Wireless proximity detector that goes off when blocked

[Wiginometry]

I'm fairly new to the field of circuit building. I noticed that there are many simple proximity detectors that can be made really cheaply, however there all of these detect when objects get close I'm trying to build a very simple circuit that will be just as cheap but detects when an object is no longer there
Eg.such as when the cookie jar is low on cookies
I don't believe in prefab microcontrollers for my simple projects as I'm incredibly cheap so any help would be greatly appreciated

 

[kellys_eye]

Weigh scales - alarm on minimum weight.

 

[Wiginometry]

Weigh scales - alarm on minimum weight.

While that's an outstanding idea if I wanted to make sure grandma takes her pills. I'm going more along the lines of the hopper is almost empty. essentially something that goes off and tells me "hey dum dum put more pellets in me so you can stay warm"
But most proximity sensors are set to go off when something gets to close I need it to go off when it's not blocked
Similar to a magnetic door or window alarm just without the magnet

 

[kellys_eye]

Proximity detectors rely on the physical attributes of the object being detected - the human body is a massive sack of s**t - errr, I mean water which is easily detected (it alters tuned circuits by capacitance) but the likes of cookies (as your example) hold little water or any property that has an effect, resistively, capacitively or inductively. The only useful attribute is weight.

You might get away by using ultra sonic range finding modules to detect the 'absence' of an object but these work on the 'hardness' of the object.

It doesn't matter if a proximity detector has something that gets too close or even too far away - both scenarios will create a signal that can be used to activate an alarm (or whatever) thus satisfying your 'detect-it-if-it's-not-there' requirement.

Is your requirement specifically for wood pellets? I can't see any other reliable method than weight if this is the case. Such detection isn't as difficult to implement as you might think.

 

[Wiginometry]

HaHa I like you bud you hit the nail on the head with the human body!!!
Anyway woodpellets is one of the things I'm trying to detect absence of.

the other however is me wanting to make a simple module to stick in a wheat/bean drill that is pulled behind a tractor I dislike the old style float gauge that it has and wanted to make something that would be a bit more accurate that would alert me when the drill is almost empty so as to not waste so much time guesstimating ... though I'm fairly decent at that....

Essentially it's not necessity that's the mother of invention but rather shear laziness

 

[kellys_eye]

There's a possibility of detecting lack of content using sound - much the same way a car engine management unit can detect knocking, you can detect a change in sound from the lack of material sloshing around the hopper. It would take some experimentation to determine the type of signals involved though - nothing 'off the shelf' as a solution so to speak.

Flow rate perhaps? If you know how much has gone in and how fast it's going out then a simple timer would alert to the potential for a refill.

Any cheap Mexicans down your way?

 

[Wiginometry]

Lol Nah darn construction guys snatch them up to quick

I'll have to play around with that idea though.... the one with the sound that is.
flow rate could vary if one of the flow tubes gets plugged.

 

[ChosunOne]

I'd be inclined to install a photoelectric sensor (photo eye) near the bottom of the hopper or jar or whatever, as long as what you're detecting the absence of, is opaque. You may need to install an LED aimed at the sensor from the opposite side of the vessel, if you want it to work in the dark. It's pretty easy to find a photoelectric-based device that can be adjusted so that a little light trickling through your cookies doesn't activate it.