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What is the voltage from a USB 2.0 port on a desktop? 5V?
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Can a potentiometer be used to regulate light from a LED?
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If so, assume can find one for the voltage from a USB port, yes?
Reason I ask, I have a microscope that uses a LED for the illumination. Unfortunately, unlike most, this scope does not have a diaphragm below the stage to regulate the intensity of the light, which is important, nor can one be added.
The area in the base can be accessed to change bulbs, so it would be easy to splice into the two incoming wires to attach a pot.
- Oh yeah, when I was young, we had rheostats. Are potentiometers the same thing? If not, then a rheostat is probably all I need for this fix, nicht whar?
1. What is the voltage from a USB 2.0 port on a desktop? 5V?
Yes. 5V. Note that there is a max current draw of 500 mA which you won’t want to exceed.
2. Can a potentiometer be used to regulate light from a LED?
Yes. A typical circuit would be a potentiometer and a resistor in series with the LED. When the potentiometer is at a dead short, then the series resistor alone determines the current through the LED, which will give you your maximum brightness. As you increase the resistance from the potentiometer, the brightness of the LED will decrease.
Some LEDs have the series resistor built into them.
3. If so, assume can find one for the voltage from a USB port, yes?
Yep. If you get a pot that’s too high in resistance it will cut the brightness of the LED down too quickly. Without knowing the specs for your LED I’m guessing a 5k or a 10k pot should do the trick.
4. Oh yeah, when I was young, we had rheostats. Are potentiometers the same thing?
Pretty much. A potentiometer is typically constructed using a wiper that slides across some resistive material. A rheostat is usually constructed with a wiper that slides across a coil of resistive wire. Rheostats in general are designed to carry more current than a potentiometer. But, basically, either way you are talking about a variable resistor.
Fantastic, engineer_comp_geek, or may I call you geek for short? 
You made my day. Many thanks for that excellent information.
I would use an LM317 configured as an adjustable current source.
The LM317 - which is sold at Radio Shack for $2.29 - is actually a “programmable” linear voltage regulator. The term “programmable” means you set the output voltage using a couple resistors.
Here’s a link to a datasheet.
The neat thing about the LM317 is that you can also configure it to be a constant current source, which is ideal for powering an LED. And it only requires one programming resistor when configured as a constant current source.
Here’s a schematic of the LM317 being used as a constant current source to drive an LED. For your application, you would simply replace the fixed resistor with a fixed resistor and potentiometer (wired as a 2-terminal variable resistor) in series.
The current can be calculated as follows:
I = 1.25/(Rf + Rp)
Where I is the current in amps (A), Rf is the resistance of the fixed resistor in Ohms, and Rp is the resistance of the potentiometer in Ohms. If you want (for example) a maximum of 18 mA through the LED, then you would use a fixed resistor of around 68 Ω and a 1000 Ohm potentiometer. When the potentiometer is set for 0 Ohms, the current will be around 18 mA. When the potentiometer is set for 1000 Ohms, the current will be around 1 mA.
Here’s what I like about using a constant current source vs. a simple series potentiometer: as the battery voltage decreases, the current through the LED will also decrease if you use a simple series potentiometer. If you use a constant current source, the current will remain steady (down to a certain battery voltage). You can think of a constant current source as being a variable series resistor, and the resistance is automatically lowered as the battery voltage goes down to maintain a constant current.