A set Base voltage and emitter resistor can set the current through a PNP Bipolar Junction Transistor (BJT).
A trimpot can set the base voltage anywhere from one supply rail to the other.
Since the PNP Emitter heads to the positive supply, the Base needs to be approx. 0.6V lower than the positive supply in order to start Emitter to Base conduction. Below hat point, the PNP transistor will pass current as needed to keep the Emitter 0.6V higher than voltage set at the Base.
The voltage difference across the Emitter resistor will set how much current flows through it, the PNP BJT Emitter to Collector, and even an LED if it is the load between collector and negative supply.
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