I am going to use a 2N2222 transistor to light up an array of LEDs. I should hook no more than 15-20 LEDs in each transistor.What's the correct the value of the resistor that goes between the transistor base and the Arduino pin?
OP also mentions "in each transistor".How many transistors.At those base currents, you also have to watch total MCU current.Leo..
So Vbe and Vce is the dropped (dissipated) voltage and in the case of NPN transistors it is kind of a "constant" valuing 0.7 right?
In your circuit, with the diode/resistor pairs on the collector side, Vbe will be approximately 0.6 Volt (or could assume 0.7 Volt too). Reason...... Ve is grounded at 0 V. The 0.6 V is Vbe(saturation) from manufacturing specs guide for the 2N2222. It's just minimum value assumption for Vbe..... assuming 0.7 V is fine too. It is linked to the 'diode' drop of the transistor's own 'pn junction'.Vce is the voltage (Vc - Ve).
What is Vc and Ve? Voltage-collector and Voltage-emitter?