slightly OT, need some electronic circuit help

Michael Torrie torriem at
Mon Oct 8 15:36:15 MDT 2012

On 10/08/2012 03:18 PM, Shane Hathaway wrote:
> Ok, it seems like you want 2 op-amps wired as comparators.  One op amp 
> decides whether to move the motor forward, the other decides whether to 
> move it backward.  (You can get multiple op-amps on a chip, so this 
> doesn't increase the parts count.)  You'll have to calibrate the circuit 
> so that the op amps don't activate simultaneously.
> To keep things simple, let's say each op amp drives a DPST (or DPDT) 
> relay.  That solves your voltage issue: it isolates the control circuit 
> from the motors, so you can apply reverse voltage to the motors.  You 
> can also use a voltage regulator in case the supply is too noisy.

Good to know I'm on the right track. I was thinking along these very
lines in fact just when I noticed your e-mail.

> Each op amp should perform a slightly different comparison.  When you 
> want the middle position, the op amp that controls forward movement 
> should compare the motor sense voltage with a voltage slightly lower 
> than the control voltage, while the other op amp compares the motor 
> sense voltage with a voltage slightly higher than the control voltage. 
> You can adjust the voltages using resistor-based voltage dividers.
> Does that make sense?  The comparator circuit on this page is a good 
> reference:

Absolutely makes sense!  This is the track I'm heading down.  Thank you
very much for your thoughts on this.  I knew there had to be a
relatively easy way of accomplishing this.


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