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.

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.

Main Power Failure Alarm and battery backup circuit

Main Power failure alarm and battery backup Circuit

This Main Power failure alarm circuit tells us when the electricity that powers a system or circuit, is gone.

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

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

Motorcycle Alarm Circuit with transistors and relay

Motorcycle Alarm circuit with transistors and relay.

This alarm has, as main elements, 3 transistors, a relay and various sensors, including motion sensors.

Electronic Doorbell Circuit using a 555 and CD4017

Electronic Doorbell Circuit using a 555 timer and a CD4017 decade counter

Electronic Doorbell circuit that lets know that someone is at the door. Made with 555 timer and 4017 decade counter allowing to change the tone of each note

Doorbell for Deaf People Circuit using LEDs

Doorbell for Deaf People.

When a deaf person is alone in a house, it would be very difficult to realize that someone is knocking on the door without any visual warning signal.

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.

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

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.

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