We have seen many alarm circuits but Fire alarm was one of the most important among all. In many buildings,industries and houses fire alarm circuit have been installed in order to alert people them in case of a fire break out in the buildings. So now we can study how this circuit and what are the principles of this circuit briefly.
WORKING OF FIRE ALARM CIRCUIT:
This circuit uses IC1 (NE 555), Thermistor, Resistors, Capacitors and Transistors in it.The IC1 is configured as a free running oscillator at audio frequency.Timer IC (IC1) can produce continuous signal when used as a Astable Multivibrator. Thermistors are the special purpose ICs whose temperature varies significantly with temperature and here we are using NTC (Negative Temperature Coefficient) Thermistors. These Thermistors exhibit high resistance when temperature is low and low resistance when temperature is low.Thus in this circuit this was used to sense the rise in temperature in the surroundings.
Initially when there is a fire break out the resistance of the Thermistor decreases and this allows the voltage to the base of the transistor through the diode D1. The function of the Diode is to allows the current in one direction through it. In this process the capacitor C1 get charges up and increases the time for which the alarm in ON.The larger the value of C1 larger the positive bias applied to the Transistor T1. As the collector of the Transistor T1 is coupled to the base of the Transistor T2 the transistor provides a voltage to the pin 4 (reset) of the NE 555 IC.It is the function of R4 which keeps the IC inactive unless positive voltage is given through transistor.It should be selected in such a way to keep the IC inactive.So thus it makes the Speaker to give sound when there is a fire breakout in the building.
The same reverse process happens when there is reduction in temperature it cools out the Thermistor therefore it gives the high resistance path to the voltage flow thus inhibiting it from reaching the Transistor therefore it keeps the IC1 in inactive state therefore the alarm remains off.
Chad,
Yes you are partially right. The signal from thermistor will be very low to trigger the 555 directly. So a simple NPN and PNP transitor pair was used to do the job. When the voltage at base of T1 goes beyond 0.7v it gets activated, then it pulls the base of T2 to ground. Since T2 is a PNP transistor it gets activated when base voltage goes below 0.7v. This activates that and finally it trigger 555
Thanks for the help! I see that now. If the npn signal is too low by itself , could a 2 stage npn be used ? I’m just wondering if it’s by choice or a have to situation. are there advantages over either way?
sir i dn’t know anything why circuit is not working………i made same connection as you give…. give me some information like on how much temp does thermistor starts working….
Boughella, Have given enough theoretical part in the description. If you want some tweaks in it read Mr. Bill’s comment above you might get extra piece of info.
Yes. you can use the cap value you specified, it only will alter the frequency of the output signal. And as far as diode is concerned you can use 1N4001 also, it was used to permit the current flow in one direction.
This is an “Over-Temperature-Alarm”, NOT A FIRE ALARM. A fire alarm typically detects combustion byproducts, not heat. Also, if your thermistor will drive a 470 ohm resistor, why do you have a two transistor circuit between it and the input of the 555? You are actually driving less impedance with the second transistor than the thermistor is… And, the transistors provides a double inversion, so the input to the 555 is the same phase as the voltage at the thermistor. If you are doing a voltage shift here, try doing it with a change in your thermistor/resistor pair. What is D1 used for? All it is doing is providing a voltage level shift of one diode drop, and if you really need this, you can accomplish it with your thermistor/resistor pair. Also, a larger C1 will not provide a greater voltage at the base of the first transistor. Also, the combination of C1 and the impedance of the 33K base resistor and the beta times the emitter resistor in the first transistor, has a time constant of less than a second. I’m sure that the thermal time constant of your thermistor is eons longer than this, as will the fire be… Additionally, it might be a good design practice to place a reverse voltage protection diode across the speaker. The reverse voltage that occurs on the speaker when the current is suddenly reduced to zero may be enough to provide a voltage across the output driver transistor to destroy it. Remember the equation V = L di/dt, and in this circuit, di/dt can be a very large value. Finally, if all you need is an over temperature alarm, you can replace the first seven parts in your circuit with a temperature sensor. This is a little device that has a bi-metal strip in it that provides a switching contact that makes or breaks at a given temperature. They are used in lots of electronics equipment. One application is to turn on a cooling fan in an electronic system when the temperature reaches a design threshold. Finally, you can replace the remainder of the parts in the alarm circuitry with one piezo-electric sounder. These are used in fire alarms, and can be purchased as a tiny module that can panel or PCB mount. So, this alarm can be reduced to a temperature sensor/switch and a piezo-electric sounder. You will still need the LED/resistor combination if you want a circuit on indicator… So, four parts… instead of nineteen. Now, with all that said, if your intent is to provide a student an exercise in building a circuit that provides a temperature alarm, then go for it. However, redesign the entire circuit. i.e., a circuit that is intended to provide a learning exercise, showing how different electronics parts function, both alone and with others, should employ good design practices. I might suggest that you change the theme of this website. Your circuits are not very good, and in every circuit that I have reviewed, considerable improvements can be made. Maybe you should present your circuits, with an invitation to the visitor to provide improvements to your circuits, and/or other circuits that will provide the same functionality. That way you will engage your visitor as well as provide yourself with a learning experience. (I apologize if this post I’m making is a little rough, and needs a little editing. I’m in a huge rush, but I still want to provide some feedback to you…
Am glad to see that you took some time to provide us extra piece of information here. Actually there are several ready made sensor modules which will do a lot better job than this one but this circuit was posted in the intention to help viewers to build this by their own from scratch. As you said design practices are necessary to hone the circuit designing skills.
And about inviting the visitors to improve or add extra piece of information seems to be good idea and surely will try to implement it here. Many thanks for your suggestion regarding this site and circuits as well.
Is it okay if I replace the 8ohm 1w speaker with 8ohm 0.5w speaker?
Why is the IC 555 used in astable multivibrator mode?
Hi! Can you please tell what is the value of thermsistor?
Noor,
It’s a standard 10K NTC one. You might want to substitute the resistor R8 with a POT and adjust the pot to calibrate it for room temperature
So when T1 is switched on, the collector goes High and switches on T2? If so , how dose T1 collector go high? Transistors are a hard concept for me.
Chad,
Yes you are partially right. The signal from thermistor will be very low to trigger the 555 directly. So a simple NPN and PNP transitor pair was used to do the job. When the voltage at base of T1 goes beyond 0.7v it gets activated, then it pulls the base of T2 to ground. Since T2 is a PNP transistor it gets activated when base voltage goes below 0.7v. This activates that and finally it trigger 555
Thanks for the help! I see that now. If the npn signal is too low by itself , could a 2 stage npn be used ? I’m just wondering if it’s by choice or a have to situation. are there advantages over either way?
circuit is not working…using 9v battery..
give me reason why..?
Sab,
Whats the problem you facing?
sir i dn’t know anything why circuit is not working………i made same connection as you give…. give me some information like on how much temp does thermistor starts working….
Frank I mead the current and the voltage
I need the theoretical part of this circuit if you have send me please
Boughella,
Have given enough theoretical part in the description. If you want some tweaks in it read Mr. Bill’s comment above you might get extra piece of info.
Hello..
Thanx for giving my answer
.
I hv another question ..
I hv 9V battery ,i want it convert into 6V. so what value of RESISTANT should be use???
Use 7806 6V regulator IC for that purpose
Hello sir..
Are u tried this firealarm by own??
Can i use 0.01u or 0.22u in the place of C2 0.04.
1N4007 in the place of D1 N40001.?
Yes. you can use the cap value you specified, it only will alter the frequency of the output signal. And as far as diode is concerned you can use 1N4001 also, it was used to permit the current flow in one direction.
This is an “Over-Temperature-Alarm”, NOT A FIRE ALARM. A fire alarm typically detects combustion byproducts, not heat.
Also, if your thermistor will drive a 470 ohm resistor, why do you have a two transistor circuit between it and the input of the 555? You are actually driving less impedance with the second transistor than the thermistor is… And, the transistors provides a double inversion, so the input to the 555 is the same phase as the voltage at the thermistor. If you are doing a voltage shift here, try doing it with a change in your thermistor/resistor pair.
What is D1 used for? All it is doing is providing a voltage level shift of one diode drop, and if you really need this, you can accomplish it with your thermistor/resistor pair.
Also, a larger C1 will not provide a greater voltage at the base of the first transistor.
Also, the combination of C1 and the impedance of the 33K base resistor and the beta times the emitter resistor in the first transistor, has a time constant of less than a second. I’m sure that the thermal time constant of your thermistor is eons longer than this, as will the fire be…
Additionally, it might be a good design practice to place a reverse voltage protection diode across the speaker. The reverse voltage that occurs on the speaker when the current is suddenly reduced to zero may be enough to provide a voltage across the output driver transistor to destroy it. Remember the equation V = L di/dt, and in this circuit, di/dt can be a very large value.
Finally, if all you need is an over temperature alarm, you can replace the first seven parts in your circuit with a temperature sensor. This is a little device that has a bi-metal strip in it that provides a switching contact that makes or breaks at a given temperature. They are used in lots of electronics equipment. One application is to turn on a cooling fan in an electronic system when the temperature reaches a design threshold. Finally, you can replace the remainder of the parts in the alarm circuitry with one piezo-electric sounder. These are used in fire alarms, and can be purchased as a tiny module that can panel or PCB mount.
So, this alarm can be reduced to a temperature sensor/switch and a piezo-electric sounder. You will still need the LED/resistor combination if you want a circuit on indicator… So, four parts… instead of nineteen.
Now, with all that said, if your intent is to provide a student an exercise in building a circuit that provides a temperature alarm, then go for it. However, redesign the entire circuit. i.e., a circuit that is intended to provide a learning exercise, showing how different electronics parts function, both alone and with others, should employ good design practices.
I might suggest that you change the theme of this website. Your circuits are not very good, and in every circuit that I have reviewed, considerable improvements can be made. Maybe you should present your circuits, with an invitation to the visitor to provide improvements to your circuits, and/or other circuits that will provide the same functionality. That way you will engage your visitor as well as provide yourself with a learning experience.
(I apologize if this post I’m making is a little rough, and needs a little editing. I’m in a huge rush, but I still want to provide some feedback to you…
Am glad to see that you took some time to provide us extra piece of information here. Actually there are several ready made sensor modules which will do a lot better job than this one but this circuit was posted in the intention to help viewers to build this by their own from scratch. As you said design practices are necessary to hone the circuit designing skills.
And about inviting the visitors to improve or add extra piece of information seems to be good idea and surely will try to implement it here. Many thanks for your suggestion regarding this site and circuits as well.