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

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Chicago357:
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!

bob91343:
A simple latching circuit will work.  The simplest concept is a relay that, when it closes, shorts the thermostat with one of its contacts.  To reset, break that short with a pushbutton or whatever.  For simplicity, have the relay coil the same voltage as the lamp that indicates the machine has run.

IDEngineer:
This screams for a simple, mechanical, non-electronic solution. Perhaps a bimetallic strip biased by a spring, such that it triggers one way due to temperature but the additional unidirectional force from the spring yields hysteresis that requires manual reset.

RoGeorge:

--- Quote from: Chicago357 on February 10, 2020, 05:01:46 am ---Is this where a "relay" would come in?
--- End quote ---

Yes, there are latching relays, similar with normal relays, except they remember their last state even when not powered.

Alternatively you can make a normal relay to latch, but it will come back to its normal state when unpowered.

Ian.M:
A relay and bulb will be fairly power-hungry compared to a LED and electronic latching circuit.  That means the gadget will need a much larger battery, as the relay coil will be on all the time the bulb is.  Consider using the simplest electronic latching device, a Thyristor aka: SCR (Silicon Controlled Rectifier) for the latching circuit as they hold on while load current is flowing, without needing any coil current.  Just about any TO-92 package SCR would do the job, e.g. the NTE540x series should be readily available in North America, e.g. https://www.alliedelec.com/product/nte-electronics-inc-/nte5400/70215202/ or can be found on Amazon.com.   You'll also need a few resistors, one to limit the gate current, another as a gate pull-down to prevent electrical noise causing false turnons, and one to limit the LED current.  Ask if you need help with the circuit.

One cute addition, assuming a front opening dishwasher with a door that hinges down, would be a tilt switch to auto-reset the circuit when the dishwasher is opened.

IDEngineer's idea is good, but rather than relying on hysteresis to hold the state, it would be better to use a sprung popup 'flag' released by the bimetallic strip.  You could probably hack a mechanical hot-water cylinder external (strap_on) thermostat, and simply pinch a  thin tin-plate or aluminum sheet flag cut from a flat sided can between the closed when cold contacts.  Bevel the contact edges with a fine file and 'break' the corners of the length of that edge of the flag so the flag can be easily pushed back between them to reset it.

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