Alarm Circuits – Temperature, light sensitive, door, power failure

 

Alarm Circuits

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.

Water Level Alarm with Three Transistors

Water Level Alarm with Three Transistors

Water level alarm with three transistors

Water level alarm. Simple & small alarm with few components. Has small current demand, it can work with a 9V battery for a long time

Light sensitive alarm using LDR

Light sensitive alarm using LDR

This light sensitive alarm is very simple and works using, as main components, a transistor, an LDR and a relay.

The 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 circuit with Op. Amp.

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

Rain Alarm Circuit using 555 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

Car Burglar Alarm using two 555 timers

Car Burglar Alarm using two 555 timers

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.

Fire alarm circuit using thermostats and SCR

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.

1 2

 

  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •