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I'm trying to think of a solution to a problem.

I plan to use a 10k linear fader to control a voltage controlled amplifier (THAT 2180), however I would like to be able to control a logic high and low signal when the fader is moved from the bottom (infinity.)

The fader has 0V at the bottom and +5V at the top. I would like the fader to act like a switch as soon as the fader is moved.

I'm thinking a comparator that takes a sample from the fader output would do the job. The aim would be to have a 0 or 1 logic signal at the comparator output, high or low, it doesn't matter.

Any ideas would be greatly appreciated.

Would it be possible to use either a TL071/72 or NE5532, because I've got a load of these. The power supply is - + 15V.

Hope that's enough info, and no, this isn't my homework.

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  • \$\begingroup\$ The +/-15 output swing won't interface well with 5 V logic. You'd be better using a proper 5V-powered comparator. Then it will be easy. \$\endgroup\$
    – Transistor
    Oct 12, 2020 at 13:40
  • \$\begingroup\$ Thank you! Why didn't I think of that. I may have a MAX942 somewhere, I think that's a 5v single rail. \$\endgroup\$
    – kr00ga
    Oct 12, 2020 at 13:49
  • \$\begingroup\$ You can use an op-amp as a comparator but it won't switch that fast (probably not a problem in this application) and the output isn't directly compatible with logic. You'll need to level translate. Also, certain op-amps, including your NE5532 include inverse parallel diodes across the inputs. A TL071/2 will work fine though. \$\endgroup\$ Oct 12, 2020 at 14:13
  • \$\begingroup\$ Should all movements (which are measurable) cause a detection or only moving the fader off from the -infinity position.? If the latter option is the wanted thing should a static being elsewhere than at -infinity be continuously detected or only the movement out of the -infinity? \$\endgroup\$
    – user136077
    Oct 12, 2020 at 14:17
  • \$\begingroup\$ I don't understand your requirements. \$\endgroup\$
    – Andy aka
    Oct 12, 2020 at 15:42

2 Answers 2

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The +/-15 output swing won't interface well with 5 V logic. You'd be better using a proper 5V-powered comparator. Then it will be easy.

Most comparators will accept inputs down to 0 V. Check the datasheet because that's an important parameter for your application. Don't forget to add a little hysteresis so that it doesn't chatter on you.

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  • \$\begingroup\$ The MAX942 has the benefit of in built hysteresis. Top reply BTW, sometimes you can't see the wood for the trees. \$\endgroup\$
    – kr00ga
    Oct 12, 2020 at 14:14
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The cheap classic LM339 fits the bill.

enter image description here

Note input common mode includes ground, this is what you want since you want to compare voltage close to 0V.

It has an open collector output, which means it will do voltage translation too. For example a LM339 powered from +15V/0V can output a 5V or 3V3 logic signal with just a pullup to +5/+3V3 on the output. In your case since the voltage to compare is in 0V...5V range, it makes no difference so you can power the LM339 from the nearest +5 or +15. Don't connect it to -15, its negative supply should be GND.

Don't compare the output of your fader to 0V though, instead use a low voltage like 30-50mV, to make sure it really registers even if the comparator has offset in the wrong direction or the pot doesn't go all the way to zero.

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