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Alarm Circuits
Alarms:  #'s       A - C       D - G       H - L       M - Q       R - T      
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Last Updated: June 02, 2021 01:44 PM

Circuits Designed by Dave Johnson, P.E. :
120 VAC Low Voltage Alarm   -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac.   . . . Circuit by Dave Johnson P.E.-July 09, 2006

3v Sweeping Siren Alarm   -  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 . . . Circuit by David A. Johnson P.E.-June 24, 2008

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

1 Time Only 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. __ Designed by Ron J.

1.5V Water Level Alert -  This circuit will emit an intermittent beep   (or will flash a LED) when the water contained into a recipient has reached the desired level.  It should be mounted on top of the recipient   (e.g.  a plastic tank) by means of two crocodile clips, acting also as probes. __ Red Free Circuits

10 GHz CW Radar based on a X-band Gunnplexer -  This is a modified burglar alarm, based on a X-band Gunnplexer type design.  It's interesting, because it has two detector diodes 1/4 wave apart, so you effectively get I/Q demodulation, . __ Designed by Jim Lux

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. __ Designed by Andy Collison

120 VAC Low Voltage Alarm -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac.. . . Circuit by Dave Johnson P.E.-July, 2006

1-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.

1-IC Design Monitors Ajar Doors -  This design senses an ajar door and, if the situation isn’t corrected within 20 sec, sounds a beeping alarm. Design by Fred Hicks, General Electric Co, Louisville, KY

1-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.

1-Time-Only Motorcycle Alarm -  Motorcycle Alarm Circuits:  This is a "one-time-only" alarm with an LED activation indicator.  If the bike is moved the siren will sound for about a minute.  Then it will switch off and remain off.  It won't activate a second time.  It can't be activated again until the next time the alarm is set. __ Designed by Ron J.

2 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 on the circuit will trip.  After a few minutes delay when the thief is at a safe distance the alarm will sound and the engine will fail.  The second circuit is a modification of the first offering enhanced protection in many more situations. __ Designed by Ron J.

2 Simple Circuit Relay Based Motorcycle Alarms -  Motorcycle Alarm Circuits:  These are two easy to build relay-based alarms.  You can use them to protect your motorcycle but 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 zero so they won't drain your battery. __ Designed by Ron J.

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 __ Designed by Ron J.

20 sec Fridge Door Alarm -  This circuit, enclosed into a small box, is placed in the fridge near the lamp   (if any) or the opening.  With the door closed the interior of the fridge is in the dark, the photo resistor R2 presents a high resistance   (up to 200K) thus clamping IC1 by holding pin 12 high.  When a beam of light enters from the opening, or the fridge lamp illuminates, the photo resistor lowers its resistance   (less 2K), pin 12 goes low, __ Learning Electronics

20 sec Fridge Door Alarm V2 -  The main purpose of this design was to obviate a small defect of the very popular Fridge Door Alarm circuit, available on this website since 1999 and built by a lot of hobbyists.  Unfortunately, that circuit stops operating when the battery voltage falls below about 2.62.7 Volts.  This is due to the 4060 CMOS IC used.  In some cases, devices made by some manufacturers   (but not Motorola's) fail to operate even at nominal 3V supply voltage. __ Learning Electronics

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 __ Contact: Collin Mitchell

2-Terminal Temperature-to-Current Thermometer Suitable for Use at the End of Long Wire Runs -  By 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, Mar 16th 2011

2-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 minutes and prevent the alarm from activating a second time. __ Designed by Ron J.

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

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 __ Designed by Andy Collison

3v Sweeping Siren Alarm -  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. . . Circuit by David A. Johnson P.E.-June, 2008

4 Digit KeypA - D -  This is an enhanced 4 digit keypad which may be used with the Modular Alarm System __ Designed by Ron J.

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 __ Designed by Radioland.nt.au

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.

4-Digit Keypad Controlled Switch -  Universal Keypad-Controlled Switches:  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. __ Designed by Ron J.

5 Digit Alarm KeypA - D -  This is an enhanced 5 digit keypad which may be used with the Modular Alarm System.  This switch will suit the Modular Burglar Alarm circuit.  However, it also has other applications.  The Keypad must be the kind with a common terminal and a separate connection for each key.  On a 12-key pad, look for 13 terminals.  The matrix type with 7 terminals will NOT do.  Choose the five keys __ Designed by Andy Collison

5 Zone Alarm -  This is a complete alarm system with 5 independent zones suitable for a small office or home environment.  It uses just 3 CMOS IC's and features a timed entry / exit zone, 4 immediate zones and a panic button.  There are indicators for each zone a "system armed" indicator. __ Designed by Andy Collinson

555 Door Ajar Flasher -  A magnet is placed on the door and a magnetic reed switch on the door casing so when the door is closed the circuit is disabled.  When the door is open, the LED flashes VERY BRIGHTLY—when closed, the circuit is disabled in such a way that it draws virtually no current for maximum battery life.  Door Ajar LED Light Schematic Bill __ Designed by Jim Keith

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.

5-Digit Alarm Control Keypad -  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.

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 __ Designed by Andy Collison

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

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

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