohankumar

Still 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 . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Sun-Up Alarm -  The SunUp Alarm can be used to provide a audible alarm for when the sun comes up or it can be used in a dark area and detect when a light comes on.  It can also be used to detect a light beam, headlights etc.  The circuit works as follows.  The phototransistor is very sensitive to light.  (Any phototransistor will work fine) __ Designed by Randy Linscott

Sweeping Siren Alarm-3 volts -  A while back I was challenged by a visitor to the website.  He needed a very loud sweeping siren type audio sound generator powered by 3v.  He tried some of the commercial sirens but they were not very loud when powered by 3v.  He also said that those . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2008

Telephone Dialler for Burglar Alarms -  Esy-to-build circuit dials a pre-programmed number via a modem and sends a warning tone if your alarm is triggered __ SiliconChip

Temperature Monitor using 741 opamp -  A simple op-amp circuit that will trigger a relay when a preset temperature is reached.  Please note that there is no hysteresis in this circuit, so that if the temperature changes rapidly, then the relay may switch rapidly. __ Designed by Andy Collison

Temperature Monitor with Alarm -  This temperature monitor alarm circuit continuosly monitors the room temperature and beeps when the temperature drops below 20 degrees centigrade.  This capability to constantly monitor the room temperature can help lower your air-conditioning costs by reminding you to turn off your airconditioner.  An optical display like a __ Designed by Popescu Marian

Temperature Monitoring with an NTC Thermistor While Measuring Load Current & LTC4151 Supply Current -  The LTC4151 is a high side power monitor that operates over a wide voltage range of 7V to 80V.  In default operation mode, the onboard 12-bit ADC continuously measures high side current, input voltage and an external voltage.  Data is reported through the I2C interface when polled by a host.  The LTC4151 can also perform on-demand __ Linear Technology/Analog Devices App Note, Apr 26th 2010

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

Temperature to Frequency Converter -  3% Linearity from –20°C (Fout = 20kHz) to 75°C (Fout = 25kHz) The LTC6990 is a precision silicon oscillator with a programmable frequency range of 488Hz to 2MHz.  It can be used as a fixed-frequency or voltage-controlled oscillator (VCO).  The LTC6990 is part of the TimerBlox family of versatile silicon timing devices. __ Linear Technology/Analog Devices App Note, Jun 16th 2010

Temperature-to-Frequency Converter -  Precision oscillator structure uses SCR effect to discharge integration capacitor.  A temperature dependent device (LM134) effects the integrator trip point to form a variable interval.  An output transistor detects the reset action and forms a logic pulse. __ Linear Technology/Analog Devices App Note, Jun 21, 2011

Theft preventer alarm -  This circuit utilising a 555 timer IC can be used as an alarm system to prevent the theft of your luggage, burglars breaking into your house etc.  The alarms goes ON when a thin wire, usually as thin as a hair is broken. __ Designed by Radioland.nt.au

Thermal Proximity Detector using a RPW100 -  Schematic only __ Designed by va3iul

Thermally Controlled NiCad Charger -  A voltage controlled current (maximum of 1A) is used to charge a NiCd battery.  The control voltage is derived from the temperature difference between the battery and the ambient air.  The charge current is reduced as the battery heats up. __ Linear Technology/Analog Devices App Note, Jun 21, 2011

Thermo Alarm -  The Th1 thermistor is a regular NTC (Negative Temperature Coeficient) of about 1000 ohms.    But other types will work too.    Adjust the sensitivity with the 20K trim-pot P1.  When the thermistor is at operating temperature, the resistance is high The 330K resistor (R3) is to introduce a little bit hysteresis back on the basis of Q2 to prevent relay chatter __ Designed by Tony van Roon  VA3AVR

Thermocouple Thermometer with Cold Junction Compensation -  The LTC2997 is a high-accuracy analog output temperature sensor. __ Linear Technology/Analog Devices App Note, Jul 7th 2011

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

Thief warning Device -  schematic only, no description included

Three Flashing LED Doorbells -  Battery Powered LED Circuits:  These circuits combine a buzzer with one or more LED displays.  When the push switch is operatedthe buzzer will sound and the LEDs will flash.  When the switch is releasedthe buzzer will stopbut the LEDs will go on flashing for another 30 seconds or so. __ Designed by Ron J.

Tilt Sensor Alarm -  Ultra simple circuit of the tilt sensor alarm presented here can be fabricated using readily available inexpensive components.  The circuit is a true transistor based design.  Home made Tilt sensor for this circuit is an ordinary little glass/plastic bottle with two metal needles inserted through its cap, and a small quantity of __ Designed by T.K. Hareendran

Time to Dust -  I have used the circuit below for many different applications, which require ultra low power drain.  It uses a single CA4013 flip/flop.  One section forms a pulse generator.  The generator produces 100uS pulses at a rate of about 1 per second . . . Hobby Circuit designed by David A. Johnson P.E.-May, 2006

Timed Burglar Alarm -  This is a simple but effective alarm circuit which can reset its self after a time that you select.  it has normally open and normally closed triggers which make this circuit very practical.  This alarm has normally open and normally closed triggers.  It's on a 555 timer so the alarm will reset it's self after a certain amount of time.  The time is adjustable 

Timer for Geyser -  This timer circuit for geyser sounds an alarm after the set timing of 22 minutes when the water is heated up.  The circuit comprises a timer IC 555 wired as an astable __ Electronics Projects for You

Timer warns when pills are due -  05/10/01 EDN-Design Ideas Some people need to take medication at precise, regular intervals.  When you're in a hospital, a medical staff is present to ensure that you take your medication on time.  But when you're taking medication at home, you must frequently look at the clock-a clear annoyance.  When my wife was pregnant, she needed to take medication every two hours from 8 am to 10 pm.  To help her time the two-hour intervals, I built the battery-powered circuit in Figure 1.  The circuit derives its power from a 9V PP3 battery.  All the ICs are CMOS -based; the low power consumption of the circuit allows one-week autonomy.  When you press the start button, BP1, a two-hour delay commences. __ Circuit Design by J Terrade

Timer with Alarm -  This simple alarm timer circuit is made with 4060 which has an integrated oscillator with a good stability with a relatively wide frequency range.  In the circuit diagram the oscillator frequency is set by the RC network that is connected at pins 9, 10 and 11.  When the timer with alarm circuit is connected with S1, the pulse __ Designed by Popescu Marian

Timer with Musical Alarm -  This low-cost timer can be used for introducing a delay of one minute to two hours.  After the timing period is over, a musical song is heard.  The circuit is built around __ Electronics Projects for You

Tiny 2-zone Alarm  -  Since I travel a lot during summer, I decided to build a really small alarm that could easily fit inside a pocket.  However, I also demanded reliable operation, simple construction and very low power consumption.  I started with CMOS logic gates, but was soon forced to abandon the concept after a few unsuccessful (and far too complicated) takes __ copy of geocities.com/tomzi.geo page

Tiny Door Guard with Alarm -  This simple Door Chime protects the door and gives a loud Alarm tone when there is an attempt of theft.  The circuit is too simple and battery operated.  A Normally Closed (NC) reed switch and magnet is used to trigger the circuit.  Alarm generator is the popular ROM IC UM 3561.  This 8 pin IC has an inbuilt oscillator to generate __ Designed by D Mohankumar

Tone Control -  Based on the classic Baxendall tone control circuit, this provides a maximum cut and boost of around 10dB at 10K and 50Hz.  As the controls are passive, the last transistor provides a slight boost.  The output. __ Designed by Andy Collison

Touch Activated Alarm -  Not much to tell here as the circuit speaks for itself.  The 555 can be almost any type, they are all pin-compatible.  Although some CMOS types may not have enough power to drive the transistor, in that case use an ordinary 555.  C1's working voltage should be increased to 25V if you decide to go with a 12V power source.  Change the value of C1 for the desired output pulse __ Designed by Tony van Roon  VA3AVR

Touch Alarm -  This is a new type of touch alarm that uses an RF oscillator at its input.  One special feature of this touch alarm is that it can use a big-size touch __ Electronics Projects for You

Touch Alarm for Door Knob -  I designed this hobby circuit many years ago and it seems be a very popular construction project.  Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms.  A metal chain attached to a box holding the electronics is place . . . Hobby Circuit designed by David A. Johnson P.E.-May, 2000

Touch triggered alarm -  No description available __ Designed by Jospfh

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 automaticallyand/or immobilize the machineevery time you turn-off the ignition.  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.

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 automaticallyand/or immobilize the machineevery time you turn-off the ignition.  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.

Transistor based Security Siren -  This circuit was requested by several correspondents.  Its purpose was to obtain more power than the siren circuit already available on this website (One-IC two-tones Siren) and to avoid the use of ICs.  A complementary transistor pair (Q2 & Q3) is wired as a high efficiency oscillator, directly driving the loudspeaker.  Q1 ensures a full charge of C2 when power is applied to the circuit __ Learning Electronics

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.

Transistor based 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). 

Transistor Delays to Detect if a door is open for a long period -  You're charging the capacitor directly from the battery.  So the charging time is related to the product RC, where R is just the internal resistance of the battery.

Two CMOS Based 24-Hour Timers -  Timer Circuits:  A pair of multi-range timers offering periods of up to 24 hours and beyond.  Both are essentially the same.  The main difference is, that when the time runs out, Version 1 energizes the relay and Version 2 de-energizes it.  The first uses less power while the timer is running; and the second uses less power after the timer stops.  Pick the one that best suits your application. __ Designed by Ron J.

Two CMOS Based Vehicle Anti-Hijack Alarms -  Automobile Alarm Circuits:  The first circuit is designed for the situation where a hijacker forces the driver from the vehicle.  If a door is opened while the ignition is switched onthe circuit will trip.  After a few minutes delaywhen the thief is at a safe distancethe alarm will sound and the engine will fail.  The second circuit is a modification of the firstoffering enhanced protection in many more situations. __ Designed by Ron J.

Two Simple Circuit Relay Based Motorcycle Alarms -  Motorcycle Alarm Circuits:  These are twoeasy to buildrelay-based alarms.  You can use them to protect your motorcyclebut they have many more applications.  If you use relays with 6-volt coilsthey'll protect your "Classic Bike".  Both alarms are very small.  The completed boards occupy about half a cubic-inch8 cc.  The standby current is zeroso they won't drain your battery. __ Designed by Ron J.

Two-Terminal Temperature-to-Current Thermometer Suitable for Use at the End of Long Wire Runs -  Adding the LT3092 to a thermistor kit along with three 0.1% accuracy resistors and one final trim, a very accurate 2-terminal temperature-to-current converter can be built.  This circuit measures 700μA operating current at 0°C, dropping by 2μA every degree until 100°C, at which point the current measures 500μA.  The obvious __ Linear Technology/Analog Devices App Note, Oct 26th 2010

Two-Tone Siren using One 4093 IC -  This circuit is intended for children fun, and can be installed on bicycles, battery powered cars and motorcycles, but also on models and various games and toys.  With SW1 positioned as shown in the circuit diagram, the typical dual-tone sound of Police or Fire-brigade cars is generated, by the oscillation of IC1A and IC1B gates. __ Red Free Circuits

Two-Zone Burglar Alarm -  Multi-Zone Burglar Alarm Circuits:  This two-zone alarm has automatic exit, entry and siren cut-off timers. So it has a particularly detailed circuit description.  There's an optional One-Time-Only module.  It will force the siren to switch off after the first ten minutesand prevent the alarm from activating a second time. __ Designed by Ron J.

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