I'm trying to figure out what sensor is best to detect a candle flame? an IR sensor? or thermal sensor or is there any idea i don't know?

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Depending on the location of the candle (remote or reachable), and how long it will be unattended for...

A thermocouple in or just above the flame could be cheap and sufficient (depends on if the candle will burn away from the thermocouple and so give a false alarm). A thermistor could be used but will soon fail if overheated.

An IR detector (possible IR phototransistor or even a photoresistive cell) as used in opto cable links could give a useable signal (basically working as an infra-red beam alarm, the candle being the source).

A photoresistive cell could also be used in the visible light spectrum if there isn't too much background light.

If you can find them there are also Ultra-violet detectors (look like large neon indicator bulbs - glass envelope) available, requires something like 60v across them, any UV causing the gas filling to break down and so collapse the voltage - a visible light is also emitted but the voltage is not high enough to sustain the glow without the UV source (too messy to set up at home?). - Yes these are used in industry !

How about a bi-metallic strip (pulled out of an old car flasher relay) heated by the flame, the bending caused could make/break a contact.

Exotic (OTT?) a sample pump pulls a gas sample from above the flame and feeds it to a Carbon Monoxide (CO) detector. The CO will fall when the candle goes out.

HT across a spark gap placed in the flame - the flame will ionise the air and cause a current to flow across the spark gap; no flame will result in no current (a small spark gap will require several kV at a very low current).

A sealed vessel above the flame will be pressurised by the fluid (gas or liquid) trying to expand inside it - the basis of a thermometer. Temperature switches and indicators are available that use this system and it is possible to make one from a length of microbore copper tube as sold for central heating (smallest bore available is best). This is mechanical rather than electronic but something like this will be at the heart of your hot water immersion heater, central heating boiler (and its room thermostat), kettle, fridge, blah blah blah.

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What about Thermistors? Cheap and Better :wink:

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What if i use a thermistor to detect it's temperature and then an IR phototransistor covered with a layer of film? is it nice?
Anyway, a thermistor is a temperature sensor rite?

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A thermistor is a resistor, sensible to heat, which will change it's resistance factor (how elegant). No I can't write english but I hope you understand what I mean.

With some sort of trigger, like a normal transistor, the thermistor could be attached to the base in serial with a proper resistor (perhaps a variable one?) and will switch it on and off ...

Sorry, hard for me to find all the technical words for each thing, but there really are some great deisgn schemes of this out there... I just have book versions so try searching for something nice on google.

Albert

 

First you make a hole through the candel with a
hot needle, put a thin wire trought the hole and
connect it with a relay.
And put some voltage on te wire.
The relay has to swith to normaly closed and so
activating an alarm.
I think it is the same principle of the light that goes
on in your refrigerator. :mrgreen:

 

What is this Monaco? :lol:
Do you think a wire alone will switch the relay on/off depending on temperature of Candle?

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Sounds to me he's trying to burn the wire with the candle.. can work, but then the current from the battery would burn the wire before the candle would ;-)

Albert

 

LOL :lol:

Seems Monaco is talking about THERMOSTATS in the refrigerator and he thinks that is a mere piece of metal wire. No Monaco, its not a piece of wire but a Bi-Metallic strip which cuts-off the circuit when preset temperatue is reached.

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Yes, thec that's what i mean.
(when the wire is burned away it should trigger a alarm.)

No, kinjalgp i am not talking about the thermostats in
the refriderator but the light that goes on when you open
the door and see what's inside the refriderator.
The swich (close to the hinge-point of the door) activates
this light. Then you can make something that operates a
relay or relay's.
But maybe there is another sollotion:
Twist two thin wires in each other (don't remove the isolation) and connect wire 1 on
a belltransformer. Connect wire 2 on a change-over swich and close the swith.
The other end of the swith is connected to a bell and the other end of the bell is connected to the belltransformer.
When the isolation of the twisted wires are burning, you will hear the bell.
Press the switch to stop the bell.
(There is a disadvantage: the wires will still burn. burning wires could set your house in fire)

The question of ahkumm was:
how to detect a candle flame
Why are you want to detect this? Do you want to prevent that your flame
burns to much of the candel, and so burning up your homemade
cristmasdecoration with pinecones, pine branches.

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Do not take me too seriously 8)

 

Ok.
So you were talking about Push-To-OFF type switches that are used in refrigerator doors.

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"lol" up your ass kinjalp, I know I was right ;-)

Just kidding with you of course.

Albert

 

ahkumm, can you tell us more about your application ...

Is this a demo to prove you can detect when the candle gets blown out?
Could the candle be left burning unattended for several hours (thus getting significantly shorter) ?
Is the key here to say "the candle has been extinguished" or "the candle is almost burnt out" or "danger - flame still burning" :?:

The above questions may help to select the most appropriate method of detection (and get us back on the thread :wink: )

My favourite is still the thermocouple - much more robust than a thermistor (which I feel would soon 'cook' in the flame). A simple thermocouple could be home-made and feed a signal to an op-amp.

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well... for reability I would definitvely buy this component... I don't think they are _that_ expensive.. are they?

/Albert "thec" Sandberg

 

Hey guys, sorry for all the confusions....actually I wanted to build a fire fighting robot, that's why I wanted to detect a candle flame....I just wan to know which method is the best for that , simple and cheap?

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