Lentil Vadai's can be made from different type of Lentils.I have tried out with variaties of Lentils and the best Vadai's turn out with Red lentils (Mysor Dhall).
It is the same form of making with any Lentils.Here I show with Red Lentils and Toor Dhall.
The best rerults in taste will be only if you add dried Maldive fish.As a subsitute you can add tiny dried shrimps or tiny dried sprats.But nothing to beat the Maldive fish.

Step 1: Ingredients

1 1/2 Cup Red Lentils
2 Cups Toor Lentils
Dried Maldive fish
1-1 /2 spoon Paprika powder
1 tea sp Turmeric Powder
Oil to fry
Onions 2
Capsicum chillies 2
Few green chillies -option
Salt to taste
Dried red chillie pieces (Not in pic) Option
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Grinder

Step 2: Soak & Grind

Soak the lentils for about two hours.It need not be exactly two hours,can be a little bit more ,but not less that two hours.
Drain the water completely.Let it hang in the strainer for a while until the water has drained fully.
Grind the lentils in to small bits and pieces.DO NOT MAKE IT A PASTE.Suppose it is dry and you need water JUST put one or two drops only ,if needed more then gradually add in drops.If you use more water it becomes soggy and you won't be able to make little balls and also it will crack while deep frying.
The last image shows that I have taken a little bit from the red and Toor lentils and mixed together.

Step 3: Chopp

Cut and chopp the green chillies ,capsicum ,and onions in to very small pieces.If you are not a hot eater then use only capsicum.

If you want you can add more onions.

Step 4: Add

Now add all the ingredients to the Lentil dough.They are chopped onions,chillies,capsicum,Maldive fish,Turmaric powder,paprika powder,salt to taste,and mix well using your fingers.The dough should not be sticky or dry.Should be able to make little balls using your fingers..I like to eat very hot food ,so I added some dried red chillie pieces.This is an option

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Dried Maldive fish gives a very good taste.If you do not have dried Maldive fish ,then you can add tiny dried sprats or dried little shrimps.This is a must for VADAI'S

Instead of big onions use small red onions -The taste is much better.I did not have at the time of making it.

Step 5: Make Shapes

With one hand make small balls(a lttle bigger than a table tenis ball) or ovel shapes ,the size being a little smaller than an egg.Please do not make it thick or big as when deep frying the inside will be raw.