Preheat alarm added to a basic kitchen oven

[Justin] didn’t want to keep checking if the ‘oven heating’ indicator light had gone off before popping his unbaked edibles into the oven. Many models offer a buzzer to let you know when the chosen temp is reached, but for folks who own a basic oven model there’s just a light that tells when the heating element is getting juice. Not to worry, he plied his circuit design skills and built a buzzer to alert him when the oven’s ready.

It only took a few components to accomplish the task. [Justin] uses a pair of NPN transistors triggered by a photoresistor. One transistor is responsible for switching on the buzzer, the other transistor is driven by the photoresistor and controls the base of its companion transistor (see the schematic for a better understanding).

He designed and etched a small PCB to host all the parts. As you can see above, it mounts over the indicator light and is powered by a 9V battery. There’s an on/off switch to the right so the buzzer doesn’t keep triggering while cooking, and a potentiometer allows him to fine-tune the photoresistor sensitivity.


  1. Corey says:

    As a Certified appliance tech, and a HaD troll Its my pleasure to inform you all that the oven is not properly preheated when the heating light turns off, intact most ovens take 20-30 minutes to properly preheat the oven compartment. Toss that stuff back in the parts bin and buy and egg timer :oD.

    • aztraph says:

      AMEN Corey! I too am an appliance tech, and an avid baker. I advise all my customers to start the preheat at least 30 minutes, and when pressed, I tell them I wait an hour. there is no substitute for a nice hot oven.

    • saul_goode says:

      did you even *think* about the hack?

      If you had then you’d understand this hack is using an LED to detect the pre-heat light and sound an alarm when the LIGHT goes out.

      • saul_goode says:

        Wait… I see what you’re saying. You’re saying that even when the light goes out it’s still not up to temp.

        That may be, but this hack still beats putting the food in when you turn it on, or worse turning it on to wait for it to preheat and forgetting you turned it on.

        …and c’mon… when the light goes out then it’s probably close enough for a frozen pizza. It ain’t rocket scientry.

      • aztraph says:

        Saul, the purpose for preheating an oven is to warm up the inner liner of the oven, not the air, if you just heat up the air, which is what the thermostat of the oven senses. or worse, the light is on a delay timer. if you only heat up the air, you loose all the heat the first time you open up the oven door, if you heat up the walls of the oven the temperature will recover quicker and you will be happier with the final product.

        if your going to do that, you might as well set a kitchen timer for 30 minutes as a reminder.

        10 points for the hack, -5 for the ability to cook

        and he uses a photo cell, not an led, check the schematic and build.

  2. DrLuke says:

    Looks pretty crappy to be honest, I wouldn’t want to have this on my oven.

  3. shaeffer says:

    It may just be that I’m a noob, but is there any reason why he couldn’t have wired the buzzer circuit to the light and skipped the photo-resistor? (And have it tucked inside, out of the way)

  4. Drew says:

    My oven doesn’t even have a light :/

  5. kade says:

    you would think that hackaday would be able to identify a PNP transistor correctly, but clearly not…

    i guess they had a 50% chance

  6. truthspew says:

    Neat project. I have gas oven and I do enough roasting and baking in it to know it’s fault and it’s characteristics.

    Takes about 15 minutes to get to 425F. Can’t tell visually without a thermometer, which I keep in the oven.

    That said, I made some sour cream and cheddar shortbread biscuits last night. I turn the oven on when I start the kneading process on the dough and by the time it’s complete, the dough rolled out, cut and placed on the sheet the oven is ready.

  7. Ren says:

    I LIKE it!

    SWMBO (She Who Must Be Obeyed) and I are usually busy in the kitchen, and often don’t notice when the preheat light extinguishes.

    Instead of building this circuit though, I think we’ll just use the temperature probe with the alarm setting. (Something I HaDn’t thought of doing before reading this article.)

  8. vonskippy says:

    Single or is your significant other blind?

  9. cryogen says:

    Creative, but hideous. The WAF on that is a 0. However, this could probably be cleaned up significantly without having to break into the oven circuitry. Take a fiber optic line (covered in matching black tubing) and make an adapter to fit over the light and then pipe the light to the device in a less obvious spot.

  10. Shawn says:

    You mean that’s not factory?

  11. saul_goode says:

    @aztraph – I understand the purpose of preheating. And as I semi-stated earlier… for a frozen pizza this would be better than fine. If baking a cake or something more serious then a more conventional ‘set it and do some prep’ cycle would be better.

    And I know what he used and how, I simplified it in my example to an LED which COULD replace the photoresistor in this build. LEDs can create AND detect light.

    My comments were due to people not being able to just say ‘Neat.’ People have to pick things apart because of their vast amount of experience ‘as a certified appliance tech’.

    Something does not have to be perfect for EVERY application to be a good hack.

    • saul_goode says:

      …Better criticism would have been-

      ‘You shouldn’t store spices along the top of the oven controls. The exposure to the increased heat will dry them out and a) make them taste funny b)reduce the quality c) make them less potent.’

      …but they have bargain brand labels so I’m still not sure that’s all too much of a concern.

      …just sayin’.

  12. Peter Jenkins says:

    Even if you don’t trust the light to tell you when the oven is preheated, there’s still an awesome utility to this:

    You can turn on the oven, flip the switch, and a little while later it will start buzzing until you acknowledge it. That way you don’t walk away and forget you turned the oven on.

    Very nice!

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