Fire Alarm with thermistor

[ImpulsE041993]

hello is der sumbody know how the circuit flow in this fire alarm?and also all d conditions on how the circuit behaves??? i got this circuit @ **broken link removed**

 

[Reloadron]

hello is der sumbody know how the circuit flow in this fire alarm?and also all d conditions on how the circuit behaves??? i got this circuit @ **broken link removed**

What part of the provided explanation are you not understanding?

Circuit Operation:

Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio frequency band. Switching transistors Q1 and Q2 drive multivibrator IC1. The output of IC1 is connected to NPN transistor Q3, which drives the loudspeaker (SPKR) to generate sound. The frequency of IC1 depends on the values of resistors R6, R7 and capacitor C2. When Thermistor TH1 becomes hot, it provides a low-resistance path to extend positive voltage to the base of transistor Q1 via diode D2 and resistor R3. Capacitor C1 charges up to the positive voltage and increases the ‘on’ time of alarm. The higher the value of capacitor C1, the higher the forward voltage applied to the base of transistor Q1. Since the collector of transistor Q1 is connected to the base of transistor Q2, transistor Q2 provides positive voltage to reset pin 4 of IC1. R5 is used such that IC1 remains inactive in the absence of positive voltage. D2 stops discharging of capacitor C1 when the Thermistor connected to the positive supply cools down and provides a high-resistance (10k) path. It also stops the conduction of Q1. To prevent the Thermistor from melting, wrap it up in mica tape. The circuit works off a 6V-12V regulated power supply. D1 is used to indicate that power to the circuit is switched on.

Ron

 

[koolguy]

hello is der sumbody know how the circuit flow in this fire alarm?and also all d conditions on how the circuit behaves???

Hi,

better to use comparator then 555 chip start with 741 op-amp.
make Wheatstone bridges measure input terminal with multimeter( + and -ve of amp inverting and non-inverting)
add series resistance with the input as:
R1 + Rrse....
a changes in thermistor resistance will change output due to op-amp high gain...!!

 

[Reloadron]

This link will help explain what RITESH KAKKAR is getting at with the use of comparators. However, the original circuit using the 555 as an astable multi-vibrator drives a speaker for an audible alarm when the temperature limit is exceeded. The comparator circuit method would require an audible device and driver. Based on your original post I believe you are learning in the early stages?

Ron

 

[ImpulsE041993]

how dis circuit behaves when there is no fire?

 

[ImpulsE041993]

What part of the provided explanation are you not understanding?



Ron

i want to know f how this circuit works when there is no fire detected, i mean the circuit flow

 

[xo0x]

when there is no fire the alarm will not sound.

xo0x.(2hrs!!)

 

[Boncuk]

The circuit will be silent if there is no fire.

With the given values the alarm will sound at 300Hz. Changing the values as of the attached circuit the alarm sounds at approximately 3KHz.

Current consumption is about 10mA if you use a low current LED as indicated in the schematic.

Current flow will increase to approximately 40mA when the alarm is activated.

Boncuk

 

[Reloadron]

The circuit does not sense fire or smoke. It senses heat so detection of fire is by sensing heat. When there is no heat exceeding a preset limit T2 is off and pin 4 (the 555 reset pin) is low so the 555 is inhibited from operation. Thus no sound. The frequency of the sound can be changed as Boncuk mentions. The 555 functions as a astable multi vibrator so when it is on there is sound and when it is inhibited (Reset low) there is no sound.

Ron

 

[4pyros]

how dis circuit behaves when there is no fire?

Nicely, it just sits there using up power.

 

[lilimike]

It might be a good occasion to point out that this circuit should not be used as a home safety device to alert in case of a fire. I think by the time this device gets activated and sounds the alarm, all occupants of a house will have died of asphyxiation caused by the smoke.

Mike

 

[davenn]

It might be a good occasion to point out that this circuit should not be used as a home safety device to alert in case of a fire. I think by the time this device gets activated and sounds the alarm, all occupants of a house will have died of asphyxiation caused by the smoke.

Mike

very good point !! nothing beats proper smoke alarms

Dave

 

[Boncuk]

very good point !! nothing beats proper smoke alarms

Dave

Smoke alarms are only useful if a proper smoke venting system is installed!

That circuit just senses overtemperature - and that's what the thread is about.

Boncuk

 

[davenn]

Smoke alarms are only useful if a proper smoke venting system is installed!

That circuit just senses overtemperature - and that's what the thread is about.

Boncuk

no he's talking about a fire alarm ... read the subject line and depending on how the OP intends to detect a fire ( increase in temp) means that a smoke alarm will probably give a much earlier warning

smoke alarms work in any situation where smoke is generated ... vented or not !! dont you have them in your own home ? you should
my alarm often warns me of my poor cooking and great charcoalling abilities



D

 

[Boncuk]

no he's talking about a fire alarm ... read the subject line and depending on how the OP intends to detect a fire ( increase in temp) means that a smoke alarm will probably give a much earlier warning

smoke alarms work in any situation where smoke is generated ... vented or not !! dont you have them in your own home ? you should
my alarm often warns me of my poor cooking and great charcoalling abilities

D

I read the subject line thoroughly. The OP is talking about fire alarm. No doubt that smoke goes along with a fire normally.

So why bother with a smoke detector if the OP wants a fire alarm based on temperature measurement?

My home has both, smoke detectors and a smoke venting system.

Further there is a circuit which allows to cook only if the kitchen exhaust fan is running, the kitchen door closed and the window open.

Frying red chili almost bursts my lungs. But it's the Thai way to cook.

You're not the only poor cook on earth. So don't be sad.

 

[lilimike]

Boncuk,

I understand your point, not sure you understood mine.
I was just pointing out that using the fire alarm (temperature sensing) as the only home safety device is dangerous because fire burns and smoke kills and the bad part is the smoke comes a long time before the temperature sensing device will sens any temperature rise.
Unless of course if the smoke comes from the device itself but then again you would get no alarm and lots of smoke.

Mike

 

[Boncuk]

Boncuk,

I understand your point, not sure you understood mine.
I was just pointing out that using the fire alarm (temperature sensing) as the only home safety device is dangerous because fire burns and smoke kills and the bad part is the smoke comes a long time before the temperature sensing device will sens any temperature rise.
Unless of course if the smoke comes from the device itself but then again you would get no alarm and lots of smoke.

Mike

Hi lilimike,

I understood your point fully, having been working in the field of air movement in large buildings for 18 years. Smoke venting is very important without any doubt and that's why I used smoke venting fans which have been tested to run for three hours (encapsuled with active fresh air cooling) at temperatures of 600degC.

However, the subject is fire alarm, which can be triggered by extremely high ambient temperatures. Not every hobbyist can afford a smoke detector and that's certainly a point to think about as well.

Here is a drawing of a smoke venting fan.

Regards

Boncuk

 

[davenn]

So why bother with a smoke detector if the OP wants a fire alarm based on temperature measurement?

because smoke begins long before flames and by the time heat from the flames is detected its already too late, you have died from smoke inhalation
smoke detecting is an earlier ansd much safer warning

Not every hobbyist can afford a smoke detector and that's certainly a point to think about as well.

good grief!!! complete and operating smoke detectors are only $10 !!


D

 

[Boncuk]

good grief!!! complete and operating smoke detectors are only $10 !! D

Good point!

When I was young I had to distribute 500 daily newspapers for one month to make $10.

 

[Reloadron]

The OP has started a few threads and each was more of a how does it work circuit explanation. I might be wrong but I do not see the OP actually building any of this stuff.

Just My Take
Ron