Track Those Leftovers With This Little Timer

We’ve all at some point in our lives opened the fridge door and immediately wished we hadn’t. A miasma of stench envelops us as we discover that last Saturday’s leftovers have been forgotten, and have gone off. If only we had some way to keep track of such things, to avoid such a stench-laden moment. Step forward [ThinkLearnDo], with a little timer designed for exactly that purpose.

The operation is simple enough, press the button and place the unit on top of the container with the leftovers in it. If you haven’t eaten the leftovers within a week, the LED will start blinking. The blink is a subtle reminder to deal with the old food before it becomes a problem.

Onboard is a Holtek HT68F001 microcontroller with a coin cell for power, not much else is needed. The Holtek is an unusual choice, one of several brands of super-inexpensive Chinese microcontrollers we see less commonly than ATmegas and STM32s. This is exactly the place where such a minimal computer fits perfectly:  a way to add a little bit of smarts to a very cheap item with minimal strain on the BoM.

If these chips interest you, a while back we covered a run-down of the different families including the Holtek and the famous 3-cent Padauk chips.

27 thoughts on “Track Those Leftovers With This Little Timer

  1. Use an esp8266 and send reminder messages to your cell phone.

    The little Holtek is cool though, but I have over 100 18650 laying around, so I could take a different route.

    1. Here’s a question though – do you need the complexity of a microcontroller to do this? Could it be done with a 555? I know Hackaday love a good 555 project!

      1. Actually a 555 isn’t so good for long times. Exar had a device, I think the XR2240, that was an IC timer followed by a long divider. So you didn’t need large values for the timing components.

        I doubt it’s still available, but a 555and cmos cku ter would do it.

      1. It’s not so bad as you would think, mu 433MHz temp/humidity sensor doesn’t have any problems getting the signal through to the RFXcom receiver. I use it for alerting when the door isn’t properly closed and the temperature gets to high.

  2. I thought about building something like this many times. How to proof it against the temperature and moisture in the fridge? How many would I need? I’d need it programmable for different numbers of days…

    …and then I went back to using a dry-wipe marker.

    The reason boxes sometimes don’t get dates on them isn’t a lack of blinkenlights, it’s laziness.

    1. Very good points for reverting to marker with different days. As far as temp/moisture, I think the Fridge condenser pulls moisture out of the air inside to generate the cool air, but you could always throw the apparatus in a ziplock. The temp I would assume wouldn’t break anything but it might affect battery life. With a super cheap chip it is worth the experiment though.

    2. Spray the pcb with conformal coating and it will survive the refrigeration environment no problem. Coincell batteries don’t enjoy being refrigerated but they do last if you keep the load low enough.

      Source: I design refrigeration sensors (including Coincell based ones).

      1. “Spray the pcb with conformal coating and it will survive the refrigeration environment no problem.”

        Spray the food with PCBs, and it will survive long after mankind vanishes!

    1. Masking tape and a sharpie? I have reusable coloured discs representing each day. When the day comes around again, the food is too old.

      Congrats on generating more waste.

  3. My nose is very well suited for detecting fridge food thats beyond eatable, but i can see the usecases in other scenario’s, for instance a reminder for medication you have to take every 24 hour.

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