Never Forget To Turn On The Cooker Hood Again

The cooker hood is a wonderful invention for removing excess fumes and steam from the kitchen. But like all electrically-powered devices, it only works when it is turned on. This was the problem facing [Peter], whose family are enthusiastic cooks who frequently forget to hit that switch. His solution? An automatic cooker hood switch that comes on when the cooker is in use, and stays on long enough afterwards to fully dissipate the fumes.

At its heart is a current transformer on the 3-phase stove power line, and we’re treated to a lesson in reading from these devices with an Arduino. They have a shunt resistor across which to produce a voltage, and their AC output is placed upon a reference DC voltage to supply the microcontroller pin. The impedance is quite high, so when the sensor had to be placed a distance from the microcontroller it necessitated an op-amp buffer. The readings then cause the Arduino to trigger a pair of relays to switch on or off the cooker hood. We can imagine that the family kitchen is thus a much pleasanter environment for it.

Cookers can also provide quite a hazard when they are left on. To that end, we’ve also featured a cooker alarm in the past.

Header image: Pbroks13, CC BY-SA 3.0.

34 thoughts on “Never Forget To Turn On The Cooker Hood Again

  1. Wouldn’t really work for the gas hob in your little photo there though, the current draw would be very low for lighting the gas.

    Great idea for those with electric cookers tho!

    1. The article says there was 3-phase power available for the stove, and doesn’t have that picture of a gas stove. Draw your own conclusions- 2020 wore me out explaining things.

  2. My hood is very loud. The fume extractor in my kitchen is very quiet, though. You can still hear the neighbors yelling when it’s on. That said, I would really prefer it to turn on when there is smoke, not every time the stove is on.

      1. That sounds like a good idea. Judging by how greasy and sticky the hood becomes, some extra steps will be probably needed to keep it working reliably after a few months. Moreover, when smoke or steam hits the inside of the hood by natural convection, it might be a tad too late. Sounds like a challenge!

  3. Is an arduino even necessary? other than keeping the hood on for some time after the cooking? Seems like with the amount of current that an electric stove would pull and an appropriate CT, one could just directly trigger a mosfet to turn on directly off the CT? One could probably implement the delayed shut off with an appropriate resistor/cap across the CT to hold a voltage for some 5-10min after the stove was shut off and the voltage dropped from the CT.

  4. My experience with hoods of the type shown (that’s the type most commonly found in US homes and apartments) is that they are very noisy and almost completely useless. Most of the vapor/smoke never gets sucked into the thing, and in only a few installations does the air get vented outside the home. In most it just goes through a grease filter and blows back into the kitchen. For all these reasons, I almost never turn mine on.

    I had a down-draft cook top that vented outside the house once. It was even worse. Most of the smoke/vapor never got drawn downward into the exhaust and just filled the kitchen.

    Range hoods and bathroom exhaust fans that sound like vacuum cleaners are two of my biggest peaves about the way houses are built. I don’t want to listen to those things, especially considering how ineffective they are.

    Now get off of my lawn!

    1. Yes – I’ve lived in 5 places so far with stove hoods, and all of them have been completely useless. I can blow out a candle beneath them and watch the smoke literally just go right by where the smoke should be being pulled into the hood. Surely we can’t be the only ones with this problem – does the average person just not think about it or care? That seems crazy to me – it’s such an obvious failure you can watch in realtime.

      One of my wants in an eventual house is a commercial grade extractor venting to outside so I can for once cook smash burgers without smoking the entire house out.

      1. One of the first things I did when I moved into my house was to replace the crappy little fan with a 600cfm fan on a dimmer. It’s loud when fully on, but we rarely need to fully vent the kitchen ~every minute. When it’s on low, the biggest issue is it being left on overnight because it’s so quiet

    2. What you should consider is the fact that maybe you did not open a small window to let in fresh air. If the hood is trying to vacuum your home, there is a point where the pressure outside is exact the same as the sucking power of the fan and the airfow stops. Same with all things with chimneys without their own air inlet from the outside like fireplaces.

      Its just like electronics: you need two poles to make a circuit.

      That is why new homes have hoods without an external outlet. And of course you clean your filter regulary, like your keyboard. Don’t you?

      1. This is a wise observation. We spent a few years replacing all the windows and doors on our house, sealing cracks in the rim-joists, and spray-foaming around the inside of that wood structure for insulation. The result now is that our home is extremely well sealed, when you turn on a fan that vents outside the fan just can’t move air because there’s no place for ambient air pressure to displace what the fan is trying to move out. Crack a window for a few minutes when you turn the fan on and it immediately fixes the problem.

        The crazy thing is that we ended up needing to install a heat recovery ventilator (HRV) because there was nowhere for humidity inside the home to go. This is ventilation that brings in fresh air from outside, exhausts stale air outside, and the two pass through a heat exchanger so the outgoing air heats (or cools in the summer) the incoming. The HRV works great but it was an unexpected expense as part of sealing up the house!

        1. I had a friend who built a house with that type of system. The bathroom and kitchen exhaust worked through it. The blower and heat exchanger were in the basement, far from the bathroom and kitchen. When the system was on, you didn’t hear it but it cleared vapor from the shower quite well. IRIC the kitchen still had the standard too-small hood to capture much of the smoke/steam from the stove.

          I don’t understand why the crappy little exhaust fans aren’t placed at the exhaust side instead of placing them in the kitchen or bathroom at the intake side. It would be a lot quieter…

          1. The reason exhaust fans are placed at the intake instead of the outlet is because they are much more efficient that way. They are much better at creating positive pressure than negative. When you attempt to “suck” air in, you are actually creating a low pressure zone and allowing atmospheric pressure to move the gasses. The problem is that these gasses try to come in from everywhere into the fan, even around the fan blades. So if you placed the fan at the outlet and tried to suck air through the ducting, you’d get very little air to move through the ducting because of air friction while recirculating the air around the fan. If you instead place the fan at the intake, you are fee to suck in all the air you can get from the room. Then you create a high pressure zone that pushes the air through the ducting and out.

            There are compromises one could make, like placing the fan somewhere midway along the duct, and using a higher tolerance fan with less air circulation, but all of those add to the cost with a marginal effect on the noise.

    3. You need to purchase a silent rangehood – that is what they are called in Australia- we have a Schweigen one that has the motor on the roof of the house and draws 1600 cu/m3 of air extraction. No fumes, no grease, no grime and no noise. They also make a bathroom exhaust unit that is also strong and silent.

  5. Put an inductive current sensor around the cord to the stove. Circuitry as appropriate to close a relay to turn the fan on, with a capacitor and appropriate circuitry to charge it and when the stove is off, make the capacitor drain slow enough to hold the relay on for some time so the fan continues to run for a bit.

  6. I think a CO2 sensor would be a nice option. I’ve been on an indoor air quality kick lately and any time a gas stove or gas oven on, without an effective exhaust fan, the CO2 rises across the kitchen, very clearly (it’s where my CO2 sensor is mounted now). The ideas about VOC or PM sensors wouldn’t turn on when a pan is being heated. The temperature based concept, mentioned above, seems pretty neat. Cheap and I don’t see why it wouldn’t work.

  7. The USA needs to get the Schweigen brand of Rangehoods/ extractors from Australia…they would solve all the issues that everyone is complaining about … silent in the kitchen….super strong to take ALL the smoke smells and grease out of the kitchen ..easy to clean filter…..15 minute delay timer that automatically shuts off 15 minutes after you finish cooking….LED lighting……fantastic brand and product.

  8. I like to cook and spent some time upgrading range hoods in several places where I lived and wanted to share some thoughts. The most important one is to have a good range hood itself. No amount of automation will fix a bad hood. A powerful and quiet one is a must. I had one hood that could hold a 12 inch glass pot lid with the sucked air and it was a pretty cool trick to show how a large lid just gets stuck to the top. It really had to do with the round filter it had in addition to a powerful fan, common baffle filters cannot do that.

    Building code for new construction in my area requires powerful hoods to have a makup air system. This has to do with the negative pressure created inside the house if the hood is running and all windows/doors are closed. Living in a place that didn’t have a makup air system definitely felt with added pressure on opening the outside door when a powerful fan hood was running. The way a makup system works: there’s a current sensor on the hood fan itself and that turns on the fan to bring outside air into another part of the house. There’s also a 40A heater to warm up the outside air if it is too cold. Since range hoods have different speeds, the current sensor gets adjusted to run the makup air only on high speeds. Unexpected uses for this system system became to quickly cool down the house in summer if the air outside is cooler than the inside.

    As for the specific timers and automation for the hood, I came to appreciate a simple mechanical switch that turns the fan at different speeds. It is hard to automatically determine the proper speed and have an intuitive manual override. Sometimes I want a high speed even if nothing is cooking (say, to vent a pressure cooker) or to just air out the smells from cooking something else. Sometimes I leave the fan running on low for a long time. Sometimes I turn the fan off or put it on low even while cooking just because I may need to get on my phone and want to minimize all the noises. Those are not the exceptions but rather common cases. A system that shorts the switch because it determined that the fan needs to be on would not work for me personally. I did have a hood that had some integrated timer and I used it a couple of times to run it for 30 minutes, but the extra thinking required of turning that timer on and thinking is in on the permanent on or on a timer was just not worth it. Having a well-placed, intuitive switch for the hood fan pretty much meets my requirements. And the lack of that intuitive switch is likely the reason why people don’t turn it on, not the lack of automation.

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