Alarm Circuits and Projects


Alarm Circuits and Projects

Alarm circuits can be very simple or very complex, but most of them are made up of at least 3 parts:

  • The sensor stage: These components can be of various types and are responsible for detecting the problem.
    The problems can be detecting an intruder, smoke, high or low temperatures, inadequate levels of liquids, absence or presence of an object, etc.
  • The trigger circuit: This part of the alarm is responsible for receiving the signal sent by the sensor, amplifying it to activate an output component such as a relay that in turn activates…
  • An audio output device such as a siren, buzzer, bell, etc. or a visual output such as turning on lights, etc. It could also be an output that combines the two options.
  • A fourth part, essential in an alarm system, is a Power source that in all cases must be accompanied by a backup rechargeable battery.

Some simple alarm circuits of various types are shown on this page.

Temperature Alarm Circuit using a diode

This temperature alarm circuit uses a common semiconductor diode as the temperature sensing element. The circuit can be used as a high or low temperature alarm.

Car Burglar Alarm using two 555 timers

This is a simple Car Burglar Alarm circuit that uses a dual contact relay and two 555 timers, each of them working as a monostable multivibrator.

The alarm will activate the car horn, approximately 5 seconds after either door is opened and has not been properly closed.

Light sensitive alarm using LDR

This alarm activates when the illumination level changes from almost total darkness to a moderately high illumination level. When this happens the alarm is locked and the alarm signal, be it a light or a sound, remains active.

Temperature Alarm using thermostats

Temperature Alarm using thermostats

This simple alarm with thermostats circuit can be used on places where we must detect overheating. It uses 3 thermostats, and a one 2 contact, relay

Fire alarm circuit using thermostats and SCR

Fire alarm circuit using thermostats and SCR.

This circuit uses several thermostats in parallel for its operation. They can be as many as you want and must be placed in the places where you want to sense the temperature.

Power Failure alarm using transformer and relay

Power failure alarm using transformer and relay

This power failure alarm circuit is very simple and is very useful when used with electric ovens, a refrigeration systems or to prevent a thief from cutting off the electrical power because it immediately warns of the problem. It also serves you when the fuses have blown.

Door opening monitor

Door opening monitor

The circuit allows detecting the opening of one or more doors with an audible and a visual warning.

Fridge Door Alarm Circuit

Refrigerator door alarm circuit with two 555 timers

This refrigerator door alarm circuit alerts you when the door has been left open. Very important, because the power consumption increases when this happends. It uses the popular 555 timer IC

Rain Alarm Circuit using 555 IC

Rain alarm circuit using 555

This Rain alarm circuit uses the water property to conduct electricity. To implement the water sensor we use two pieces of metal, which are placed close enough for a drop of water to let current flow between the metal plates

Temperature alarm circuit with Op. Amp.

Temperature alarm circuit with 741 operational amplifier

This temperature alarm circuit with operational amplifier activates a relay when the temperature of the place to be controlled exceeds a set level. Useful for an oven temperature control or a cooling system

Shadow detector alarm circuit with two LDRs

Shadow detector alarm circuit with two LDRs

This circuit detects if there is a difference of lighting between two LDRs, and sends an audible and visual warning. Circuits that use two LDRs are more reliable than those that use only one.

Fuel Pump Cut-off Circuit

Fuel Pump Cut-off Circuit

Fuel pump cut-off circuit for the car prevents the activation of the pump. Useful to increase the security level of a car which already has an alarm installed

Burglar alarm using thyristor and relay

Burglar alarm using thyristor and relay

This is a simple Burglar alarm uses a thyristor as the main element and a relay as the output device. The relay can activate a buzzer or a siren.