Dishwasher Liquid Level Sensor: How ?

Hi,
I have a ten year old KitchenAid dishwasher for which I have just replaced the automatic dispensing soapdish assembly. This is the kind that has the little flap door cover on it that opens (via a solenoid) at the appropriate time in the wash cycle to allow the dry soap to fall out into the wash.
Anyway, it also has a dispenser for some liquid that you can add, which we never have. Probably for some wetting agent liquid that they sell. It is dispensed at the correct time, also, by this same solenoid which in addition to opening the soap dish cover actuates a small piston type plunger which squirts a small amount of the liquid out.
Have just noticed for the first time that there is a light on the control panel that indicates this liquid dispenser is empty. Probably was not functional/broken in the original soapdish dispenser assembly.
Can't for the life of me figure out how they determine if the liquid dispenser is empty, though. The only thing going to this assembly are two leads for the soap door solenoid.
But there are also two leads going to a very small, looks like a resistor, mounted on the outside wall of the liquid dispenser. Can't come into contact with the liquid, as it's a solid piece of plastic between. Plastic is black, so not an optical sensor.
All I can come up with is that it is a resistor, and part of an electrical-bridge where the difference in thermal conductivity of the heat radiating from the resistor is different if there is, or is not, liquid on the other side.
Somehow, I don't think that this is how it works.
Any thoughts on how they know if the liquid is empty in the chamber ?
Thanks, Bob
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Robert11 wrote:

you know you can buy those little plastic packed cubes of dishwasher soap.. just toss it in and shut the door
when the dispenser loses it's prime, it turns the light on
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Hi,
Thanks for reply.
Npt sure what you mean by: "when the dispenser loses it's prime" , [it turns the light on] A few more details, please.
Thanks again, Bob
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Robert11 wrote:

it means it's sucking wind
when the timer calls for the "rinse aide" it sends electricity to a solenoid switch, which opens the dispenser
I don't really know that much about it, yours is probably a photosensor
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Robert11 wrote:

i take that back, you said there was 2 leads going to the dispenser so thats connected to the solenoid
far as the light? probably just a simple float type swith, like in the back of a toilet bowl
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could be a thermistor which is a resistor that changes resistance with temperature, then it could woek just as you say, the liquid keeps it cool, when it runs outm the thermistor heats up....so the thermistor can be both the heater and sensor in one component.
Mark
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