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NPN Bipolar Junction Transistors with resistance on the emitter side has the basic electrical property of setting the base voltage minus the approx. 0.6V base to emitter drop across the resistance.
The resistance can usually just be the load. But, some loads may through off the voltage, so it’s a good idea to include a resistor (10k should work well) parallel to the load.
The trimpot makes for an easy to use voltage divider. But, it can’t provide hardly any current without that voltage being dropped by quite a bit. The transistor base to emitter needs almost no current from the trimpot to set it’s emitter voltage to approx. 0.6V less than the trimpot.
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