How can I make low sodium (140mg or less per serving) pancakes / waffles?

Reduce or leave out the salt. It is not essential to the chemistry of the recipe. I will not speculate on salt substitutes, as that is a health and medical issue, off topic for this site.

The pancakes will then not taste as good, but that is unavoidable. Perhaps you can serve them with a highly flavorful accompaniment, like a reduced peach chutney or similar, to make the entire dish more interesting.

Not sure what kind of advice you expected.

Edit: Based on the discussion of baking soda (sodium bicarbonate) usually used in pancakes: as Kate Gregory points out, there may be alternate chemical leaveners.

Another possibility is to use mechanical leavening, by beating the eggs, fat, and any sugar well until they reach the ribbon stage (probably more than necessary, per Harold McGee, but still a clear indication); at this point they will have the maximum amount of air incorporated. This batter cannot be held, but it should rise reasonably well, even without baking powder.

There are also recipes that use whipped egg whites to leaven pancakes, but they produce a different, fluffier product that may be called Fluffy Pancakes or Swedish Pancakes depending on the recipe.

I agree with SAJ14SAJ. You can leave out the salt or use a small amount of it, without a problem. What you can add to minimize the loss of flavour, is a pinch of vanilla (for neutral-tasting pancakes) or a different aroma, for instance almond extract (for a variation).

A random web site I have no reason to trust claims that half a teaspoon of baking soda contains 616 mg of sodium. This more reputable site says 150 for 1/8 tsp, which pretty much agrees, and also says 100-200 in 1/4 tsp of baking powder. Let's take those as correct for now. My favourite pancake recipe uses 2 tsp of baking powder to make about 12 pancakes, so that's 800-1600 mg, meaning a person could have one (or possibly two if the sodium content is more to the low end) of those pancakes and meet your requirements, assuming you didn't add any salt.

If you would like to serve them 2 or 3 pancakes (which I think is a more reasonable serving size) then you probably want to look into low-sodium and sodium free baking powder and baking soda. Discuss with the pancake-eater's dietician whether the substitute is actually ok or not, (sometimes low-sodium also means low-bunch-of-other-stuff) but that same Low Sodium FAQ lists several brands of baking soda and baking powder with no sodium at all. Using those brands would let you serve all the pancakes you want!

My husband has had a low sodium diet due to kidney and heart disease since 2008 and I've faced this challenge already. Baking soda is the culprit - not added table salt. There are several low or no sodium recipes online for pancakes but be sure to select using a no sodium baking powder. One of my favorite websites is lowsodiumcooking.com. There is a no sodium / gluten free baking powder online called Featherweight Baking Powder by Hain Foods. I purchase mine on Amazon as I cannot locate it in the Boise area. In the past I've tried to use a product found at Walmart - Rumford Reduced Sodium Baking Power but the content is still quite high for our needs at 35 mg per 1/8 teaspoon. Most pancake recipes call for a minimum of 2 t of powder which equates to 280 mg per serving size. With syrup is added to the equation it just adds up too quickly for us. If kidney disease is also an issue, do not use a salt substitute as the potassium levels are increased.

Assuming you are making savoury crepes/pancakes/waffles then missing salt could a big compromize on the flavour/taste. There's no substitue for salt but low-sodium table salt(that contains Potassium chloride). (So I still prefer sea salt). Lemon juice activates the same taste receptors as sodium, so adding a spritz of lemon to your food in place of salt makes biological sense. Anyway it's not a substitute. However for savoury bits, if you can use very little portions (puree) of herbs (e.g. tamerind) that could save the palate...with that sour-saltyness. I use this herb and some others with absolutely less salt to even marinade fish/meat, make pickles.

Low Sodium Salts:

* Morton Lite Salt, 50% less sodium than table salt
* Diamond Crystal Salt Sense, 33% less sodium

But hey! For Sweet pancakes, salt is not a must. Infact never used. As everyone else pointing out you may leave it out for sweeties.

You can buy no sodium baking powder and baking soda as far as salt goes I just leave it out but I do whip my yolks and whites separately.