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

[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!

 

[Visitor]

Look at figure 2 in this article. I think it's exactly what you need. SCR Principles and Circuits

S1 will be a normally closed pushbutton switch and S2 will be the temperature sensor. When the temperature sensor closed (switched on), it will trigger and latch the SCR, illuminating the LED, until S1 is pressed to briefly remove the power.

 

[Les Jones]

Just use the contacts of the thermostat to trigger a small SCR such as a 2N5060. The light (LED ?) must draw more than the holding current of the SCR for it to remain latched on. To reset it just break the circuit to the light with a normally closed push button. You could use a small relay but the SCR is cheaper and simpler.

Les.

 

[rjenkinsgb]

Looking at the original question, the requirement is to trigger the indicator light after the dishwasher cycle has completed and it has cooled to a certain point.

That will need an "edge trigger" or two-stage sequence, so it's not falsely latched on when the device is initially set and the machine has not heated up.

It can be done with a 4093 using one part as a debounce and that capacitively coupled to two more sections configured as a bistable.
I'll try to draw up a legible version of the circuit to post soon.


Edit:
This is my concept circuit, using a CD4093 quad schmitt NAND gate and a few other components.
Ignore the extra bits on the gate symbols, that's just the way that program represents them..

V1 at the left is the power source, eg. a 9V battery.
V2 is the thermal switch.

The two gates at the right form a bistable or flip-flop circuit, which can be set to either state by taking one or the other input low.
(More info here: https://en.wikipedia.org/wiki/Flip-flop_(electronics)#SR_NAND_latch )

When power is switched on, C3 holds the lower input low for a significant time (longer than the other capacitors take to charge and everything settle), which forces the bistable to the state where the lower output is high - so no voltage across the LED.


Initially the thermal switch is off, so R1 charges C1 and the output of the A1 gate switches low after a short time.
(C3 should hold the other input of the bistable low for longer than that, so nothing happens).

When the thermal switch closes, C1 discharges, A1 output goes high.
Nothing else happens.

When the switch opens as the machine cools, C1 charges again, A1 output switches low and a pulse passes through C2, triggering the bistable to change state so the upper output is now high and the lower one, connected to the resistor and LED, is now low - the light comes on.

It will stay in that state until power is removed.

It needs an on-off switch in the battery connection & it should have a capacitor between power and ground.
R3 and R4 should also be a rather higher value in retrospect, 47K or 100K would be reasonable.

The only steady-state current draw apart from the 4093 (which takes miniscule power) is R1 while the thermal switch is closed, and the LED circuit once the device is triggered.

An ultra-bright LED could run from a higher value resistor to reduce current consumption further; I have some white ones that are painful to look at with less than 1mA current..

If anyone sees any daft mistrakes or has any improvements, please comment!

Edit 2 - I just realised the spare section of 4093 could be used to make the LED flash on completion, rather than just light continuously.
Use this circuit: https://lh5.ggpht.com/_ghw-EsIYo58/TTtzsVVgCnI/AAAAAAAAAFQ/u9XaBTjbBOs/s640/AboutSchmitt02.gif

And connect the "control" input to the upper gate output on the latch.
The LED & resistor again between the [added] gate output and positive power.

Something like 1M and 1uF should give a moderately fast flash.

 

[Visitor]

I think the above is over-solving the problem. The need is not so much to tell if cycle is complete but rather if the dishes in a cold dishwasher are clean or dirty.

 

[Visitor]

A relay could also be used instead of an SCR, at the cost of greater battery drain. This circuit is functionally the same as the SCR circuit, but perhaps easier to understand or to help explain the SCR circuit.

 

[crutschow]

A relay is more expensive and takes more power than an SCR.
I vote for the SCR circuit.

For maximum battery life, use a high brightness LED as an indicator and operate it at just a few mA (as determined by the battery voltage and the resistor in series with the LED).
Good battery life can be obtained with 3 AAA alkalines in series.

Are you sure the surface of the dishwasher will reach 85F on a winter day, when the house may be cool (say 68F).

 

[alec_t]

If anyone sees any daft mistrakes or has any improvements, please comment!

The units for C1 and C3 are not specified. Presumably you intended uF, not F ?

 

[rjenkinsgb]

The units for C1 and C3 are not specified. Presumably you intended uF, not F ?

Yes, absolutely - all uF values!

 

[AnalogKid]

A CMOS 555 also will do the job and does not need a buffer to drive an LED. And. I kinda like the idea that the LED does not come on until after the dishwasher cycle has finished, but there will be a significant delay between when it is ok to open the door and when the door has cooled enough to trigger the latch (whatever it is).

ak

 

[gophert]

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!

the part your daughter found at Alliedelec.com is perfect and will do exactly what you want because it turns on at about 85°F and stays on until ambient temperature falls below 67°F. You may have to cool the sensor in tap-water to reset it. It may contain a bi-metallic coil and a magnet to maintain the contact as it cools.

 

[Reloadron]

I,m going with the thermal switch, SCR and a LED simple and should work just fine.

Ron

 

[gophert]

I,m going with the thermal switch, SCR and a LED simple and should work just fine.

Ron

common, SCRs are not suitable with batteries. Too much holding current.

How about this...
- diodes are the temp sensing elements.
- one row of 3 diodes faces the dish washer to sense the warmth
- one row of 3-diodes monitors room temp
- micro-current JFET op amp (TLV2241) is used to trigger the LED.
- the on state of the op amp gives positive feedback to keep latch the output (with a diode) until power is turned off/on.
- make sure the two 220k ohm resistors are well matched.
- highlighted resistor may need to be tweaked from 1k to 5.6k.
- the transistor (e.g. 2N3904) is needed because the drive current of the microamp op amp is so low (voltage sag below LED forward voltage).
current draw in off state is 20 uA

current draw in on-state with high-brightness LED is 390 uA.

 

[Reloadron]

common, SCRs are not suitable with batteries. Too much holding current.

You have a good point there in that it's not like we just fire the SCR with a Gate pulse. Using a button type surface bi metal little thermostat it will remain on as long as the temperature is above its threshold. I have not looked at the data sheets for any low current, low voltage SCRs. Now if I were to use PN junction to grab temperature I would likely use 2N3906 transistors with the collector and base tied. I have seen that done quite a bit to sense temperature and since that design you posted looks at a differential you could probably just use two transistors. Linear Technology has a pretty good paper on the method.

Ron

 

[gophert]

You have a good point there in that it's not like we just fire the SCR with a Gate pulse. Using a button type surface bi metal little thermostat it will remain on as long as the temperature is above its threshold. I have not looked at the data sheets for any low current, low voltage SCRs. Now if I were to use PN junction to grab temperature I would likely use 2N3906 transistors with the collector and base tied. I have seen that done quite a bit to sense temperature and since that design you posted looks at a differential you could probably just use two transistors. Linear Technology has a pretty good paper on the method.

Ron

Thank you. Interesting write up.