Download Simple Fire Alarm Circuits at Low Cost

Simple Fire Alarm Circuits at Low Cost
Introduction:
Just imagine when an apartment is caught fire in the second floor! The people in the second floor can easily
recognize the fire and can inform to their neighbors and run for their lives; but people in the first, third and
fourth floors may not know the fire accident and busy with their work. This will be great danger to the people
live and property also. So there should be an alarm to inform to all people in the apartment in which the fire has
taken place and act immediately. Here fire alarm plays a key and important role. It will detect the fire
automatically and inform to the people with an alarm. Now-a-days, fire alarms are must for the apartments,
theaters, hotels, restaurants, coal mines, petrol bunks or gas stations, etc.
Here are two simple fire alarm circuits which will use different concepts and different components for
generating the fire alarm. The main basic concept is almost same that sensing the fire or heat by the
component and giving that information to the remaining circuit to give alarm.
Fire Alarm Circuit Using Germanium Diode:
This is a simple fire alarm circuit using Germanium Diode and 555 timer. In this circuit Germanium Diode play
very important role in detecting the fire. This circuit is very easy to construct, cost effective and implementable.
Block Diagram of Fire Alarm Circuit Using Germanium Diode:
Here is the
simple fire alarm circuit which costs less than 100 rupees. The key component in the circuit is DR25
(germanium diode) whose resistance will decrease with increase in temperature. The conduction of germanium
diode will start at 70 degrees. So we may use germanium diode as a heat sensor. When the temperature is
more than 70 degree, the germanium diode will conduct and trigger the NE555 timer through a transistor. The
NE555 is configured in astable multivibrator and make the buzzer to alarm when germanium diode conducts.
So that we can get alert and act according to the alarm.
Circuit Diagram of Fire Alarm Using Germanium Diode:
Circuit Explanation:

The DR25 germanium diode is heat sensor which will conduct when temperature is increased at certain
point. The DR25 is made reverse biased in the circuit. It will conduct only when it is more than 70degree of
room temperature.

The DR25 is connected to the transistor in reverse bias, which has high reverse resistance (more than 10K
ohm) and does not make the transistor to turn off which is connected to the reset pin of 555 timer. The
reset pin of 555timer will be in ground level when the transistor is turned off. Here, the 555 timer is
configured as astable multivibrator.

When more than 70degrees in room temperature occurred, the resistance of DR25 diode drops to 1k ohm
which will make the transistor to turn off and make the reset pin to go high. This will generate the output at
pin3 and make the sound through the alarm.

We can use 3 or more diodes in reverse bias connected in parallel and placed in different room. If there is
fire accident, it will sense and make the alarm.
Note:
1.
If DR25 germanium diode is available, you can still use AC128, AC188 or 2N360 germanium transistors.
Use base and emitter junctions in place of cathode and anode.
2.
Diode must be connected to the circuit in reverse bias.
Fire Alarm Circuit Using LM341:
Here is another small project on fire alarm. When a fire accident is happened in home or office, it will detect
the fire (intensity of fire in the room) at instant and give the alarm so we can be ready and act accordingly.
Block Diagram of Fire Alarm Circuit Using LM341:
The thermistor is the main component which detects the fire by sudden change in the room temperature
because of the heat generated by the fire. The thermistor will detect the heat and give the information to the
LM741 OP-AMP. The op-amp will make the NE555 to generate pulse which has been given to a buzzer to
buzz.
Main components:
Thermistor: The resistance of the thermistor will change according to the temperature. There are two types of
thermistors of which one type of thermistor will increase its resistance with increase in temperature called as
positive temperature coefficient and the other type of thermistor will decrease its resistance with increase in
temperature called as negative temperature coefficient. In this project, we use the thermistor which will have
lower resistance with respect to the temperature (negative temperature coefficient thermistor). Thermistor has
large applications in consumer electronics, food preservation sectors etc.
LM741: LM741 is an operational amplifier which will work according to the difference in the input voltages.
LM741 has following features like high current driving, voltage gain, noise amplification and also provide low
output impedance. LM741 can also used as a short circuit protection.
Circuit Diagram of Fire Alarm Using LM741:
Circuit Explanation: There are two modes in this project:

Mode 1: How the circuit will work if there is no fire

Mode 2: How the circuit will work if there is fire.
Let us now study about these 2 modes in detailed way.

Mode 1 – When there is no fire: The thermistor will have high resistance and there will be no difference in
the voltages coming to the LM741 at pin2 and pin3 so there will be no output which will help to trigger the
timer.

Mode 2 – When Fire started: The thermistor resistance will go down to the heat produced by the fire and the
resistor R3. And thermistor will act as a potential divider circuit resulting a low voltage at pin2 of LM741 as
the temperature goes high. The reference voltage is given by the R4 to the pin3 when there is a difference
in the voltage. The pin6 will trigger the timer configured as astable multivibrator. NE555 timer will activate
the speaker which makes the alarm. The variable resistor R5 will vary according to the difference in the
voltage of the op amp. According to the resistor R5, the timer will generate pulse. If temperature goes high,
the speaker will make high noise.
Fire Alarm Applications:

Fire alarms can be used in any place like parks, theaters, hotels, restaurants, boats, ships, etc.

Fire alarms can be also used as temperature sensors in some applications, when the temperature goes up
abruptly it can inform.

Fire alarm can be used in our home for safety purpose and it is a very good precautionary measure.