Alarm Circuits
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.
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.
Over-temperature alarm using operational amplifier
Over-temperature alarm using operational amplifier.
This over-temperature alarm circuit uses a common semiconductor diode as a temperature sensing element.
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.
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
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.
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.
Multitone generator circuit – Open door indicator using CD4001
Multitone generator circuit / Open door indicator using CD4001
This very simple multitone generator circuit has many applications but here it is shown as door indicators, generating a different tone for each door of our house
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.
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
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
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 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
Over temperature alarm circuit using Thermostat
Over temperature alarm circuit using Thermostat
This alarm circuit uses the 67L080 thermostat (TH) and can be assembled easily, due to the small number of components required.
How to Make a Car Temperature Alarm using 741 and 555 ICs?
How to make a car temperature alarm?
This car temperature alarm check the car engine temperature and warns us, by means of a sound signal, when the temperature exceeds a seted value
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
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.