got it working

Rewired it from scratch. Measured output. Yeh. Saw volts as low as 1.2 and change. Connected the barn door tracker motor. 4 revs in 1 minute 6 seconds. In the ball park now, unlike the first attempt.

Still, I'm down at the very end of the pot. No more room...

Reviewed Ian's notes. "V_OUT... with the pot turned down to 0 [should] be 1.25." My results agreed with this. But I wondered if I could get it lower. He showed the formula:

Output voltage = 1.25 x (1 - R2/R1) 

where R2 is the pot and R1 is the... resistor [on the output side].

I punched that into Excel and got crazy numbers. Rising exponentially?

He sent me a link. Www.reuk.co.uk/LM317-voltage-calculator.htm. But it didn't work. Crikey!

Fortunately, the 404 page was smart and offered a search. Looked up the LM317. Ah ha! Formula was wrong too!

Output voltage = 1.25 x (1 + R2/R1)

Then I found the calculator!

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

And learned, quickly, why it doesn't matter that it's a 1k pot...

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LED seems to be responding better too...