Yes. Restats or variable resistors will slow the motor but at a loss to torque. So you wouldn't be able to turn as much weight. Also isnt good for the motor and may decrease its life.
PWM is better. The wave visual works for guys like us who study sine waves. A more rudimentary way is to imagine hard wiring power and ground to the motor. Then install a switch on the ground leg of that and rapidly flicking that switch 1000 times a second. The voltage remains the same. All its doing adding and removing the ground at set intervals.
A note on the sensor. You are going to need a decent quality capacitive sensor. Lead is non ferrous (magnetic). So looking for sensor rated to detect it will help. They work by using a magnetic feild to induce a voltage and then detecting that. Materials that are non magnetic will respond with a a weak signal. For more information see link
https://www.se.com/uk/en/faqs/FA22749/
NPN or PNP doesn't matter. Thats just how the transistor in side the sensor works. NPN mean Neutral-Power-Neutral and it will send out a ground (this is the most common as controlling ground is safest). PNP stands for Power-Neutral-Power it will send out a power signal when tripped.
Search for an NPN sensor rated for nonferrous metals will enhance your success.
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