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Alarm Schematics / Circuits

12 Volt Alarm Power Supply

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.

4 Digit Keypad

5 Zone Alarm Circuit

5 Zone alarm circuit MIT

A CMOS Based Motorcycle Alarm  - 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.

A Simpler CMOS Single Zone Alarm - 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.

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.  The circuit rectifies and filters the power line signal.  I set the resistor values, so the DC voltage produced is close to 1% of the RMS value of the line.  Thus, a 120vac line would yield about 1.2v DC.  That voltage is fed to a pair of voltage comparators.  The outputs of the comparators decide if the sampled voltage is above, below or acceptable.

Alarm Circuit - Surround your Entire Yard with this Laser Alarm

Alarm control keypad

Alarm monitors rotational speed of DC motor - 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.

Alarm Power Supply

Alarm tells you to close the refrigerator door - EDN- Design Ideas,  A simple gadget that you place inside the refrigerator alerts you by beeping if the door stays open for 20 seconds or longer.

American Police Car Siren (linscott) - Scroll down to locate this circuit. The 555 on the right is wired as an alarm tone generator and the second 555 timer on the left is wired as a low frequency astable timer which generates a ramp waveform of about 6 seconds that is buffered by the transistor and again used to frequency modulate the tone generator. The transistor is used to help strengthen the signal to the speaker.

An Expandable Transistor Based Burglar Alarm - This transistor based alarm features automatic Exit and Entry delays, together with a timed Bell Cut-off and system Reset. Along with the Exit/Entry zone, the basic alarm board has one Instant Zone. This will be adequate in many situations. However, the modular design means that it's easy to add as many zones as you like to the system. Details of a Four-Zone expansion module are provided.

An SCR Based Burglar Alarm

Anit Theft Motorcycle Alarm

Auto Burglar Alarm (Randy Linscott) - This alarm circuit is based on two 555 timers. The alarm will sound your car horn if anyone opens the car door while the circuit is armed. The timers will allow you to leave the car without sounding the horn. To turn the circuit on S1 must be closed.

Automatic Intruder Alarm

Automatic Night Lamp with Morning Alarm

Battery Low Voltage Beeper - This circuit provides an audible and visual low voltage warning for 12V battery powered devices. Idle current: 6ma Low Voltage Warning current: 15ma.

Beeper bug transmitter for fox hunt & vehicle tracking

Big Ben Sound - This circuit produces  famous Big Ben sound. It produces  "ding dong" sound when switched ON. Basically  circuit alternates between two frequencies which are adjustable. This produces  "ding-dong" sound. The first C (left) oscillates at about 1Hz. The second IC's tone is modulated by  changing voltage at  output of  first IC.

Motorcycle Brakelight Flasher

British Police Car Siren - The first circuit simulates the siren of a British police car. It uses two 555 timers in the circuit. The 555 on the right is wired as an alarm tone generator and the second 555 timer on the left is a 1 Hz astable multivibrater.

Broken Charger-Connection Alert - Detects if a device is not properly connected to its supply.  Suitable for battery chargers, portable appliance supplies etc.

Car Alarm

Car Alarm & Immobilizer - This circuit features exit and entry delays, an instant alarm zone, an intermittent siren output and automatic reset. By adding external relays you can immobilize  vehicle and flash  lights.

Car Anti-Theft Wireless Alarm - This FM radio-controlled antift alarm can be used with any vehicle having 6to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in  vehicle at night when it is parked in  car porch or car park. The receiver unit with CXA1019, a single IC-based FM radio module, which is freely available in  market at reasonable rate, is kept inside.

Car Back-up Alarm - The brake lights of  automobile trigger this circuit on and off.  This save  annoyance of  alarm when it is not needed.  This is an older circuit which was published in Popular Electronics Magazine, but still a good circuit today.

Car-Police Siren - This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail.

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, the wire is connected to one of the steel posts, held in the ground.  The typical earth ground resistance will be in the 1000s of ohms range, which is low enough to keep the n-channel FET turned off in the circuit below.  If the wire is cut or broken

CMOS 4060 Burglar Alarm - 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.

CMOS Based Vehicle Anti-Theft Alarm - This car alarm circuit features Exit and Entry delays, an instant alarm zone, an intermittent siren output and automatic Reset. By adding external relays you can immobilize the vehicle and flash the lights.

CMOS Single Zone Alarm

Cold Activated Switch (Andy Collinson) - A simple thermistor triggered cold switch with adjustable threshold.

Cold Activated Switch - The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees celsius. A suitable bead type thermistor is found in the Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range of temperatures. A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents relay chatter when temperature is near the switching threshold of this circuit.

Combination Digital Lock - A multiple input combination lock using CMOS counter IC's. Flexibility and code change is allowed by changing output connections.

Combination Lock PIC16F84

Combination Lock-Digital

Controller Provides Multiple Alarm Driver Formats - EDN-Design Ideas: DC/DC converters for use inside the telephone handset require operation from the high-source-impedance phone line. Additionally, the CCITT specifications call for maximum on-hook power consumption of 25 mA. The dc/dc converter in Figure 1 is 70%-efficient at an input power of 25 mA, providing 5V at 3.4 mA. Controlled, low-peak switch current ensures that the ­48V input line experiences no excessive voltage drops during switching.

Countersurveillance Monitor - This simple-to-build, yet effective circuit can help you search out and remove electronic eavesdropping devices.

Cut Phone Line Detector (Aaron Cake) - A while ago I got an email asking for the schematic of a circuit to detect cut phone lines. It didn't take me long to find this circuit in Electronics Now. When the circuit detects that a phone line has been cut, it activates a MOSFET which can be used to drive a relay, motor, etc. It can also be connected to a security system.

Cut Phone Line Detector (Tony van Roon) - This circuit speaks for itself.  When the phone line is okay, Q1 acts as a short with a very high ohms value via R1, R2, and R3 for a total of about 66 MegOhm so very high. When the phone line is cut (Tony van Roon's open), it activates the MosFet (Tony van Roon's Q3) via transistor Q2 to drive the load.  Substitutes are fine, none of the components are critical.

Doorknob Alarm  - 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 placed around the inside doorknob of a wood door. Anyone grabbing the knob from the outside is detected by the electrical capacitance change that occurs from the human hand contact between the knob and the box. Almost all of the commercial devices sold use a more expensive and power consuming radio frequency circuit approach to detect the capacitance change. But, a very inexpensive and micro power technique can also work. This circuit schematic should dramatically reduce the cost of the device and allows it to operate for many years from one set of batteries.

Electric Field Disturbance Monitor - This schematic is the motion discriminator alarm and battery monitor sections of the field monitor that is discussed in more detail a Electric Field Disturbance Monitor. (this link is off-site)

Electric Field Disturbance Monitor - This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at Electric Field Disturbance Monitor. (this link is off-site) The system can detect human and animal motion by the electric fields they disturb.

Machine Power Loss Beeper - For some medical equipment it is important for an operator 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.

Micro Power Over - Temperature Alarm - The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a cheap but accurate thermistor as the temperature sensor.

Power Reminder Beeper - This circuit produces a short beep once each minute. It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on. It is simple enough to be packaged inside a small plastic box. It might be ideal for computers, printers or some test equipment that shouldn't be left on all night.

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. The circuit draws very low current from a +3v battery and could be housed in a package similar to a small ashtray. The assembly might be placed in a suitable out of the way area to collect dust. It would alert a maid when it was time to dust the room. The circuit detects dust with an infrared LED that is pulsed so its light shines onto a smooth flat plate. Any dust settling onto the surface is detected by a phototransistor, mounted at a 90 degree angle from the LED. When the dust reaches a particular level, sufficient light is reflected into the phototransistor to change the logic state of the circuit to an alarm condition. The alarm output could be connected to a beeper, a flashing LED or to one of the LED flashing circuits in this hobby circuit section.

Unplugged Power Cord Alarm - 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. The device was designed to snap onto the outside of a power cord, where it senses the AC electric field emitted. No direct connection to the internal wires of the power cord is needed. I also included a low battery voltage monitor.

Water Seepage Alarm - 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..

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