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If you can’t power something directly with a signal that has a voltage close to what you want, then a Bipolar Junction Transistor (BJT) wired as an Emitter Follower (aka. common collector) might work for you.
Keep in mind that NPN BJT emitter followers output approx. 0.6V less than the signal.
A potentiometer makes for a good voltage signal, but a terrible voltage source.
It relies on resistance to divide a voltage. By taking current from a trimpot (trimmer potentiometer) you throw off that voltage.
An emitter follower is when a load is between an NPN BJT emitter and ground, while the collector is connected directly to the positive supply.
A middle range voltage at the base starts conducting current through the base to emitter. However, once a little bit of that current starts flowing, a lot more current can flow from collector to emitter (and through the load).
Therefore, the collector current flows as needed to almost completely stop the base to emitter current from flowing. To do so, the collector needs to pass the current that the load demands when it has 0.6V less than the NPN base voltage across it.
- PNP BJTs use the opposite polarities but the basic principles are the same. They output 0.6V higher than the signal at the emitter, instead of lower.
- An extra 10k resistor is commonly added from emitter to ground. It helps hold the voltage steady when there is a tricky load. A different value can be used.
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