Alarm Circuits

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.

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.

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.

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.

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.

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)

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.

12C508 Smart Lock: The base sends a data sequence (the password) which is recognized by the key. The key checks the sequence and -if it recognizes it -sends to the base another data sequence, as an answer to the password. At the Time when the base gets a correct answer (which means that the key has been introduced), the relay is put into action, and it can command a number of consumers.

4 Digit Keypad:

5 Zone Alarm Circuit:

Active IR Motion Detector:

Alarm Control keypad:

Alarm Loop Sensor (LM3915):

Alarm Touched Triggered:

Almost Ultrasonic Motion Sensor:

Anit Theft Motorcycle Alarm:

Anti Theft Security for Car Audios:

Auto Burglar Alarm: This alarm circuit is based on two555 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. To set the alarm, open S2 ( it is normally closed ) this will give you about5 seconds to get out and close the door. The exit delay Time is set by R1 and C1. If anyone opens the doors for more then two seconds the horn will sound until power is removed from the circuit. The2 second Time is set by R2 and C2. If you open the door, you must deactivate the alarm by closing S2. This very basic circuit could be used for a home also.

Automatic Entry Control System:

Automotive Alarm System:

Basic RF Transmitter for PIR Sensors:

Battery Low Voltage Alarm #2:

Battery Low Voltage Alarm:

Battery Low Voltage Beeper:

Capacitance Proximity Alarm:

Car Burglar Alarm:

Car indicator alarm:

CMOS Single Zone Alarm:

Cold Activated Switch:

Combination Lock (PIC16F84):

Combination Lock (Simple Version):

Combination Lock: This circuit is very basic to build. To open a the lock which is connected to the K1 Load. you must press each momentary switch in the correct sequence. The sequence used in this circuit is S1,S2,S3,S4. If any of the other switches are pressed the circuit will reset and you will need to start over. Depending on how you wire the switches, you can use any4 switch combination

Cut Phone Line Detector #1: Can be used to alarm security system

Digital Combination Lock:

Digital Entry Lock:

Dummy alarm:

Electronic Card Lock System:

Electronic Door Codelock:

Electronic Lock:

Enhanced Alarm Keypad:

Four Digit Keypad:

Freezer Door Alarm:

Frost Alarm:

Fuse Monitor / Alarm: a simple way to see if a fuse has blown without removing it from its holder

Gate Alarm Circuit:

Glitch Detector:

Home Security Project:

Infrared Alarm: This circuits consist of two parts, the first part will transmit a signal, and the second part will detect that signal and trigger a relay. To adjust the circuit, hold down S1 while pointing LED1 at the receiver. Adjust R6 until you hear the relay click. You can increase range by using a high output LED for LED1. Bright light will stop the receiver from responding to the transmitter.

Infrared Transmitter / Detector:

Intruder Radio Alert System:

Knock Alarm:

Loop Alarm:

Low Power RFID Transponder:

Luggage Security System:

Micropower Wireless Motion Detector Transmitter:

Miniature Loop Alarm:

Modular Burglar Alarm:

Moisture Alarm:

Motorcycle Alarm:

Novel Buzzer:

Optical Proximity Detector: A "reflector" isolator (A) detects the presence of an object by bouncing light off of it. This technique is useful in circuits that detect when an object is close enough to the sensor (B).

Oscillating output improves system security: EDN-Design Ideas / Many electronic-control systems have digital outputs that use transistors. One method of improving the security in these outputs is to use an oscillating signal to represent a logic-high state instead of a fixed voltage level (Figure 1). This type of signal, a dynamic variable, can drive the circuit shown in Figure 2...

Passive Infrared Pyroelectric Infrared Motion Sensor:

Perimeter Monitor: Using a single cable such as speaker wire or doorbell cable, this circuit can be remotely positioned, for example, at the bottom of a garden or garage, and used to detect all sound in that area. The cable can be buried in a hosepipe or duct and is concealed out of sight. The mic is an ordinary dynamic mic insert and should be housed in a waterproof enclosure with the rest of the circuit.

Personal Alarm: Small portable, anti-bag-snatching unit. Also suitable for Doors and windows control

Personal Silent Alarm System:

PIR Motion Detector Light Controller:

PIR Motion Detector:

Power Supply Failure Alarm:

POWER SUPPLY FAILURE ALARM: This circuit employs an electrolytic capacitor to store adequate charge, to feed power to the alarm circuit, which sounds an alarm for a reasonable duration when the supply fails.

Proximity Alarm #1:

Proximity Alarm #2:

Proximity Switch:

Pyroelectric Infrared Motion Sensor:

Radio Wave Alarm:

Refrigerator Door Alarm:

RF Alarm:

Security Alarm:

Simple Alarm System:

Single Button Lock Provides High Security: EDN-Design Ideas / PDF contains multiple circuits - scroll to find this circuit.

Single Zone Alarm:

Snore Alarm Electronic Device:

Speed Limit Alarm:

Sun Up Alarm: The Sun - Up 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) The sun shining on this device will provide a high to one of the NAND gates. This will cause another NAND gate to oscillate which will drive another gate to output a100hz tone. The transistor provides drive for the speaker.

TinyTwo Zone Alarm:

Touch Activated Alarm:

Touch Activated Switch:

Touch Triggered Alarm:

Toxic Gas Detector:

Turn Signal Alarm:

Ultimate Battery Low Voltage Alarm:

Ultrasonic Motion Detector:

Ultrasonic Remote Control & Alarm:

Universal Alarm System:

Wailing Alarm:

Warning Strobe Flasher for Alarm Systems: