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 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

Secret_Panic_Switch  -  Design an electronic switch which would be positioned behind a wall at several key positions and would be activated when an adult human hand was pressed against the wall at that position.  The best way to detect the human hand against a wall was with a capacitance switch.  The QTFS3X from Faraday switches looked like a perfect match.  The switch could be powered by a DC voltage down to 3v.  It required only 30ua of current.  It was quite small and would easily fit behind the wall. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 18 May 2011

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.