AnsweredHot!logarithmic potentiometer

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brownt
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2018/04/16 19:21:26 (permalink)
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logarithmic potentiometer

Using a 50K MCP4341 digital pot and I would like the output to be logarithmic. The input value written to the pot is 0 - 127.
 
I have tried this, log10(value) * 60, which gives a fast rise at the start and then a slower rise at the end. That is what I want, but I wonder if there is a better way than *60.
 
Also I would like the option to have a slow rise at the start and a faster rise at the end. How is that done.
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qɥb
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Re: logarithmic potentiometer 2018/04/16 20:39:59 (permalink) ☼ Best Answerby brownt 2018/04/16 23:19:31
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brownt
Using a 50K MCP4341 digital pot and I would like the output to be logarithmic. The input value written to the pot is 0 - 127.
 
I have tried this, log10(value) * 60, which gives a fast rise at the start and then a slower rise at the end. That is what I want, but I wonder if there is a better way than *60.

So you are trying to scale a 0-127 value to give you a 0-127 output with a log curve?
You can't take LOG10 of zero, so "log10(value+1) * 60" would be safer, although that only gets to 126, not 127.
 

Also I would like the option to have a slow rise at the start and a faster rise at the end. How is that done.

You change from "log" to "antilog", i.e. by using a power function.
something like:
(1.039 ^ value)-1
will give an output of 0 to 127 for inputs from 0 to 127.
 

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qɥb
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Re: logarithmic potentiometer 2018/04/16 22:24:15 (permalink)
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n.b., just a lookup table might be better than all this floating point math.
 

This forum is mis-configured so it only works correctly if you access it via https protocol.
The Microchip website links to it using http protocol. Will they ever catch on?
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