I want a Tiny85 based device under the lid of a jam jar. I want it to turn on some LEDs when I touch the lid. It's powered by a coin cell.
How about a piezo disk to sense vibration? You may need to tap lightly rather than just touch the lid. If it's -too- sensitive it might detect a door slam so there's a range where it'll be just right.
Is this a metal lid? Most jam jars I've seen around are metal. If yes then make your own capacitor.
About a billion years ago one of the first factory jobs I had was to make 'Bolometer elements.' I'm not even sure what the heck the elemts were for was but it was some sort of ancient (meaning WWII era) technology that was still in use in the late 70s. *ALL* the devices they had in stock were *BAD* because the guy who'd been making them just did whatever he felt like and had no idea what the actual process was. So, I had to research from scratch how to make the friggin things. Pretty cool for an 18 year old kid eh? LOL!!!
The most IMPORTANT aspects of that Bolometer element was two capacitors about 1/4" across, built by spraying silver paint on opposite sides of a piece of mica, and a platinum wire (wow now just popped into my head, Wollaston wire,) about 1/8" long. That made a very precision R/C circuit. I had to research at the local community college library how to calculate the dimensional specs for these precision caps and resistors.
The critical aspects were that the silver/mica caps and the Wollaston resistor had to be within a certain range of values. The guy who -had- been making them made boxes full of faulty parts had paid close to no attention to those values.
There's a formula that I don't remember any more that references the square inches of dielectric between two plates, vs the plate thickness. I'm sure you can find that somewhere.
You need a know thickness dielectric, but very stable, nylon, mica, maybe some other plastics. But they used mica because it was very stable, pure nonconductive dielectric, and fairly precision in thickness for repeatability. In your case precision is not much required, just 'a' capacitor of some vaguely known value. Whether you'll need to make something the size of a dime or a half dollar I have no clue - it depends on the thickness of the dielectric - but in general 'thinner = more capacitance.'
Glue down that dielectric on the underside of the lid with maybe CA which works best when clamped super-thin. That's to avoid much variation between the -calculated- dielectric thickness vs -assembled-.
Then glue another piece of conductive plate, any thin metal really, to the dielectric.
Now find a capacitive sensor circuit. Something that doesn't measure absolute capacitance but 'registers' when the capacitance changes 'a bunch.'
When the jar is untouched the capacitor will charge up to some level by static electricity, will vary with humidity. Again the exact value doesn't matter.
Your body will normally be at some voltage level drastically different from the jar capacitor, either higher or lower but probably generally much higher. When you touch the lid the electrons will kick off the sensor circuit and register in the Arduino.