detect kitchen stove in use, turn on fan

[Roland Alden]

I want to detect cooking on a cooktop/stove so that a circuit can automatically turn on the fan of an overhead ventilation hood. I have looked at heat detectors and smoke detectors made for fire detection and find they are not sensitive enough. They are cleverly designed to not be fooled by the rate of rise from a stove or steam or even the smoke coming off cooking food. In spite of the fact that everyone in the smoke alarm industry says don't install a smoke alarm in the kitchen, I find them to be relatively immune to cooking smoke. I have looked at HVAC thermostat type controls thinking a probe inside the hood and another probe away from the cooktop but up on the ceiling might work. A comparator would fire if the difference between the two probes was > n and the hotter probe was the one in the cooktop/chimmney. However, no HVAC thermostat actually can do that comparison; they have different ideas about what to do with what they are measuring. I have also looked at motion detectors and maybe PIR detectors but the commercially available ones are 1) very wide angle and would trigger even if someone were just in the kitchen and not cooking; and 2) are designed to not be fooled by heat that is not in motion. Preceisly wrong for detecting a pot on a stove coming to a boil. I have thought about detecting that the stove is "on" by installing either a flow meter on the gas line or draw on the AC line. This would work except for the fact that I need the detection location to be up on the ceiling and not down on the floor because of wiring limitations. I could go with wireless I guess. However just turning on the stove and then having the fan start is not as good as actually waiting until some significant amount of heat/smoke starts to rise. If you are just boiling a cup of water it might be annoying to have the fan come on the moment you hit the on switch. There are some solutions I could hack together with some fancy laboratory sensor units feeding data to a computer but these are all overkill / too expensive. All I need to do is close a dry contact relay up on the roof. Any ideas?

 

[Roff]

Try searching for ir thermometer with digital output. Melexis has some products that might work.

 

[MikeMl]

How about a current transformer feeding a low-voltage AC relay.

 

[Roland Alden]

You mean on the stove itself? To detect that it has been turned on? Actually right now I hava gas cooktop so I would need a flow meter. It would work when I get the big $ induction model my wife wants; but it suffers from the problem I mentioned. I don't really want the fan to come on instantly before there is really a reason to turn on (humidity/smoke rising off cooktop).

 

[Roland Alden]

Looks like Exergen Corporation makes some interesting products. A K type thermocouple / 3.5 – 14 Kohms, varies depending on temperature range. What I would need to add to that would be a circuit which watched for sudden temperature rise over an adjustable period of time (could just a trim pot on the pc board) and then send 12v to my relay (or close a relay).

 

[Roff]

The reason I like the Melexis parts is that they can remotely sense temperature, and they have a PWM output which can be easily processed in either analog or digital form. They're not inexpensive, and you would probably need some sort of lens, although I'm not sure.

EDIT: The response time is pretty much instantaneous, but you could easily filter or delay it. The sense the actual temperature of what they are pointed at, so you wouldn't have any trouble detecting a hot stove. Nothing else on the stove would get that hot unless your house were on fire.

 

[Roland Alden]

The important thing is that absolute temperature is not really that important. It is rise over a brief period that matters. I am sure it is easy enough to calibrate the sensor and make a circuit that triggers when it reaches some temp but what I need is a timer that stores a value, waits some amount of time, gets a fresh value, and fires if they are far enough apart in the correct direction.

 

[Roff]

The important thing is that absolute temperature is not really that important. It is rise over a brief period that matters. I am sure it is easy enough to calibrate the sensor and make a circuit that triggers when it reaches some temp but what I need is a timer that stores a value, waits some amount of time, gets a fresh value, and fires if they are far enough apart in the correct direction.

I understand what you're saying, but if you're looking directly at the stove temperature (as opposed to the temperature in the flue, or hood, or whatever you have), then, as I said, absolute temperature is enough information, as the stove will be much hotter when it's on than when it has been off for a while. For example, the one Melexis part I looked at had range that topped out at 125C. All you would have to look for is a temperature above, for example, 100C. How would the stovetop get to 100C if it wasn't on?
I'm not trying to talk you out of the path you seem to be on. I just want to be sure you understand that there is an alternative.

 

[Roland Alden]

...All you would have to look for is a temperature above, for example, 100C. How would the stovetop get to 100C if it wasn't on?

I agree and see your point. I am now wondering if I can get a sensor that is robust enough to mount inside the hood, perhaps at an outer edge, where it would be subject to a certain amount of smoke/grease over time? Or if I should really try to mount it significantly outside and aim it at the cook top? An entirely different option would be to mount it in a j-box near the cooktop and possibly aim it upwards and have it measure the temp up inside the hood itself as opposed to the pots on the cook top. In all cases I need a circuit that will close a contact or allow some current to flow when a (slightly) adjustable setpoint is detected.

 

[Roff]

Are you able to do the circuit design?

 

[Roland Alden]

No I'm not smart enough to do that although I could probably solder together the parts on a breadboard well enough. I was also thinking someone might know of some kit that would actually have what I need. Maybe something like this **broken link removed** could be adapted or there might be some flavor of such a thing that I could handle as a DIY project. Of course the idea is sheer genius and I assign all patent rights to whomever will build me one for $20