From: bill.sloman@ieee.org   
      
   On 12/02/2026 2:56 am, john larkin wrote:   
   > On Wed, 11 Feb 2026 23:38:42 +1100, Bill Sloman    
   > wrote:   
   >   
   >> On 11/02/2026 6:39 am, john larkin wrote:   
   >>> On Tue, 10 Feb 2026 07:49:22 -0900, Christopher Howard   
   >>>  wrote:   
   >>>   
   >>>>> Standard apprach for low-accuracy approximation is diode-resistor   
   >>>>> network.  But such network gets inpractically large if you need   
   >>>>> good approximation.   
   >>>>   
   >>>> Thank you. Looking at all the suggestions put forward, it seems like —   
   >>>> for my application — that just getting the AD633 might be best choice.   
   >>>> My analog computer is short one multiplier for a simulation I wanted to   
   >>>> play with, and I wonder if there was some easy trick to get a reasonably   
   >>>> accurate square function without getting a decent multiplier. In a lot   
   >>>> of basic physics simulations, one has to square velocity to get   
   >>>> things like drag or lift forces.   
   >>>>   
   >>>> Line approximation with a resistor-diode has some appeal. Maybe it could   
   >>>> be accurate enough for my little educational experiments? I think, to   
   >>>> pull it off, I would need to use trim pots — two per segment, for the   
   >>>> biasing and the attenuating.   
   >>>>   
   >>>> The schematic from the neurological paper seemed to be a line   
   >>>> approximation solution with some of the diodes in the op amp feedback. I   
   >>>> didn't try to get the whole research paper so I'm not sure how one would   
   >>>> work out the correct resistor values.   
   >>>>   
   >>>> I downloaded the information on the MC1496-D and LM13700. I see the   
   >>>> basic idea is that these chips also produce products of signals, but   
   >>>> beyond that I can't make any intelligent comments at present. My analog   
   >>>> computer is usually dealing with signals in the range of a few hundred   
   >>>> Hz up to a few kHz. I am curious if maybe something could be done with   
   >>>> the gain control pin on the LM386 chip (I have quite a few of those). It   
   >>>> looks like, using a series RC network, that the gain can be set anywhere   
   >>> >from 20 to 200.   
   >>>   
   >>> A simple opamp circuit with some resistors and diodes can have a   
   >>> several-segment approximation to a square root. I did that in a   
   >>> steamship throttle control once and it worked well enough.   
   >>   
   >> Didn't you have a DEC digital processor to play with for that gig?   
   >>   
   >> The DEC PDP-8 I got to play with for my Ph.D. project could be persuaded   
   >> to do multiplication and division - I used it for signal averaging - and   
   >> square root extraction shouldn't have been difficult.   
   >   
   > Yes. I had a PDP-8 12-bit machine running FOCAL, and simulated the   
   > steam valve, the turbines, the prop,  the hull, and the ocean. I   
   > plotted the responses on a teletype machine. We showed it to the   
   > shipyard and the owners and they liked it and we got tho sell them the   
   > engine room consoles. That was for the 32,000 hp, LASH ships. I think   
   > I was a sophmore at Tulane at the time.   
   Focal wasn't up to much. I programmed my machine in assembler   
     - Macro-8 - and used interrupts to get tolerably precise timing.   
      
   > Sea trials were fun. People treated this gawky kid like a god and I   
   > got my own cabin.   
      
   The chemistry department wrote me off as gadget-happy. which wasn't   
   entirely fair. My gadgets did work   
      
   > The actual control loop was feed-forward from the throttle to the   
   > steam valve position, with a diode nonlinear function generator, and   
   > limited-influence RPM feedback. That limited the number of people who   
   > might be killed if the feedback tach failed. Which one did, but that's   
   > another story.   
   >   
   > Ships are most all diesels now. Steam plants are efficient but complex   
   > and it's hard to get crews who can run them.   
      
   My father got an Argus mini-computer from Ferranti to run his continuous   
   counter-current digestor a bit later. It had enough spare capacity to   
   run a couple of paper machines as well, where it was lot more valuable -   
   the programmed transition from one weight of paper to another went a   
   whole lot faster under computer control.   
      
   Eventually the digestor went back to manual control - the control crew   
   had learned how to do it right from the computer program, and didn't   
   need the extra help, and the computer took over a few more paper machine..   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)