[sdiy] LM3914 current control was: new project working!!!

[Michael Ruberto]

hi,

well I have spent several hours experimenting and here's what I've got. I 
can vary the output voltage coming from the resistive DAC by more than 6 
volts using a CV. the CV is run into an inverting follower then through an 
18K resistor to the 3914's pin 7 (REF OUT). I had to change all the 
resistors between the outputs and +V rail from 10K to 1K. now what this 
allows in regards to my original idea is the elimination of the divider and 
multiplier stages. instead of the divider there will be a VCA controlling 
the input signal amplitude before it enters the input of the 3914. the CV 
controlling that VCA will also be used (inverted) to control the current 
flow of the 3914's outputs. now the tricky part will be obtaining a balance 
between input amplitude and output voltage so the output will increase as 
the signal amplitude is decreased in such a way as to partially negate the 
effects of attenuation by the VCA.
I imagine it will be impossible to get it mathematically perfect but that's 
ok since I just want to be able to process a CV at the input and still have 
a decent output range at low bit counts.

this scheme also saves on parts costs since it now only requires a 3080, a 
3914, and a quad op-amp. tomorrow I will work on getting the balance right 
and hopefully find time for a schematic if it all works.

~M

PS: Ian, I suspect this current control may also be useful with your 
PPPPPulser device. my brain is a bit fried so I will just have to leave it 
up to your imagination until I can think more about it.

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