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It looks the same, but what has changed is on the inside.

  • Prototyping, Electronic Gadgets, Experiments, Fridge Door Sensor

After surviving for around 35 days on the fridge, the LED light of the fridge door sensor that we created seemed to be getting a little dim, and the alarm was getting softer. The battery seemed to be getting weaker.

This posed the question: Could we get the device to last at least a year before the battery needed to be replaced? Before we could get there, we first needed to understand how much power was being consumed.

And so we wired up the multi-meter to do the tests:


Before: Theoretical 41 days on battery for Version 0.1

When the door was closed: 2mA
When the door was open: 10mA
When the door was open and alarm buzzer sounding (after 20s): 12mA

Doing some quick calculations, assuming standard 2000mAh AA batteries, the device would theoretically only last for around 41 days with the door closed, and shorter still if the door was left open for long.

Hence, we got to work to replace the components to less power hungry ones. After testing individual components, it turned out that the Hall-Effect Sensor was consuming most of the power when the fridge door was closed; hence we swapped out the Hall-Effect sensor for a reed switch instead. We also replaced the LED with a lower powered one, added some resistors, and also updated the software logic and the sleep mode for the ATMEGA328P microprocessor chip to achieve more power savings. This resulted in a ten-fold reduction in power consumed especially when the fridge door was closed. And since the fridge door should be closed for most of the time, the impact was significant!

Fridge Door Sensor v0.2
Version 0.2: It looks the same, but what has changed is on the inside

After: Theoretical 555 days on battery for Version 0.2

When the door was closed: 0.15mA
When the door was open: 8mA
When the door was open and alarm buzzer sounding (after 20s): 11mA

Doing some quick calculations, assuming standard 2000mAh AA batteries, the device may now last 555 days with the door closed!
Now if we just needed it to last a year, we should still have 686mAh [2000-(0.15*24*365)] of power left for when the door is open or when the buzzer sounds.

Whether it lasts a year or not will depend on how long the door is left open.

If the alarm were to be left sounding non-stop, the device would probably only last around 7 days. Though this would mean that the fridge door is left open for days with the buzzer beeping and all the food inside would be rotten!


Ideas for future work

  • Automatically notify the owners' mobile phone(s) or connected device(s) when a fridge door is left open.
  • Detect when there is an action to close the door. So the alarm can turn on if shortly after the event, the door is still not closed. Instead of waiting for 20 seconds.
  • Package it smaller and neater. For production use, the ATMEGA328P chip could also probably be replaced with a micro chip.
  • The data could also probably be visualized to see patterns of fridge usage, and trends of when the door is left open.

But for now, it is to be seen whether version 0.2 will really stay up and running for the next 365 days!

It looks the same, but what has changed in this iteration is on the inside.
May what is on the inside keep changing for the better!

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