Low cost fire alarm circuit.
Description.
When there is a fire breakout in the room the temperature increases. This ultra compact and low cost fire alarm senses fire breakout based on this fact.
Transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases. The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3(AC 128). So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie,light,bell,horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched.
Circuit diagram with Parts list.
Notes.
- The Preset R1 can be used to desired temperature level for setting the alarm ON.
- This is not a latching alarm,that is;when the temperature in the vicinity of the sensor decreases below the set point the alarm stops.
- The circuit can be powered using a 9V battery or a 9V battery eliminator.
- All capacitors are electrolytic and must be rated at least 10V.
- The load can be connected through the C,NC,NO points of the relay according to your need.
- The calibration can be done using a soldering iron, and a thermo meter. Switch ON the power supply.Keep the tip of soldering iron near to the Q1. Same time also keep the thermometer close to it. When the temperature reaches your desired value adjust R1 so that relay gets ON.Done!
Hi guys.. the circuit may work but take note you can’t use it as a standard fire alarm in rooms especially ventilated one,you need to heat the room first or the room is already on flames to activate the alarm and you can’t control the fire anymore. For kitchen you can used heat sensing alarm like this and other fire alarm circuits using heat sensing component such as thermistor where smoke alarm will be not suitable but for rooms use smoke alarm which is cheaply available for safety reason.
hi!
it’s realy works.
hi!this is swapnali(TE E&TC)student..i want miniproject with report or with more ideas.so please sir help me…
it will be helpfull 4 us if u provide a vedio
Hi Zack you can replace it with 2N2905. the symbol shown for Q3 is wrong it should be similar to Q1 as it is also a PNP transistor
sir i hv try to find Q3 AC128 but unvailable,can i change that AC128 to another trans?
help pls
can i supply this ckt by 24 volts? and regarding the n.c – n.o output of this ckt is it respect to ground or to the com. terminal of the relay.
hi, i want to know how to connect the detector to the ckt. from the base of bc177 respect to the ground?
Hi Elgardo you are right Q3 is wrongly shown as NPN transistor it should be PNP with collector to relay and emitter to positive. otherwise the circuit is correct and will work as per the design.
Sir,
I am studying the circuit diagram and I got confused.
Is it the collector of Q2 will be connected to the base of Q3? (It says on the text :The emitter of Q2 (BC 108)is connected to the base of Q3(AC 128).)
Or is this a typographical error?
Please help.
Sir,
I am from India,
I want to start my own company based on Development Of projects….
please give me some guidance and valuable time if possible then mail me on
dhananjay_gangapurkar@yahoo.in
Thanks For reading…
Keep saying some good thought for me………….
it’s very nice…!
Hi Sarah for fire alarm DC is preferable, so that they can work during power cut also. You can use rechargeable batteries with a suitable charger for 9 volt. this will be fail safe, whenever power supply is available it will be charging the battery as well powering the alarm. during power failure the battery will take over.
is there any other way to give 9v instead of using transformer
sir i didn’t understand the terms NO,NC, C; and the component K1, and is R1 is a rheostat or resistior, waiting for your reply
i didn’t under stand the R1(it is a resistor or rheostat), what are the components used as K1 ,NO,Nc,C
This circuit will not work properly as soon as you plug it. You need to calibrate the circuit before using. Calibration can be done by adjusting the POT R1.
I tested this circuit and there is no problem with it.
sir i tried your ckt…but after lots of attempts…still the ckt is not responding to the heat.when i connect to the supply the relay gets on (showing that current is flowing through the ckt)..but after that it doesnot respond to heat at all.please help!
BC 477, BC478 etc can be used in place of BC177
sir i am from bangladesh.your circuit is very easy to establish.but i face a problem to find transistor Q1(bc177).its unavailable.equivalents of this transistor bc178,bc179 are also unavailable.please help me to find another transistor.
hi the circuit was nice but i have a small doubt that is “what is the relation(formula) between Temperature and Resistance R1″.
i mean how can we know the resistance(R1) value for our desired temperature.
hi sir..
im quennie
from the philippines and
i just want to ask if you have
a diagram of the ic you used..
i mean if you have some diagram
to guide on so that i can place the materials in their
right position and place..
thank you sir
and god bless!
I think your article very useful for people who want to know about alarm work especially electronic
Hi deepak,
which transistor you used in place of Q3 ?
sir the circuit u provide for fire alarm is not working according to the circuit.
actully I am a student and working on this fire alarm project. I joined all the connections according to ur circuit but it is not working plz help me…
at place of Q3(AC128)i used another transister like the same of this.
plz sir help me to complete my project ..
waiting for reply..
thank you.
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we wnt to ask abawt low cost fir alarm braes lik thts model in yor websitik
may ourdr is:500 panel 2zon
500 panel 4zon
300 8zon
200 12 zon
100 18 zon
5000 smok detecotor
2000 hetar detecotor
2000 strop siren
500 alarm beel