Creating a simple device that tells you when your dishwasher has been run

Thread Starter

Chicago357

Joined Feb 9, 2020
3
Hello -- My daughter is doing an invention fair project in which she aims to create a simple analog device that can magnetically attach to the exterior of a dishwasher and that will contain a light indicating when the dishwasher has been run. This light will go on when the surface temperature of the dishwasher reaches a minimum temperature and then must remain on until it is manually reset (eg, whenever someone comes along to empty the dishwasher).

She has found a small surface mount disc thermostat that closes on rise (normally open) at 85 fahrenheit (https://www.alliedelec.com/product/selco/ca-85/70098628/) that seems like it would provide a good temperature sensor. The only problem is that it has auto-reset, meaning it will re-open once the temperature drops back to normal again. She can't find anything equivalent to this that has manual reset. Of course, if she uses an auto-reset switch, then the light will go off as soon as the temperature of the dishwasher drops back down, which basically makes her gadget pointless.

Is there a way to set up the circuit in such a way that would allow the light to stay on (with manual reset) even after the temperature drops (and the temperature-sensing switch goes back to open)? Is this where a "relay" would come in?

Thank you for any help or suggestions you can provide!
 

WBahn

Joined Mar 31, 2012
25,225
I don't think you will be happy with 85° F as the set point. Does your house (or the house of anyone that might use this device) ever get above that temperature on a hot summer day?

A part of your daughter's process should involve some investigation of what the temperatures of dishwasher doors actually get to, keeping in mind that there are lots of different dishwasher makes and models and they are going to vary quite a bit.

They make purely mechanical thermal switches that have a popout flag when they trip and that must be manually reset. Perhaps designing the device around that is something to be considered since it gets rid of anything electronic and the associated power requirements.
 

WBahn

Joined Mar 31, 2012
25,225
Another thing to consider is the target user of the device. Why would someone want this, how are they expected to use it, and how might they actually use it?

For instance, it seems to be targeted either at people who are likely to forget that they ran the dishwasher or people for whom it might be useful to know at a glance that someone else has run the dishwasher.

Okay, so now how does this device help them and how are they expected to use it? Let's say that someone runs the dishwasher today and I come home tonight as see that the sensor is tripped? What does that get me? Let's say that it allows me to look in the dishwasher first for my favorite coffee cup. After getting the coffee cup out, what am I expected to do? Reset the device? Or leave it tripped since most of the dishes that were washed are still in there? So when does it get reset? Only when the dishwasher is completely emptied?

How reasonable is it to expect that people will do what is expected of them consistently and reliably, and what are the consequences if they don't? For instance, that they will only reset it when they empty the dishwasher AND that they will always reset it when they empty the dishwasher? How many times will it take people NOT doing what is expected before people lose confidence that the device is telling them anything useful? Either that it is showing that an empty dishwasher (or a dishwasher that is full dirty dishes) has been run because someone forget to reset the sensor or that it is showing that a dishwasher full of clean dishes hasn't been run because someone took out one thing and reset the sensor?

Considering these kinds of issues and addressing them could set your daughter's entry apart from the competition, most of whom will be too caught up in the tech to consider the human factors.
 

Thread Starter

Chicago357

Joined Feb 9, 2020
3
@WBahn Thanks so much for this helpful reply! I was not aware of the type of device you described. That would be an interesting approach and might allow her to incorporate other user-friendly elements in her product design rather than having her attention be consumed by the electric circuitry. Do you happen to know of any websites that would sell such a thing or brands that make them? I just did a quick search using the term "mechanical thermal switch with popout flag surface mount" but couldn't find any results that looked like what you were describing.

Separately, I agree completely of course with your point about different dishwashers having different surface temperatures, and room temperature varying a lot by house. I'm not sure she'll be able to solve for that in the context of this project, but certainly that would become a high-priority refinement as soon as she has created something that works on at least one dishwasher (ours). :)
 

Thread Starter

Chicago357

Joined Feb 9, 2020
3
Another thing to consider is the target user of the device. Why would someone want this, how are they expected to use it, and how might they actually use it?

Considering these kinds of issues and addressing them could set your daughter's entry apart from the competition, most of whom will be too caught up in the tech to consider the human factors.
@WBahn Wow, thank you very much for these additional thoughts! Those are indeed very good points for her to consider -- both in her product design and in her discussion of the product and its own limitations. Very helpful!
 

Alec_t

Joined Sep 17, 2013
10,902
I don't think you will be happy with 85° F as the set point. Does your house (or the house of anyone that might use this device) ever get above that temperature on a hot summer day?
Agreed. A way round that would be to have two temperature sensors; one for ambient and one for the machine. A latching indicator could then be triggered only when the machine temp goes significantly above the ambient temp.
 

GetDeviceInfo

Joined Jun 7, 2009
1,680
Maybe, a magnetic disk that has two colours, one on each side. Whoever starts the washer, flips it to the clean color. Who ever empties the washer, flips it to the dirty side. Marketing may say use a catchy phrase on each side.
Putting dirty dishes into a cycled machine just seems like such a giveaway.
 
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