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Sometimes schematic diagrams include additional information to know about the circuit. Such as how much current you can expect through the circuit.
A capital i (sometimes looks like a lowercase L), inside of a bubble, placed in series with one or more components, is one way to draw attention to the current flow in that (part of the) circuit. I is the symbol for current when doing formulas.
It is important to learn ohms law formulas. To calculate how much current (I) in amps that you can expect through a resistor, you take the voltage across the resistor, and divide that by the resistance of the resistor in ohms. The value that you get is in units called amps (A).
I = V/R
If you see a value given in the bubble (usually in mA), then that should be approximately how much current you can expect from that circuit. There’s a lot of variables in electronics that may cause measurements of the circuit to be slightly higher or lower than calculated. Most circuits don’t need to be precise.
Diagram above shows the calculated current through various circuits. 20mA is the absolute max you want to put through an indicator LED. It’s better to raise the resistance to lower the current to 10-15mA for some LEDs than to aim for 20mA. And you can even aim for as low as 2-5mA for other LEDs that are still bright at lower currents.
1mA is the same as 0.001A. One milliamp (1mA) is one one-thousandths of an amp. Whereas 1 amp (1A) is the same as 1,000mA.
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